Radio Boulevard
Western Historic Radio Museum


WWII Radio Equipment - Part 2

WWII Airborne Radio Communications
 Airborne Radio Navigation Equipment

U.S. Navy and U.S. Army Air Forces


PBY Catalina with DU-type Direction Finder Loop Antenna installation (1938 photo)

Airborne Radio Communications Gear & Airborne Radio Direction Finding-Navigation Equipment

  U. S. Navy

Navigation methods and equipment described in this next section are extremely dated. All air navigation methods and the equipment in use today have evolved and advanced tremendously compared to those methods and equipment used over 80+ years ago. Satellites and computers are driving all navigation today. Pre-WWII, the only satellite orbiting the Earth was the Moon and most computers were slide rules. I've included some basic information about pre-WWII (and WWII) air navigation methods to show how this radio equipment was used and what the operators were trying to accomplish at that time. While the following equipment can still perform its design functions and demonstrate the principles of Radio DF navigation using a "Radio Compass," all of the various Range Beacon signals and other Air-Nav signals referenced in the following write-ups are no longer being transmitted (and haven't been for over half-a-century.) The only "old-time" Air-Nav that's still around are Non-directional Beacons and these can easily be received by this equipment. However nowadays, NDBs are also considered antiquated and they are seldom, if ever, used for air navigation. Many airports still keep their NDB in operation as "aviation tradition" and its signal is provided mainly as airport identification (at least in the USA.)

Navy Department - RCA Manufacturing Co., Inc. - Contractor

 MODEL  DZ-2 - Aircraft Radio Direction Finder
NAVY Designation: Type CRV/CEX-46152  

(CEX example shown was manufactured by Emerson Radio & Phonograph Co., CRV was used for RCA Mfg. Co.,Inc. builds)

RCA Manufacturing Co., Inc. built the earliest versions of this Radio Direction Finding receiver for the Navy for use in air navigation and search-rescue. The DZ-2 dates from 1939 but there was an earlier DZ-1 (almost identical but with upper frequency end of 1500kc, the DZ-A was similar to DZ-1.) The subsequent contracts were built by other companies during WWII (up into 1942.)

DZ-2 General Description - Using 8 tubes in a superhet circuit and tuning from 15 to 70 kc and from 100 to 1750 kc, the DZ-2 used a rotatable dual loop antenna (the VLF/LF loop is used on Bands 1 & 2 while the LF/MW loop is used on Bands 3, 4, 5 & 6) and a fixed vertical "sense" antenna (usually a "T" wire from the cockpit to the tail) to determine "true" direction (called Uni-lateral Reception/Sense.) Non-directional and Bi-lateral (bi-directional) options were also provided. The DZ-2 also featured a BFO (CW/MCW toggle switch in CW) and a switched audio filter ("SHARP" switches in a bandpass filter with a CF of 1020hz for CW reception.) The receiver used a cushioned shock mount.

POWER was provided by the aircraft battery/charger system buss running at approximately +28vdc cable-routed to the DZ-2 POWER connector and internally to the front panel POWER switch. When in the "ON" position, +28vdc was routed to the tube heaters and to pin B of the DYNAMOTOR connector which then routed the voltage down the three conductor DYNAMOTOR cable to the external dynamotor box. The dynamotor box had only one three-pin connector. The DZ-2 "ON" position turns the motor section of the dynamotor on and then the generator output section routed the +230vdc B+ back up the DYNAMOTOR cable via pin A to power up the DZ-2 B+ requirements. Pin C provided the common chassis ground return.

photo above: DZ-2 top of the chassis

The loop antenna (CRV-69065) was installed in an aerodynamic housing externally mounted on the aircraft fuselage with cables for signals and loop rotation routed to the aircraft interior for operator control. A dual-scale azimuth compass was provided and the compass was fitted with a relay-operated articulated mask that allowed only the correct scale to be viewed depending on the selection of BI-LAT (bi-lateral or bi-directional) or UNI-LAT (uni-lateral or single null cardioid pattern) functions and also on the frequency band selected. The two loops were mounted one loop within the other and offset at a 90?angle to one another (with the VLF/LF loop being the outer loop and the LF/MW loop being the inner loop.) The two loops were center-tapped and also shielded from each other. The corresponding two azimuth compass scales were also offset 90? The relay-operated compass mask would show the correct scale for the loop in use (dependent on the FREQUENCY RANGE selected since the VLF/LF loop is oriented 90º from the MW loop) and then would also change again when UNI-LAT was selected since that required a 90?repositioning of the loop from the Bi-directional null position. (more details in "DFing with the DZ-2" below.)

photo above: DZ-2 bottom of the chassis. The long rectangular gray box at the lower right is the optional line filter for the +24vdc input voltage (aircraft batt-gen buss.)

Tubes used in the DZ-2 were mid-thirties glass envelope types with 6 volt heater requirements. RF-6D6, LO-76, Mixer-6C6, IF1-6D6, IF2-6D6, Det-6C6, BFO-76, AF Out-41 were the tube used. V-101 is actually a neon overload protection bulb on the RF amplifier grid. Noticeable was the tuning range gap from 70kc up to 100kc. This was to allow for the IF amplifier stages that operated at 89kc. The first IF amplifier stage input transformer had a tertiary winding that was switched in above 200kc. The tuned frequencies below 200kc employed somewhat loose coupling in the IF to provide sharp selectivity (also, this transformer was tuned to 88kc.) Above 200kc, the tertiary winding was switched in and that increased the coupling and broadened the IF bandwidth. The two remaining IF transformers were tuned to 89kc and were of standard configuration. The DZ-2 BFO was set to produce a 1000hz beat note on Bands 1 and 2 but to zero beat the IF on Bands 3, 4, 5 and 6.

No AVC was employed in the DZ-2 since the receiver was strictly for DF and the use of AVC could affect the operator's ability to find the nulls in signal levels. Audio output impedance was 600Z ohms. Dual phone jacks allowed the operator not only use 'phones but to also insert an audio output meter to monitor the receiver output level to aid in tuning in nulls. The audio output transformer T105 had an internal and separate choke in series with the transformer plate winding to provide isolation for the screen voltage to the type-41 tube and the plate winding of the audio transformer. 

DFing with the DZ-2 - NON-DIR (non-directional) reception used just the sense antenna since it was omni-directional and usually allowed finding and tuning-in the desired signal frequency easily. BI-LAT (bi-directional) used just the loop antenna since it provided a "figure 8" pattern. UNI-LAT (uni-lateral) used both the loop and the sense antenna combined within the receiver's RF amplifier input section to provide a "cardioid pattern" that indicated true direction. DFing with the DZ-2 involved first using BI-LAT to determine approximate bi-directional signal minimum levels or nulls with one of the two nulls then selected and the bearing noted. The DZ-2 was then switched to UNI-LAT which resulted in the selection of the uni-lateral scale on the compass (the scale mask was shifted to show 90º offset.) The loop was then rotated to the same bearing that had been noted in BI-DIR on the formerly showing azimuth scale (essentially, rotating the loop 90º.) This orients the loop so that the minimum response of the cardioid pattern is either pointing directly towards the signal source or pointing 180º in the opposite direction away from the signal source. The UNILATERAL ADJUSTMENT control was then rotated while noting the signal level listening for a noticeable drop or null in the signal level. If a minimum null wasn't found, then the loop was rotated 180?and the UNILATERAL ADJUSTMENT again rotated listening for a minimum null response. IF the minimum null response heard was when the loop was at the first compass bearing (same noted Bi-directional bearing) then that compass reading was the correct bearing direction (the null towards the signal source.) IF, however, the minimum response was found when the 180?rotation was selected, then the actual correct bearing was due to the 180?rotation resulting in the loop null then pointing towards the signal source at that bearing on the compass

photo above
: The data plates on top of the DZ-2 cabinet. Note that the "contractor" is RCA Mfg. Co., Inc. but this receiver was actually built by Emerson Radio & Phonograph Corp.
The DZ-2 Loop Compass (Loop Drive Assembly) CRV-69064 - Photo left - The dual scale consists of two 0 to 36 indexed scales offset by 90º, one or the other of which is covered by an articulated mask (in the photo the upper scale is showing 90º and the masked lower scale would be showing 0º.) The box connector (behind the extension tube collar) is for the relay cable that connects to the base of the aerodynamic housing of the loop. That two pin receptacle is connected internally to the seven pin loop cable connector which has the five wires necessary for the dual loop connections and the two wires that provide the "switched" +24vdc to operate the Loop Compass' internal solenoid that positions the scale mask depending on the DZ-2 frequency range selected or whether UNILATERAL is selected.

What is shown in the photo is only the Loop Compass/Drive Assembly. Not shown are the extension tube and the aerodynamic housing containing the dual loop assembly (mainly because I don't have those pieces.)

The Loop Compass/Drive Assembly is shown as actually setting on its "hand wheel." When installed in the aircraft, the aerodynamic housing and loop antennae were mounted externally on top of the fuselage and mounted to the aircraft's "skin" with a reinforcing plate. The extension tube was secured to the bottom-plate of the loop mount and, if a long extension tube was required, it was also secured with support brackets to the airframe. The Loop Compass/Drive Assembly was mounted to the extension tube using the collar nut, internal collett and secured with set screws. The extension tube was the proper length for the final compass position to be  at "eye level" for the radio operator to easily see the scales and manipulate the hand wheel, the brake and the vernier control knob. When installed, the compass housing didn't rotate but was kept in a stationary position by its mounting to the extension tube. Inside the extension tube is the loop shaft that is mounted to the loop antenna assembly at the top end and then "keyed" to the compass drive at the lower end. The "keyed" loop shaft always maintains its correct relationship to the compass scales. The loop shaft is secured at the lower end of the shaft with a collett and nut underneath the housing.

Once the Loop Compass/Drive Assembly and the Loop Antenna Assembly were completely installed on the aircraft, operation was as follows,...if the brake was in the OFF position, then the vernier drive was disengaged and hand wheel could be used to turn the loop to any position quickly and the compass would show the direction bearing of the loop relative to the aircraft fuselage. If the brake was placed in the ON position, then the hand wheel was locked and the vernier drive was engaged and the vernier knob could be turned to allow the operator to slowly and accurately rotate the loop and observe the compass scale while listening for a null.

Loop Response Patterns - What happens when setting these various loop positions is that the bi-directional and the uni-lateral/cardioid patterns rotate along with the loop rotation. When rotating the loop 90?after finding one of the bi-lateral nulls, the resulting loop position may have the cardioid null pointing towards the signal source but it could also be pointing away from the signal source at a 180º opposite bearing. The loop response with the peak signal amplitude pointing at the signal source is just about the same signal amplitude strength as the signal response off of the sides. Comparing the two uni-lateral loop positions, one being the first bearing and then 180º opposite the first bearing and then noting which position shows the minimum signal response then indicates the desired "true" direction with the cardioid null pointing at the signal source in that loop position. See drawing to the right showing the relationship of the loop patterns.


Next came the more difficult part,...determining the DF "true" bearing,...the calculations.


Basic Pre-WWII Airway Navigation - Figuring a true bearing was more involved than just reading the loop compass scale. The loop was normally tuned for a null response since this was much more accurate than trying to determine the "antenna position to maximum signal" response (the null on a cardioid pattern is very deep and very apparent compared to the very broad signal peak, therefore the null was easier to find and much more accurate for direction finding.) However, once the correct "null" was determined, the resulting loop compass reading was relative to the airplane fuselage (the nose of the airplane was 0? so the navigator also had to know exactly the bearing of the course that the airplane was flying (using the aircraft's magnetic compass.) The aircraft flight bearing (and any deviations) were added or subtracted as necessary from the loop compass scale reading to arrive at true bearing and direction of the received signal.

Some types of deviations could occur from the aircraft structure in relation to the location of the loop antenna on the aircraft fuselage and could come from the wings, vertical stabilizers, other antennae or engines and prop wash. These were called quadrantal errors and some compass instruments could mechanically adjust out these types of errors since they were essentially caused by the aircraft structure itself and didn't change (with the exception that some structures might be resonant to the frequency of operation.) Quadrantal errors tended to become more of an issue as the frequency of operation increased. At LF or even MW, quadrantal errors were stable and minimal.

A specific aircraft's quadrantal errors were first determined upon completing the installation of the DF equipment and with the aircraft on the ground. A technician would position himself and a portable transmitter out 1000 feet from the nose of the airplane (usually referenced as 0º.) Test signals at various frequencies were transmitted and the results at the airplane equipment logged. Then the technician moved to 30º still 1000 feet out and another series of test signals were transmitted and readings taken at the airplane. The technician then moved to 60º at 1000 feet, so on and so on, until an entire circle had been made around the airplane with signals transmitted and readings logged.

Once the ground testing was completed and the results logged then further testing was performed with the aircraft aloft at 5000 feet elevation. Since there couldn't be a technician with a portable transmitter at that altitude, a fixed ground beacon signal was used and the airplane flew in a circle out at a specific distance (at least 1 mile) around the beacon antenna. Readings were logged every 30?until all twelve positions were logged. At this point all quadrantal errors were known and either logged or if the DF compass allowed, the errors were adjusted out.

The most important bearing deviation was from "magnetic variation," which is the deviation of magnetic N from true N, and it depended where on Earth the aircraft was flying. When calculating magnetic variation it was important to know whether the deviation was to the West or to the East and the important rule was "always add Westerly deviation and subtract Easterly deviation." As an example, Dayton, Nevada has a magnetic deviation of almost 20?E, while a location like New York City has a magnetic deviation of about 12?W. Magnetic variation was always shown on the navigation charts being used (shown in 1938 chart below as the dotted arced red vertical lines with degrees and direction indicated - also shown in red within the latitude and longitude markings on the map's edges.)

All of the deviations were generally known in advance since magnetic deviation is charted for physical locations on Earth and the aircraft deviations or quadrantal errors were measured at the installation of the DF equipment in the airplane and either logged or adjusted out. In addition to these calculations, the navigator had to also factor in the air temperature, barometric pressure (dependent on altitude,) the aircraft's speed and wind drift.

photo above
: 1938 Air Navigation Chart for Buffalo, NY area

Also, there were different charts for daytime or nighttime flying with different map projections, e.g., Mercantor for nighttime flying and Lambert for daytime flying. Mercantor projections are purposely distorted to present square map grids but as the distance increases the distortion errors increase and a correction factor had to be added or subtracted to the course. The nighttime charts were simplified with no landmarks shown other than those that would be visible at night.

Navigators usually were equipped with several types of tools that were usually kept in a kit and included many types of scales or articulated pointers, usually made of transparent plastic, for placement on top of charts. Also included were various types of calculators (sometimes called "computers" but most were like circular slide rules and able to perform several calculations or conversions simultaneously.) Some tools had combinations of transparent articulated pointers and sliding scales for calculating variables or position. These calculators, scales and conversion tables helped to speed-up the process and hopefully increase navigation accuracy.

photo above: A "Time-Distance" computer, Type D-4, from WWII, for the USAAF. Diameter of this device is 4 inches. There are more computation scales on the backside of this Type D-4 computer.
Uni-lateral Directional Uses - Uni-lateral was used when an unknown signal from an unknown location needed to be DF'd. Most likely the signal was from a downed aircraft's life raft with the raft's occupant(s) running the emergency transmitter (a Gibson Girl, for instance - covered in another section further down. Even though it was basically for the USAAF they were installed in most types of aircraft.) An exact bearing/direction was needed to find the emergency transmitter and rescue the downed pilot (and crew, depending on the type of downed aircraft.) Time was usually critical, so with "true" direction known, the airplane could fly that course and eventually find the raft and occupants. Sometimes, if there was enough time, a second bearing from a different position might be taken to provide "triangulation" information for a more exact location of the emergency transmitter. If there was a second airplane involved in the search, the second bearing could be performed almost simultaneously and the bearings mutually shared via radio for the quickest rescue which, if there were "high seas," could be performed by a nearby ship since landing a PBY in rough ocean conditions was fairly difficult and consumed a lot of fuel.

Bi-lateral Directional Uses - Other times, when the aircraft was just flying towards a "known location" beacon, then Bi-lateral was used and the loop locked in position perpendicular to the fuselage (athwartship) and the airplane course determined by steering the airplane in the direction of the bearing of the minimum signal response. An Audio Output Meter could be plugged into one of the DZ phone jacks to use as a visual indicator of minimum signal response. Since the Range Beacon's call and location were shown on the navigation charts, Bi-lateral allowed "beam navigation" and kept the airplane on course within a defined "Airway" to the airport or city that the beacon was transmitting from. The "Airway beams" locations and bearing directions were also shown on the navigation charts along with the "between the beams" signals of "A" and "N" Morse identifiers to indicate via the radio signals and by the navigation charts where the airplane was in relation to the four fairly narrow navigation "beams" from the Airport Range Beacon transmitter/antenna system. The "A" and "N" Morse identifiers would combine when the airplane was "on the beam" and a continuous tone was heard although this information was only transmitted for several seconds, then a long pause and the information sent again (this format was continuously repeated.) Additional information shown on navigation charts included marine beacon calls and frequency, weather station calls and frequency, AM Broadcast station calls and frequency, general terrain and elevations, latitude and longitude, major visual navigation points (roads, railways, lakes and rivers,) other visual indicators such as tower lights that included rotating lights, lights that blinked Morse, or other types of lighted beacons.

More DFing information on "Right-Left" of course indicating circuitry in WWII Ally Radio Equipment and in the section on the British DF Receiver, the RAF-Marconi R1155.


DZ-2 SN:1486 - Retrofitting Dynamotor Operation and Building Proper Cables

DZ-2 SN: 1486 - I got this DZ-2 in 1995. It came from my old radio collector friend from years ago, Fred Winkler (1926-2001.) Fred had changed the tube heater wiring from the original series-parallel to parallel to operate the tube heaters on six volts AC using a filament transformer as the heater voltage source. He had a regulated and adjustable +200vdc power pack that was vacuum tube based. However, at one time Fred had been collecting rejected Ni-Cad batteries from old rechargeable electric shavers. He got the old batteries from someone he knew that did the warranty servicing on the electric shavers (this was in the mid-1980s.) About half of the batteries he collected were defective but the other half were still in good operating condition. Fred made up a battery pack that contained enough of the Nicads to provide +200vdc. At one time, he operated this DZ-2 with a six volt DC battery for the tube heaters and the +200vdc Nicad battery for the B+. I remember that Fred operated the DZ-2 was out of its case most of the time. When doing that sort of operation, Fred apparently would write the call letters and dial setting of various AM-BC stations he heard in water-based ink on the tops of the aluminum shields (lucky it was water-based ink that cleaned off easily without damaging the aluminum matte finish.)

DZ-2 Rework Needed - I used the DZ-2 a few times in 1995. I even installed the AC PS components into a metal cabinet and cleaned up the power connections but that was as far as I went at that time. Fast forward 25+ years, interest in pre-WWII air navigation had me thinking about putting this DZ-2 back to its original configuration to operate on +24vdc for the tube heaters, to have the B+ provided by a dynamotor as original and to utilize the proper connectors for correctly built cables. A replica MW loop would be built to allow DF testing.

DZ-2 Chassis Rewiring - Some of the original tube heater wiring was still present but two original wires from the harness had been cut too short to be reconnected to the proper tube socket. Several of the original inter-connecting series-parallel wires were entirely missing. Luckily, I had a box that had many pieces of the exact type of vintage wire that was original, even with the correct insulation color and tracer color. It was pretty easy to remove the non-original parallel wiring because it was a "twisted pair" with yellow and green rubber insulation on the wires. The retrofit back to series-parallel heater wiring was made very easy because the DZ-2 manual has the detailed wiring diagram with numbered identification of each wire. This isn't a schematic but a pictorial representation of how the wires are connected and routed within the receiver. For the two harness wires that were too short I used the exact type of wire for the extensions and covered the soldered splice with vintage black sleeving that was then "tucked" out of sight. There were two shunt resistors that were originally IRC brand 68 ohm 1W resistors on V-105 and V-106. Of course, these resistors had been removed when the DZ-2 was wired for parallel heaters. I couldn't find two IRC brand 68 ohm resistors but I found two matching 68 ohm BED resistors that looked convincingly original when installed. I did a DCR test to see if everything measured correctly and after that test I connected up +28vdc to pin A of the POWER receptacle and negative chassis ground to pin B. The tubes didn't light-up. It seemed that wiring for the change for 6.3vac parallel filaments had bypassed the fuse block but my return to original series-parallel wiring now had the fuse block connected. It turned out that the old fuse was blown (probably a long time ago.) The fuse was the larger type found in some older military equipment. A standard fuse won't "snap in" the holder clips. I had a selection of the correct type in the old parts bins so with a good fuse installed, the tubes illuminated and looked a correct orange color (I know,...I should measure the heater voltage on each tube but the "color test" is okay for now.) For the final test, I connected up +200vdc to the DYNAMOTOR pin A. With a short antenna on the Sense antenna terminal I picked up several AM-BC stations indicating that the DZ-2 was functioning well enough considering I had it connected up using test clip leads. Proper cables should provide an improvement.

Building Proper Cables - The correct types of connectors were purchased off of eBay but the proper type of cables had to be built. The reason for "custom" building is that the cables need to be shielded and require specific gauge wires depending on the current the wire has to carry. Specific DZ-2, multi-conductor, shielded cables are obviously not being manufactured by anyone anymore. Even in WWII, these cables were "custom" manufactured and then supplied in bulk lengths with the DZ-2 equipment for "custom" installation within the aircraft with the "installers" building the interconnection radio cables. So "custom" building the cables is the only option for correct operation of the DZ-2 in its original configuration.

POWER cable - The manual is vague about the wire gauge used in the cables. The most common spec shown in the cable drawings is 41 strands of .010" diameter wire (30 gauge) which describes 14 gauge wire. I built the POWER cable using two 14 gauge wires with a rubber sleeve covering both wires. The rubber sleeve was covered with braided copper shielding harvested from old RG-8 coaxial cable. The entire cable was wrapped with black electrician's tape. The proper MS3106E16S, a 2 pin connector, was installed on one end with the shield drain wire connected to the chassis ground pin. The opposite end had large spade lugs installed (soldered) and a shield drain wire also with a small spade lug. I made this cable 50" long.

DYNAMOTOR cable - This cable was built from three conductor 14 gauge rubber insulated power cable. Again, braided copper shielded was installed with drain wires on each end. The shield and cable were wrapped with black electrician's tape. Although the original cable used two 14 gauge wires for the +24vdc to the Dynamotor +LV and the Chassis Ground along with an 18 gauge wire for B+ from the dynamotor, it was much easier to just use the 14-3 power cable (obviously the 14 gauge wire will not present much of an IR drop with the result being a very slightly higher B+ voltage at the DZ-2.) Connector type MS3106R14S was used and the cable length is 40."

LOOP cable - This cable is built from five 18 gauge wires that connect to the two loops and the common loop ground. Two 16 gauge wires are used for connecting the LP-21 compass scale mask relay which is actuated by the band switch position or when switching to uni-lateral. The connector is AN3106A16S-1S and the cable is shielded.

Dynamotor Box, DM-28 Mount, Filter Board - To operate the DZ-2 "as original" I was going to build a replica of the dynamotor-filter (identified as CEX-21562.) I had a DM-28 dynamotor but that was all,...just the dynamotor. No filter box and no filter components,...not even the mounting base for the DM-28. While the DM-28 was originally for the BC-348 receiver, it does have the same specifications as the dynamotor used with the DZ-2. Finding a DM-28, especially one that's missing the filters and mount, is fairly easy and using what is essentially a "parts" unit doesn't deprive a BC-348 restorer of a complete unit.

This was going to be a build-project that would function as the original did. It would have the correct value filter components inside the case. But it would only be a replica, that is, I'll make it look close to the original (complete and original DZ-2 Dynamotors are difficult to find.) I needed to identify the four wires that were exiting the bottom hole of the DM-28. I had to remove the end bells to verify which wires went where. I reinstalled the end bells and applied +27vdc to the +LV side. On the +HV side I had +244vdc showing on the DDM (that's with no load.)  >>>

>>>   Finding the capacitors for the filter wasn't a problem. I needed one 3.75uf with at least 50wvdc, I had several 4uf 400vdc capacitors. I also needed 1uf at 300wvdc, I had 1uf at 630wvdc. Also, two .01uf at 400wvdc, I used .01uf at 600wvdc. The inductor, L301, had to be built. The original L301 was an air core inductor wound on a .375" diameter form with 17 gauge wire. I had to use 16 gauge wire (much easier to find.) All of the inductor parameters are in the manual which makes building L301 easy, sort of. I suspected that a homebrew attempt would probably result in the L being little low so I used a .375" diameter powered-iron rod for a core to increase the L. I wound five layers each wrap insulated with one thin layer of blue masking tape. Each layer was 50 turns. When completed I wrapped the choke with black electrician's tape. Measured L was 1.4mH and the DCR was 0.4 ohms. The L was about 25% higher than the original choke and the DCR was about 0.15 ohms more than the original choke. I installed the filter components on a vintage garolite terminal board and tested the operation of the dynamotor with the filter and with the dynamotor output running the DZ-2. All operations were as expected. With no antenna connected and the gain at maximum, some brush noise could be heard but the entire test operation was done using clip leads with no shielding and, of course, the DZ-2 wasn't in its cabinet either.
The dynamotor aluminum box was built using a six inch cube aluminum project box. I cut the top side down by 1.5" so the box was then 6" x 6" x 4.5" with removable top and bottom pieces. The original DZ-2 dynamotor box was 7.5" x 6.5" x 4.5" so my replica is fairly close in overall size. The DM-28 dynamotor didn't have its original mounting base so I made one out of .063" aluminum. It's mounted to the dynamotor with two spacers and two 10-32 screws. The four wires coming out of the bottom of the dynamotor pass through a .375" hole in the new base. Each corner of the base has a rubber grommet for cushioning. The dynamotor mount is mounted to four 8-32 studs with washers and nuts to allow elevating the dynamotor mount slightly above the aluminum bottom and thus allowing it to only be suspended by the rubber grommets. This also allows room for the four wires to exit the bottom of the dynamotor for connection to the filter and the box connector.

The only connection to and from the dynamotor/filter is through the three pin MS box connector that mates with the connector plug at the end of the DYNAMOTOR cable. The filter circuit mounts on two small garolite terminal boards that are mounted to the bottom screws that mount the box connector. After all of the mounting holes were drilled and a "dry run" of assembly to make sure everything did fit together, the box was painted black wrinkle finish. After a few days of curing time for the wrinkle finish, the entire dynamotor box and dynamotor unit could be reassembled and the wiring completed.

Operation is quite an improvement from the clip lead connections. Now, with the proper size shielded cables built with the correct gauge wires, voltages are at spec with minimal IR drop. With +27vdc input operating the dynamotor and the DZ-2 tube heaters and with the dynamotor providing about +220vdc B+, the DZ-2 is performing better than ever. With the antenna disconnected and the VOLUME fully advanced virtually no dynamotor noise can be heard indicating that the filter and the shielded cables dramatically reduce noise within the receiver.

However, the dynamotor RFI noise was radiating and with an antenna connected to the DZ-2 input the noise "picked up" was significant. In fact, the noise could even be received by any other near by receiver. I was relying on the cable shield and DZ-2 chassis connection to ground the case but the entire DZ-2/dynamotor system wasn't connected to a substantial ground. The solution was very simple, the aircraft the battery-charger system negative was tied directly to the airframe and all shields, chassis, etc. are also tied to the airframe. I didn't have the AC power supply negative (-24vdc) tied to the power supply chassis which then connected to the house ground. A small jumper from -24vdc terminal to the power supply chassis ground reduced the RFI noise to a whisper.

I think another improvement might come from replicating the way the equipment was originally installed in the aircraft. All of the cabinets, shock mounts and other pieces were mechanically mounted to the aircraft framework and the battery/charger system had the negative tied to the airframe. By using a 0.50" wide copper braid cable to interconnect all of the cabinets together and then tie those to the -24vdc and that then tied to the house ground might come close.

photo left
: The completed replica CEX-21562, DZ-2 Dynamotor-Filter

photo right: Inside the box showing the DM-28 and filter


Navy Department - Western Electric Company

RU-GF Series of Aircraft Radio Receivers & Transmitters

Model RU-16  Type CW-46051A    -    Model GF-11  Type CW-52063A

The RU Series of aircraft receivers and the matching GF Series of aircraft transmitters evolved throughout the 1930s. Like all pre-WWII equipment, contracts were for very small quantities so the early versions are very rare. The most commonly seen RU/GF versions are the RU-16 and the GF-11 which were produced in fairly large quantities in the very early part of WWII (but apparently not used extensively in actual service compared to the contract quantities produced.) The contracts actually date from before WWII began for the USA, April 21, 1941 with Western Electric Company as the contractor. The RU-16 and the GF-11 both operated on +12vdc implying that the installation would be in earlier types of aircraft. By 1941, +12vdc aircraft power was quickly being replaced with the improved +24vdc power.

The intended use for the RU/GF equipment was in single-seater or two-seater airplanes (radio op seated behind the pilot) but the manual also mentions "flying boats" as another possible user in the installation instructions. Each installation into an particular airplane was "custom fitted" with each of the connecting cables custom-built from supplied "bulk cable." Additionally, flex control cables were also custom-fitted and were built from supplied "bulk" flex cable material. In some single-seater airplanes, the only place to install the radio gear was behind the pilot's seat so remote controls using flex control cables and spline drive flex cables along with remote switch boxes and tuning heads were installed to allow the pilot to have the radio controls in front of him.

There was also a U.S. Army version of the RU/GF equipment, the SCR-AL-183. The receiver was designated as BC-AL-229 and the transmitter was BC-AL-230. The contracts are from the late-thirties up into 1940 with Western Electric as the contractor. This equipment is very similar in appearance to the RU/GF equipment but internally both the receiver and transmitter abound with minor differences. The SCR-AL-183 was the 12vdc version and the SCR-AL-283 was the 24vdc version. The Army versions were also intended for one and two-seater aircraft installations and are found in both black wrinkle finish and in bare aluminum.

The overall use of the equipment was very low by mid-WWII. This non-use resulted in many complete RU-16/GF-11 equipment packages being sold on the post-WWII surplus market "new in the box" which accounts for the "fairly common" status of the RU-16/GF-11. The RU-17/GF-12 were the 24vdc versions and apparently this equipment was used much more extensively during WWII and isn't encountered as often as the RU-16/GF-11. For quick identification the data plates on the 12vdc units had a black field while the data plates on the 24vdc units used a blue field.

photo above: RU-16 Receiver - serial number 8989

photo above: GF-11 Transmitter - serial number 8888

RU-16 Receiver Circuit - Six tubes are used in the RU-16 circuit which is a TRF (tuned radio frequency) receiver with tracking BFO. The tubes used are 1RF - 78, 2RF - 78, 3RF - 78, AVC - 77, Detector - 77, AF Out/BFO - 38233.  The last tube, Type 38233, is a dual triode that provides the tracking BFO with one triode and the Audio Output stage with the other triode. The plug-in coil assemblies each contain five shielded coil units - four units that determine the tuning range of the assembly and one unit for the tracking BFO coil required. The "dual frequency range" coil assemblies contained an internal switch that was operated by lever located on the front of the assembly. The single range coils had a metal handle-type strap for removing the coil from the receiver. The "dual frequency range" coils had a large protrusion that housed the range switch and also provided something to "grab onto" for coil removal so a handle wasn't used on the dual range coil assemblies. The Antenna or Loop switch could be set up to operate locally at the receiver or remotely via a flexible cable. There are two antenna inputs, A and L - L.  The A terminal could be connected to any of the typical aircraft antennae available and depended mainly on what type of airplane was involved. Most single-seater airplanes had a wire antenna from the cockpit to the tail. Two-seaters usually had an aerodynamic mast near the airplane nose with a wire running to the tail. A central wire dropped down to the rear-seat part of the cockpit for the radio gear connection (a "T" antenna.) Some installations used a trailing wire (depended on the aircraft.) It was also possible to use the DU, DU-1 or DW-1 Amplified Direction Finding Loop which worked with a sense antenna to provide a "true direction" cardioid pattern that allowed determining a correct bearing towards an unknown signal. The output of the DU/DW Loop was connected to A on the RU receiver. The complete RU/GF setup provided power to operate a DU-type loop.   The DW-1 Loop is profiled further down this page.

The L - L terminals are for a "homing loop" antenna. Homing Loops provided a "figure-8" pattern with two deep nulls off of each side of the loop. The loop would be set athwartship and then the airplane steered toward the null. The general direction was known and the "homing loop" provided accurate navigation to a specific airport beacon.

GF-11 Transmitter Circuit - Four tubes are used in the GF-11 transmitter, two type 89 tubes and two type 837 tubes. One of the 89 tubes was the master oscillator tube while the second 89 could be a MCW audio oscillator, an audio sidetone generator on CW or a Voice modulator depending the the mode selected. The two 837 tubes were operated in Push-Pull as the power amplifier. The 837 screens and suppressor grids were tied together and modulated in the MCW and Voice modes. The two 89 tube filaments are connected in series for 12vdc operation and the two 837 tubes are connected in parallel (837 tube uses 12 volt filaments.) Low voltage (+12 to +14vdc) was supplied by the aircraft battery/charging system buss and B+ was supplied by the shared dynamotor, that is, both the RU and the GF obtained their B+ from the same dynamotor (CW-21109A) and various resistor dividers within the circuits of each unit. Tuning ranges are determined by eight plug-in modules that provide a frequency range of 2000kc to 3200kc and from 3000kc to 9050kc. Eight plug-in modules were supplied with the GF-11. 

RF power output for the GF-11 was about 2 to 7 watts for all modes in the 2-3mc range and 12 to 15 watts in all modes in the 3-9mc range. The meter is an RF amp meter (the radio op tuned for maximum current to the antenna.) There were two phone jacks on the side of the GF-11 that provided meter access to measure Modulator current and also the PA plate current. The RU-16 Test Meter could be used to measure these points if desired. Necessary for operation was the Transmitter Control Box CW-23097. This box had the switch for CW, MCW or VOICE modes of operation, RADIO-ICS switch, input jacks for an external key or mike and a connector for the cable to interface with the RU-16 Junction Box for transmitter operation. On top of the Transmitter Control Box was a built-in hand key button for CW or MCW.

The Ancillary Pieces - The RU-16/GF-11 (actually the entire RU/GF series) required a large array of peripheral ancillary equipment to actually operate the receiver along with the transmitter. In addition to several peripheral boxes there was an array of specifically "identified by number" special connector plugs with special pin patterns or different diameters that interconnected the RU-16/GF-11/the Dynamotor/both switch boxes through the Junction Box. Originally, bulk cable was supplied with the equipment and each interconnection cable had to be custom-built using the correct type bulk cable with the correct connector plugs installed. Additionally, the bulk cables were "un-jacketed" to allow the cable shields to be easily bonded to the aircraft frame for lowest noise pickup. Cables were supposed to have a metal identification tag installed during construction. Since each installation was "custom-fitted" to the aircraft most of the original RU-GF cables remained in the aircraft and what is found today are mostly unused RU-GF connector plugs. Most plugs weren't identified except perhaps with a single number stamped on the shell.  Lack of specific identification complicates finding some of the plugs. The connector pin numbering is unique to each type of plug, that, and the pin pattern have to be used to identify an unmarked plug. A typical plug is shown installed in the dynamotor photo to the right. 

To actually operate the RU-16 receiver required the Remote Switch Box (photo below) that provided the switching for the BFO on/off, for selecting AVC/MVC along with ON/OFF function for the entire system, a Gain control for the receiver output, two phone jacks for the audio output, a three-circuit phone jack for the Test Meter and box receptacle for cabled connection to the Junction Box. The Junction Box is essential for the proper interconnecting and operation of the entire RU-GF system. Likewise, the Dynamotor is necessary to provide voltage to both transmitter and receiver through the Junction Box. To operate the GF-11 transmitter required the Transmitter Control Box and connection to the Junction Box. There was also an Extension Control Box that connected to the Junction Box (use was optional and intended for two-seater airplanes) and a Remote Transmitter Control switch (was user supplied and connected to the phone jack on Junction Box - operated the PTT line.) For single antenna T-R operation the Antenna Relay box was necessary. There was also an external "RU Test Meter" that could be added to the setup (plugged into the RU-16 switch box or the side of the GF-11.) There were two connectors (74 and 76) on the Junction Box that could be used to provide voltage to operate a LM-type CFI (Crystal Frequency Indicator, aka: heterodyne frequency meter) or an amplified loop antenna like the DU-series. It was also possible to power a "Homing Adapter" like the ZB Series from either connector 74 or 76. (ZB-3 "Homing Adapter" profiled further down this page.)

photo above
: RU-16/GF-11 Dynamotor CW-21109A - serial number 6142

Two "substitute plugs" were supplied and could be installed in the Transmitter Control Box connector and the Extension Control Box connector (37 and 80) of the Junction Box if a "receive only" setup was intended. These "substitute plugs" each had an internal jumper to route the circuitry necessary for "receive only" operation. Also, Remote Loop-Antenna Switch flex cables and Remote Tuning Range Switch flex cables were included in the package but their use depended on the ultimate installation requirements. These flex cable connected controls were necessary in some single-seater installations where the radio gear was located behind the pilot and only the remote controls and switch boxes for the radio operation were up front. The flex cable was also supplied in bulk and each remote flex cable had to be custom-built for the installation requirements. An external telegraph key could be used, a carbon mike like the RS-38 was needed if Voice operation was desired and a set of Lo-Z 'phones (600Z ohms) was also necessary. The ICS (Internal Communication System) provided a method for the pilot to talk to the radio op through the use of the Transmitter Control Box by switching to ICS-1 or ICS-2 and working into the Extension Control Box. ICS-1 allowed intercommunication with the radio signals still audible and ICS-2 removed the radio signals and only allowed the intercommunication (this position was "spring loaded" so the receiver signals couldn't be "locked out.") The ICS connection to the audio output grid provided the sidetone through the 'phones when in "RADIO" on the Transmitter Control Box and when in the CW mode.

The RU-16 required a Receiver Tuning Head that was connected to the tuning gear box by way of a flexible spline cable similar to old car speedometer cables. The spline flex cable was also supplied in bulk lengths and custom fitted for the installation requirements. There was also a small direct coupler tuning adapter that could be installed for local tuning (of the receiver.)

In addition to the all of the ancillary pieces necessary just to operate the RU-16 and GF-11, the plug-in coil assemblies are required for both units. The RU used four "dual frequency range" coil packs and five single range coil packs. Each coil assembly originally had a specific metal case to store it in. When all coil assemblies were available the RU-16 could tune from 190kc up to 13.575mc. The GF-11 also had its own plug-in tuning modules and each of those also originally had a metal box for protection during storage. Eight GF-11 tuning modules were supplied allowing the transmitter to operate from 2.0mc up to 9.05mc.

photo left: CW-62008A Junction Box with many of the required connector plugs partially inserted. Note that some of the plugs do have identification numbers on the shell.

photo left: A RU-16 Single Range plug-in coil assembly for tuning Range E. Note the "extraction" handle. Also, five shielded coil units that function as (3) RF coils, (1) Detector coil and (1) Tracking BFO coil. Tunes from approximately 1200kc up to 2100kc.

photo right: A RU-16 Dual Range plug-in coil assembly. This is the O-E Range coil set with a total coverage using both ranges of 187kc up to 455kc. The switch lever is located on the front-facing part of the coil assembly and is marked "FREQ - HIGH - LOW." The knurled collar allowed removing the lever so a flexible control cable could be installed for remote switching of the tuning range.

Many of the ancillary pieces are interchangeable to the RU-16/GF-11 from the RU-17/GF-12 and RU-18 receivers and possibly earlier versions.  It depends on what the piece's function was. The Receiver Tuning Head, for example, since it's entirely mechanical and the tuning dial scale is 0 to 100, is interchangeable from RU-16 to RU-17. The NAVAER 08-5Q-100 manual is very specific with two pages of possible interchangeability of the various pieces for both the RU and GF equipment. On the receiver series, the RU designations run up to the RU-19 and possibly even to the RU-20.





photo left
: The Receiver Tuning Head, Type CW-23012, in this case, for the RU-17 (blue tag) but most ancillary components were interchangeable between the RU-16 and RU-17. Since the tuning dial on the receiver and on the remote tuning head were both scaled "0 to 100," a tuned frequency versus dial readout chart was attached to the top of the receiver's tube cover plate. The fiducial could be mounted in several positions around the dial perimeter to allow mounting the Tuning Head in the best position for pilot or radio op visibility in the particular installation.

photo right: Remote Switch Box, Type CW-23096A, for the RU-16 which is required for actual operation of the receiver. There are other ancillary boxes but the Switch Box is essential to the operation of the receiver since it has the ON-OFF function for the entire system, the output control, AVC-MVC function switch, the BFO switch and the audio output jacks. METER was a jack for the RU Test Meter to observe the signal level for loop DFing.

RU-16/GF-11 Operational Transmit-Receive Station - Project

June 2021 - Certainly finding and acquiring the large quantity of necessary peripheral equipment for the RU-16/GF-11 is a daunting task. However, as more and more pieces were found, it then became much easier to visualize a completed project. The challenge for the finished project is that the working station is very low output power with the GF-11 running about 12 watts output on Voice or CW. On Voice, the screen modulation combined with the RS-38 carbon mike will make almost any QSO difficult except for those local hams that are on the Vintage Military Radio Net. CW can be used for those mil-hams that are further away.

This project started a few months back with the acquisition of an excellent, very original RU-16 that was only missing a few minor small parts. I already had the CW-23012 Receiver Tuning Head but I had to find a spline flex cable drive for it to interface with the RU-16. I then started looking for and gathering up more of the needed accessories. I found that I had a lot of the connectors in my junk boxes. A very nice condition GF-11 transmitter was donated by WA6OPE (with connector plugs.) W7MS donated a "parts set" Army BC-229 to provide the missing but very necessary slide-clip brackets for the shock mount for the RU-16. KD6TKX donated a large box of parts that included both shock mounts, the Junction Box and the Antenna Relay plus many loose connector plugs. Acquired on eBay was the Dynamotor, found only because it was listed just by the CW-21109A part number. Also, a couple of RU-16 coil sets (one NOS in the original box,) a RU-16 switch box, a second Junction Box (for needed connector plugs and other parts) and a very necessary repro manual. Around here in the various parts boxes I found a very nice condition RS-38 mike (Telephonics mfg) and a really nice CTE-26003A Navy key (also Telephonics mfg.) Also, a LM Heterodyne Frequency Meter that still had the original power input connector and the original cable with correct plugs for the RU/GF Junction Box connection (and it was already set up for 12vdc and +200vdc to +400vdc B+.) I'm still looking for the GF-11 Transmitter Control Box CW-23097, a second #233 plug (RU-16 receiver to Junction Box) and maybe a Q-G dual range coil set for the RU-16 so that 75M could be tuned. Of course, there are a multitude of other minor extras that, if found, would be nice to have. But since I do have enough of the parts necessary to go through the receiver and get it operational, that part of the project can get underway.

photo above: Some (but not all) of the RU-16/GF-11 station components. Still needed is the GF-11 Transmitter Control Box CW-23097. To be authentic, all cables have to be custom-built for proper length and also to be fully shielded. The shielded cables didn't have a jack originally and that's how these station cables will be built. A minimum of six cables are required and two more for powering various accessories like the LM CFI or the DW-1 loop antenna (not in photo.)


Navy Department - RCA Manufacturing Co., Inc.

Aircraft Radio Receiver - Model ARB - Type CRV-46151

The ARB receiver was a six tube superheterodyne receiver intended for use in USN aircraft. It was an updated version of the earlier, mid-thirties RU receiver series that required several sets of plug-in coil assemblies to change tuning ranges in addition to a baffling array of remote boxes, remote cables and a junction box for interconnection of all of the extra pieces. The ARB receiver simplified the hook-up and dramatically improved the overall performance by replacing the RU's TRF with Tracking BFO circuit with a superheterodyne circuit. The ARB receiver tuned from 195kc up to 9.05mc in four bands. Two dual-frequency IF amplifiers are utilized with 135kc used in the 195kc to 1600kc range and 915kc used from 1.6mc to 9.05mc (a dual frequency BFO was also required.) The receiver used 12 volt heater tubes (in series-parallel for 24-28 volt operation) and had one RF amplifier 12SF7, a Converter 12SA7, two IF amplifiers, both 12SF7, Det-AVC-1AF stage 12SF7 and an audio output stage 12A6. A neon bulb was used as a voltage regulator for the LO part of the Converter stage. The lower two bands could be set up for loop operation, specifically for homing DF purposes. All four bands could be used for Communications and operated with the various types of aircraft antennae available. To simplify the external power hookup, the aircraft +28vdc buss was connected to POWER for tube heaters and inside the ARB was a dynamotor that provided the +230vdc B+.

The ARB was designed for either single-seater (pilot only) or two-seater aircraft (pilot and radio op) but could also be found in larger aircraft with a crew that included radio op/navigator, pilot and co-pilot. Modes of reception were CW, MCW and Voice with the options of AVC or MVC (Manual gain control.) When in "AVC," the Volume control operated as an AF gain control with RF/IF sensitivity controlled by the AVC line. When in "MVC," the Volume control operated as a RF/IF gain control with the audio gain set to maximum. Selectivity options when in AVC were either Sharp or Broad. The "Broad" position was intended to ease tuning in the 1600kc up to 9.05mc range where signals tended to be more difficult to "tune in" due to the wide span of frequency coverage in each of the two bands (additionally, the remote receiver tuning head was particularly difficult to "fine tune" due to flexing of the cable drive which resulted in significant "backlash" if not installed correctly.)

The initial ARB receivers are pre-WWII and were used for Communications and for Homing DF. For DF, the aircraft had to be equipped with a rotatable, non-center-tapped, loop antenna that could be connected to the ARB receiver terminals marked L1 and L2 (L2 is chassis ground.) When connected in this way, the loop would be operational only on the two lower frequency bands only (195kc to 1600kc.) This type of loop antenna would have a bi-directional "figure-8" pattern and was generally set "athwartship" and the airplane steered towards the minimum signal response (homing.) The two other antenna terminals are marked AT and AF. AT indicated a "Trailing Antenna" which was normally installed in larger aircraft and consisted of a clad stranded steel cable with flight weight that could be reeled out to about 200 feet behind the aircraft when in flight. AF indicated "Fixed Antenna" which was a smaller antenna consisting of an off-center fed wire between the cockpit and the tail of the airplane or it could be a short vertical installed on larger aircraft (usually 4 to 5 feet tall maximum.)

photo above
: ARB Receiver on its shock mount (hard to see) with ZB-3 Homing Adapter on top, Operator's Control Box and Receiver Tuning Head to the right. On the Operator's Control Box, the unused connector plug marked "Control Box" would be used for the wire cable running to the Pilot's Control Box. The opening with the threaded barrel is for the Bowden cable that ran to the cockpit to allow the pilot to switch control of the receiver from "REMOTE" back to the radio operator, "LOCAL." Dual phone jacks could be used for two sets of 'phones or one jack could be used to operate an Audio Output Meter for DF purposes.

The Control Boxes - To actually operate the ARB receiver required the Operator's Control Box (Type CRV-23256) or the Pilot's Control Box (CRV-23254.) The receiver could be operated by either of the control boxes when the aircraft installation was for "dual control setup" required for radio-navigator or pilot control. There wasn't a volume control on the receiver but each control box had a volume control. Likewise, there wasn't a phone jack on the receiver for audio output and, again, the control boxes had dual phone jacks for audio output. Normally, the audio output impedance was set for LOW which was 600Z ohms. By moving a pair of jumpers in either control box the audio output impedance could be set to HI or 4000Z ohms. Either control box could also switch bands on the receiver remotely when the receiver's "MOTOR" switch was ON as this enabled the receiver's motor-driven band switch. This function could be disabled with the receiver front panel "MOTOR" switch and the band switch on the receiver then operated manually.

The Operator's Control Box had a port with a threaded barrel for installing a bowden cable that mechanically connected internally to the "LOCAL-REMOTE" switch. If the radio op wanted to pass control of the receiver to the pilot, he switched the lever to "REMOTE." This operated the bowden cable and pushed the bowden cable knob at the pilot's location to the "up" position. Through the inter-connecting wire cable the Pilot's Control Box electrically was activated and the Operator's Control Box was deactivated. The Pilot's Control Box now allowed the pilot to have control of receiver operation. Additional to this "dual control setup" was a second (flex cable connected) Receiver Tuning Head that was installed in the cockpit near the pilot. This setup allowed the pilot to tune the receiver, switch bands, control gain and select reception mode. To pass control back to the radio op, the pilot would push down the bowden cable knob and that operated the Operator's Control Box "LOCAL-REMOTE" switch via the bowden cable returning control back to "LOCAL" and activating the Operator's Control Box and deactivating the Pilot's Control Box.


 photos: The Pilot's Control Box CRV-23254 (left) and the Operator's Control Box Type CRV-23256 (right.) The Operator's Control Box adds the LOCAL-REMOTE switch.

The Receiver Tuning Head -  Receiver tuning was accomplished by using a Receiver Tuning Head (Type CV-23253) that had a conical tuning dial scale viewed behind an index window and a hand crank type of tuning control. Coupling to the receiver was via a metal flexible spline-ended cable in a metal flexible housing (similar to the speedometer cable used in older cars.) There was also a direct coupler that could be attached to the receiver tuning gearbox that allowed direct "at the receiver" tuning utilizing the receiver dial for frequency readout but the dial scale was minuscule so a magnifying lens was built into the dial bezel.

Additionally, the direct coupler had a "feed-thru" connection that allowed the direct coupler and the receiver tuning head to be connected together and to simultaneously operate the receiver tuning (this setup tends to compromise the otherwise smooth operation of the Receiver Tuning Head.)

Hint: For smooth tuning operation don't over-tighten the collar nuts on any of the flex cables or couplers. Tighten the collar nut only "finger-tight" for best results - just tight enough to keep the flex housing from moving when changing directions in tuning. Also, avoid "tight" bends in the routing of the flex cable with no radius tighter than 6" for best results. Securing the flex cables and tuning heads will also help significantly by keeping all parts in a stable, fixed position.

Ancillary Parts - The ARB used one power cable, one control box cable, one flexible tuning (spline-ends) cable, one "Pilot's" control box, one remote tuning head, a shock mount and phones. It was also possible to setup for "dual operation" by adding an "Operator's" control box and control box cable (connected to the first control box) along with a second flexible tuning cable, "T" coupler and second tuning control head for larger aircraft where either a navigator or radioman was aboard. A bowden cable was also necessary to allow switching from "Remote" to "Local" control from the pilot's position.

Although the "matching" transmitter for the ARB receiver was generally considered to be the ATD made by Bendix Radio. The ATD was a relatively low power (40W,) with four auto-selected tuning unit channels and was a fairly difficult to use piece of equipment. Later in WWII, ARB receivers were often paired with the Collins ATC transmitter that could produce 100 watts output power, was very easy to operate and provided ten autotuned frequency channels.

photo left: Close-up of the Receiver Tuning Head Type CRV-23253

Initial Concept of Homing DF with the ARB - The ARB was originally intended for use with a non-center-tapped loop antenna that allowed one end of the loop to be grounded. These loops responded in a "figure-8" or bi-directional pattern and could provide a bearing by DFing a known beacon signal to allow "homing-in" on the signal. This provided the pilot or navigator with a very accurate bearing towards the origin of the "known" signal. The bi-directional pattern allowed for two "deep" nulls off each side of the loop. The nulls were much easier to detect and much more accurate for determining the "minimum response" at specific loop positions. Since the Operator's Control Box had two phone jacks, an Audio Output Meter could be plugged in to allow an accurate visual indicator for minimum signal response. Though "true" direction might be ambiguous since there were two nulls, generally the pilot and navigator knew the approximate direction of the airfield they were flying to and the loop's null gave them a precise bearing toward the airport's Range Beacon. Pre-WWII, there were many Remote Airways Radio Range beacons along with regular Airport Range Beacons that provided navigation radio DFing signals to allow the pilot or navigator to determine the correct course (called an Airway) to fly to a desired airport. The pilot or navigator would set up the loop and receiver to allow navigating the aircraft "on the beam" to a desired airport or other location. Also, AM-BC stations were sometimes used for beacons since their signals were usually strong and consistent and the transmitting location (city) was known. All beacons and most strong AM-BC stations were shown on the navigation charts. It was also possible, if an accurate position of the aircraft wasn't known, for the pilot or navigator to take bearings on two known beacons. The bearings could be transferred to the chart as lines (although navigators usually had plastic scales and "computers" for this function.) Where the two lines intersected on the chart would indicate the aircraft's location (a form of triangulation.)
My ARB setup - Since I had all of the accessories necessary, I decided to setup the ARB as if it were installed for dual position operation. This required two CRV-23253 Receiver Tuning Heads, two spline flex cables, one right angle spline adapter and one "T" spline adapter. One position has the Pilot's Control Box with a Tuning Head and the second position has the Operator's Control Box with a Tuning Head. I used an automobile choke bowden cable that I modified to use as the pilot's Local-Remote actuation device. I had to rebuild the POWER and the CONTROL BOX cables to be the required length (and to be shielded cables.) I also had to build a shielded cable to interconnect the Operator's Control Box to the Pilot's Control Box. Although the installation has the ZB-3 Homing Adapter mounted on top of the ARB receiver, I don't have the two external control boxes necessary to interface the ZB-3 correctly. I do have the power cable routed to the ZB-3 and using the two other output connectors on the ZB-3, it can be made to operate, but the proper control and antenna relay boxes are needed for an authentic interconnection.

As a transmitter, the Collins ATC is setup to operate with the ARB. The actual intended transmitter was the Bendix ATD, which is a rare item. The ATC was sometimes paired with the ARB in some installations during WWII, so there is a precedent for the combination. 



photo above:
ZB-3 Homing Adapter
. Installing the ZB Homing Adapter on the RU receiver required removing the RU vacuum tube cover. The ZB shock mount holes and slide-clips align with the mounting pins that were for the RU vacuum tube cover. The ZB shock mount plate doubles as a RU vacuum tube cover when installed. There were two "slide latch" pins on top of the ARB cabinet to allow for the installation of a VHF homing receiver such as one of the ZB-Series (ZB, ZB-1, 2 or 3.)

 Navy Dept.- Contractor: Western Electric - Mfg by: Zenith Radio Corp.

Model ZB-3 Homing Adapter  - Type CZR-69076

The ZB-Series was actually designed in the late-thirties for use with the RU receivers but were later also installed on ARB receivers. These Homing receivers used four 954 acorn-type tubes and received direction-location signals transmitted by aircraft carriers. The carrier transmitted a homing signal using a small rotating VHF beam antenna mounted high up on the carrier superstructure that sent out a signal that was "timed" to send specific but differently coded signals every 30?of rotation (speed of rotation was fairly slow at about 3 rpm.) This would allow identification of each 30?sector by a pilot using a "homing adapter" and radio receiver to ascertain an accurate and specific direction to the aircraft carrier. VHF was used since the radio wave was highly directional with the antenna used producing a very narrow beam width that wasn't affected by propagation. The actual signal was comprised of a VHF carrier wave that contained a modulated subcarrier of 700kc. The sub-carrier was modulated with MCW signals consisted of various Morse letters that identified 12 sectors separated by 30º increments of a 360º degree circle surrounding the carrier. The ZB receiver would initially receive the VHF signal which was then down-converted so it could be received on the RU or ARB receiver in the airplane when tuned to 700kc. The pilot had to know which letter identified which 30º sector around the carrier relative to his flying position (for security, these codes changed daily.) The strongest signal indicated in which sector the airplane was flying and this allowed the pilot to calculate where the carrier was and how to approach the landing deck. Depending on the aircraft's altitude, the homing signal could be received out as far as 275 miles away. Use of the ZB Homing Adapters allowed pilots to find their way back to their carrier from long distances, or at night or in otherwise very poor visual conditions.

The ZB-3 Homing Adapters would receive the VHF "homing signal" from the carrier transmitter that usually operated around 242mc. The converter of the ZB receiver would then subtract the VHF carrier leaving the sub-carrier and modulation. The MW sub-carrier wave could then be tuned in on the ARB receiver (or the RU receiver.) If the VHF carrier wave was intercepted by the enemy, no information could be detected without the ZB-type conversion taking place. There were other ZB Homing Adapters at other frequencies, for example the ZB-2 utilized frequencies in the 34mc to 58mc range while the ZB-3 utilized frequencies in the 234mc to 258mc range with the normal operating frequency being 242mc.

The ZB Homing Adapters required a specific ZB-Control Box that routed power from the ARB receiver or from the Junction Box in the case of RU receivers. A specific ZB-Antenna Relay Box was also required and that allowed switching RF signals to the ARB (either from the Homing Adaptor or from the MW or HF aircraft antenna.)

photo above
: The ZB-3 chassis showing the voltage selector switch on the right (some RU receivers operated on 12vdc.) Also the antenna input coaxial cable going to the 1st RF amplifier stage. The next two stages to the left are the second and third RF amplifiers. The left side last tube is the converter/LP filter circuit.

The rear panel ZB connectors are routed as follows: The three-pin connector cable is routed to the ARB accessories socket or the RU Junction Box. The four pin connector cable is routed to the ZB-Antenna control box. The eight pin connector cable is routed to the ZB-control box. These connections allowed the ARB or RU to be used for "Homing" or for communications. Also included in the ZB accessories was a test oscillator with VHF output. The "snap pins" on top of the ZB are for the canvas cover (called a "skirt") that could be installed when the ZB wasn't in use.

Later versions were given the designations of ARR-1 and ARR-2. The ARR-1 is very similar to the ZB-3 while the very late versions of the ARR-2 used an on-board dynamotor and flexible cable remote tuning control.


photo right: Rear of the ZB-3 showing the various receptacles.


photo above: The DW-1 version of the Amplified Direction Finding Loop

Navy Department - Bendix Radio (Div. of Bendix Aviation Corp.)

Model DW-1 - Type CRR-50061 Coupler & CRR-69052 Plug-in Loop

Amplified Direction Finding Loop

As air radio navigation evolved, it became increasing apparent that determining "true direction" would be a great advantage, especially in air search and rescue operations that involved finding the unknown bearing to a received signal from an unknown location. By combining the loop bi-directional pattern with a "sense" antenna (usually a small vertical or short wire antenna,) a cardioid pattern would result, giving the user the ability to determine "true direction" of a signal because of its single null response.

The DU, the DU-1 and the DW-1 loop antennas were designed for use with the RU-series of receivers and the contract for the DW-1 dates from 1940. Power for the loop came from the RU-Junction Box. The DW-1 and DU-1 can also be operated much more conveniently with the ARB receiver. The DW-1 can be powered directly by the ARB accessories connector that provides +28vdc and +230vdc. Inside the DW-1 housing is a dual RF amplifier arrangement with one 12SK7 tube amplifying the loop signal and the other 12SK7 tube amplifying the sense antenna signal. The plates of the two tubes are connected together and that performs the "mixing" function that creates the cardioid pattern allowing the ARB receiver operator to determine "true direction" by the loop's position relative to the aircraft fuselage and course. The DW-1 loop is colored gray for 180º of its circumference and black for the other 180º of circumference. Gray is "inline" with 90º on the azimuth compass and is considered the "front" of the loop and is generally set up on the aircraft in that orientation. The DW-1 has a three-position switch marked R-B-D that changes the antennae as follows: R is the Sense antenna only which is omni-directional. B is the loop only which is bi-directional. D is the loop and sense antennae combined giving a cardioid pattern.

There isn't any specific information that the DU-1 or the DW-1 was ever used in combination with the ARB - this equipment set up isn't mentioned in either manual. However, electronically and physically, it's very easy to operate them together and to determine "true direction" of an unknown signal. The output of the DW-1 is connected to the AT antenna terminal with the control box set to COMMUN (for operation on all bands for AT input.) Since the combining of sense antenna and loop for a cardioid pattern is already accomplished by the DW-1 circuit only a connection to the ARB RF amplifier is necessary. Actually, if the DW-1 is powered separately, its output can be connected to any radio receiver antenna input for DFing. Additionally, two jumpers can be moved internally to change the series filament connection on the 12SK7 tubes to parallel connections for +12vdc operation of the DW-1 (along with +230vdc B+.) The parallel filaments +12vdc connections would be necessary if I used the DW-1 with the RU-16 receiver.

DW-1 with ARB Combination - Operation - Connect the DW-1 to the ARB Accessories connector with a proper cable. Connect a Sense Antenna to the A terminal on the DW-1. This can be any relatively short wire, e.g. 10 feet, oriented vertically (if possible.) Connect the DW-1 output to the AT input on the ARB. Set the Control Box to COMMUN and select the AM-BC range of frequencies. Set the DW-1 to R (Sense Antenna.) Switch ON the ARB and the DW-1. With the Sense antenna selected (R) you can tune in a relatively weak AM-BC station (don't tune in a local AM station as the signal will be way too strong for DFing.) Adjust the VOLUME on the Control Box for an average listening level. Next, switch the DW-1 to B (Loop only) and rotate the loop for the strongest signal response and then "tune-in" the DW-1 frequency dial for the strongest signal. AM-BC will be on Band 2 or Band 3 depending on where with the AM-BC band you are tuning. Once the desired signal is tuned to maximum, next rotate the loop (approximately 90º) and you should find one of the two nulls. Increase the VOLUME to be sure you have the loop exactly at the null and observe the compass bearing. Now, switch the DW-1 to D (Loop+Sense) and rotate the loop exactly +90?from the observed compass bearing. If the signal remains strong, then carefully rotate the loop another +180? and you should find the null. The null is very sharp and when the null is found the loop axis will be "pointing" at the signal and the compass gives the bearing. If you actually set-up the DW-1 oriented N-0º & S-180º then you would have a pretty close bearing to the signal (if you adjust for magnetic deviation, you'd be even more accurate.) You might have to adjust the DW-1 Gain pot for best null response. Adjust the Gain pot by switching between R and B and adjust the pot for the best balance of the two responses. Be sure to have the loop pointing at the signal source for this adjustment. This will assure that a deep null will result when in the D position. There is a moveable mask on the DW-1 compass that only allows a little over 120?of the compass to be viewed. When the "loop-only" null is determined, then the mask opening can be moved to have the "zero" index at the observed bearing and then about +90?of the observed bearing will be at about two-thirds towards the opposite end of the mask opening.

This DW-1 required a little bit of mechanical rework to get it function correctly. Also, all components were checked and found to be within tolerance. Once the mechanical problems were corrected, the DW-1 was ready to test. I have to admit that I wasn't expecting too much from the DW-1. A few years earlier I had access to a DU-1 that I was testing for a fellow LW enthusiast but the results were less-than impressive (I wasn't using the loop with an ARB receiver however.) I had also tried out this very same DW-1 at a local mil-radio ham's shack with it connected to his ARB receiver but, again, the results were not impressive. A different story with the DW-1 in operation with my recently serviced and aligned ARB receiver. The signal levels available with either the DW-1 loop or just a small sense antenna are very strong. It's very easy to set up the DW-1 with the ARB and the DF results are very apparent making it easy to find nulls and bearings. The ARB and DW-1 loop combination is excellent for demonstrating WWII DF procedure (if anyone is interested.)


Since the "Gibson Girl" has been mentioned in the write-ups on the Air-Nav receivers above and, even though it's sort of "out of place" with Navy gear (since it was USAAF,) I'm putting the "Gibson Girl" write-up here with the Navy Air-Nav gear. By the end of WWII, both the USAAF and the Navy were using the SCR-578 and post-war the designation changed to AN/CRT-3 with the AN indicating "Army-Navy."

SCR-578 "Gibson Girl" Emergency Transmitter
BC-778 Transmitter plus Accessories, Initial Contractor: Bendix Radio

AN/CRT-3 "Gibson Girl"
T-74 Emergency Transmitter plus Accessories

By the late-thirties there had been various types of emergency radio transmitters developed to aid in rescue at sea of downed aircraft crews. Most early attempts were crude and not very effective. In 1941, the Germans had come out with their "Not Sende Gerat 2" aka NS2. It actually was a compact, more effective version of their earlier fairly large and heavy NS1. The NS2 was similar to the yet-to-be developed "Gibson Girl" with a 500kc transmitter operated by a hand-crank generator, a slightly curved housing and a yellow paint job. Similar "Gibson Girl-type" accessories went with the NS2.

In 1941, the British captured a German NS2 and copied it with it becoming the British T-1333 emergency transmitter used by the RAF. Again, the T-1333 is very similar to the yet-to-be developed "Gibson Girl." When a second NS2 was captured, the British gave it to the USA with the idea that the USA had the manufacturing resources and capability to produce the huge number of emergency transmitters that would be required for the war effort. The USA engineers further refined the design concept and built a more robust mechanical configuration. The USA version became the BC-778 transmitter that, along with all of its accessories, was designated the SCR-578. Production started in 1942 with Bendix Radio (a division of Bendix Aviation Corporation) being the initial contractor.

The BC-778 was a two tube transmitter that operated MCW on 500kc. Power was provided by a dual voltage generator that was gear driven when turned with a hand crank on top of the transmitter. Voltages were +24vdc and +330vdc when the generator was driven with the hand crank turned at about 80 RPM (the German NS2 generator required 120 RPM.) The tubes were a 12A6 oscillator that was grid modulated by a 12SC7 Tone Oscillator. Power output was about 5 watts. Turned with the hand crank simultaneously with the generator was the code wheel that keyed the transmitter automatically. The code wheel sent ten "SOS" signals and then about ten seconds of "key down" to allow for DFing the position of the transmitter. The BC-778 had to be tuned for maximum brightness of the neon lamp indicator.

It was also possible to have one operator crank the generator and another operator press the KEY button and send Morse (it was very difficult for one operator to both crank and send Morse at the same time.) The transmitter had a large canvas strap the was used to secure the BC-778 between the operator's legs which then allowed for easier manipulation of the hand crank (both hands could be then used.) Under good conditions on the open sea with the search aircraft at a 2000' elevation, the signal could be picked-up out to about 200 miles. The BC-778 was packed in a weather-proof flexible case that contained many accessories to allow the proper set-up of the emergency transmitter for best signal results.

photo above: The WWII Gibson Girl BC-778-D

Accessories - The BC-778 was equipped with a reel out antenna wire that was about 250' long,...but how to get the antenna aloft for proper operation? There were two options available. If it was windy enough, a box kite could be deployed to lift the antenna into position. If there was no wind then the balloon was used. To fill the balloon with a lighter-than-air gas required using the hydrogen generator provided in the kit. The hydrogen generator was a cylinder that contained a grain-type material that when exposed to salt water (or sea water) would produce hydrogen gas and heat. The generator cylinder got too hot to hold during the production of H2 gas. The H2 gas would be used to fill the balloon using a rubber tube provided. The balloon would rise and pull the antenna up with it. Another device in the kit was a signal lamp that plugged into a socket in front of the BC-778. The signal lamp was mounted to the operator's head with a canvas strap. The lamp could send SOS simultaneously with the transmitter keying or it could be switched on "Continuous." The signal lamp was for nighttime rescue operations. Good antenna operation would depend on a proper ground and in the front panel of the BC-778 is a compartment that has a metal cable with a "sinker-threaded plug" attached to the end. This ground wire was dropped over the side of the life raft into the sea water to provide the ground. There were other tools and extra antenna wire included in the kit. The complete kit with BC-778 was designated as SCR-578.

photo left: Close-up of the Antenna Reel, the rubber insulated antenna lead wire with antenna clip and the Ground cable with threaded "sinker" plug. This is on the BC-778

photo above
: Accessories and case for the post-war AN/CRT-3. The box kite is partially assembled to show the shape. In front of the kite is the head signal lamp. To the right of the lamp are two balloons in cans. The tall canister is the H2 generator.

AN/CRT-3 - After WWII ended, the use for the "Gibson Girl" continued on. However many times the 500kc operating frequency wasn't efficient, especially if the downed aircraft happened to be on land rather than at sea. The effectiveness of the Gibson Girl on land was about 50 miles at best. See the movie "Island in the Sky" for some fairly accurate depictions of where, (the Canadian wilderness of Northern Quebec) and how the Gibson Girl, with antenna "strung up" in a tree, would have been used on land. The need for a higher frequency of operations evolved into the AN/CRT-3 which provided both 500kc and 8280kc or 8364kc operations. No tuning was required. The transmitter would send for about 50 seconds on 500kc and then automatically change to 8280kc and send for about 50 seconds before switching back to 500kc. All "MANUAL" sending was on 500kc only. The transmitter was designated as T-74 but the entire kit was designated as AN/CRT-3.

The "Gibson Girl" was used post-WWII up into the late-sixties when smaller solid-state emergency transmitters and transponders became more common place. The USSR also produced a "Gibson Girl" look alike in their AVRA-45 which was being used at the end of WWII.

photo left: The T-74 (AN/CRT-3) version of the Gibson Girl. Note that this version can be set to 500kc only in MANUAL or in AUTOMATIC send on 500kc and then switch to 8280kc automatically repeated every 50 seconds. This example still has the leg straps and it's also functional.

photo right: The top label on the BC-778 showing basic operating instructions. Note that the downed pilot is shown in a life raft (FIGURE 1.)


U.S. Army Air Forces

Aircraft Radio Communications Equipment, Radio Navigation & Radio Signal Intercept Gear

If ever there was a WWII military aircraft receiver that did "so much, so well, with so little," it was the BC-348. The receiver provided two TRF amplifiers, three IF amplifiers, Crystal Filter, front panel adjustable BFO, AVC-MVC, direct frequency readout out on an illuminated, "band-in-use" masked tuning dial and an Audio Output stage that could be selected internally for 300Z or 4000Z loads. Unlike the proceeding aircraft receivers profiled above, the BC-348 was entirely "self-contained." All that was required was to connect the receiver to the +24vdc aircraft power buss, hook up the antenna, hook up the remote standby and plug in the 'phones. Well-over one hundred thousand BC-348s were built during WWII making them one of the most common WWII aircraft receivers out there. It's unfortunate that over the past seven and a half decades hams (and surplus dealers) have managed to "cut and hack" almost every surviving example making it nowadays a real rarity to find an all original, dynamotor-operated BC-348. 

BC-224-H - 1942

Signal Corps U.S. Army  -  RCA Manufacturing Company, Inc.

BC-224 Series and BC-348 Series  -  Various Other Contractor Companies

RCA introduced the BC-224 Aircraft Receiver in 1935. It ran on the then popular 12 volt power systems used in most aircraft. The initial version of the BC-224 had the tuning dial on the left side of the front panel. This version is usually designated as the BC-224-A and the number produced was very small which was typical for pre-WWII military contracts. As aircraft power systems evolved, 24 volts became the standard voltage and that required changes to the radio equipment that was going to be installed in the newer airplanes. RCA redesigned the BC-224 to operate on 24 volts and this receiver was designated as the BC-348. The BC-224 continued to be built for installation into earlier aircraft while the BC-348 was produced for modern aircraft installations. Both receivers were built by RCA Manufacturing Co., Inc., a division of RCA-Victor that built all of the commercial and military radio equipment for RCA. When WWII began, several other radio companies became contractors for BC-348 construction,... Belmont Radio, Wells Gardner & Co., Stromberg-Carlson, to name a few. Only one contract for BC-224 receivers was built by another company other than RCA Manufacturing Co., Inc. and the last contract for the BC-224 appears in 1942. The BC-348 was produced through WWII and total quantity produced is certainly well over 150,000 receivers (over 50,000 receivers alone were produced by Wells Gardner & Company on just the "Q" model contract.)

The BC-348 operates on 24-28vdc with the high voltage (~+220vdc) provided by an internal dynamotor. It's quite likely that well-over 150,000 BC-348 (total of all versions) were built during WWII by many different contractors building many different versions within that time period. The circuit used eight tubes with the heaters originally wired in series/parallel for 24vdc operation (each of four 6 volt tube heaters in series would operate on 24vdc.) The early circuit provided two RF amplifiers, a Mixer, a Local Oscillator, an IF amplifier stage, a combination 2nd IF amp and BFO, a combination 3rd IF amp and Detector/AVC followed by a type 41 audio output stage (this was changed to a 6K6 in some versions.) These versions will have a 991 neon lamp acting as a regulator on the local oscillator and an antenna trim control. Construction makes use of four component boards resulting in extensive use of wiring harnesses. The Crystal Filter and BFO are in fully shielded cans. This early version is usually referred to as the "Grid Cap" version since these types of tubes are used. This early version was difficult to work on and was expensive to build. To reduce costs, the Q, N and J versions were introduced. These versions eliminated most of the component boards and used "point to point" wiring to reduce costs and ease rework. This version is called the "Single-ended Tubes" version since "non-grid cap" tubes are used. The later circuit used two RF amplifiers, a converter stage, three IF amplifiers, a duplex diode/triode provided Detector, AVC and BFO functions and a 6K6 provided the audio output. The later versions eliminated the shielded Crystal Filter, Antenna Trim control and the fully shielded BFO. The audio output impedance was internally selectable at "low Z" which was around 300 Z ohms or "high Z" which was around 4000 Z ohms (on later versions.) Some BC-348s will have a decal on the front panel indicating if the "low Z" was optioned.


photo left: Top of the RCA Mfg. Co. BC-224 chassis which is virtually identical to all of the "Single Ended Tubes" versions of the BC-348 chassis. Note that this BC-224 does have the proper DM-24 dynamotor installed.

A selectable crystal filter was also included in the circuit (if working correctly, the crystal filter can narrow the IF bandwidth to about 1kc wide, which counters the popular contemporary opinion that the BC-348's bandwidth is "broad as a barn.") The dual dial lamps were adjustable for brightness and were wired in series through a potentiometer and fixed resistor. Frequency coverage was from 200-500kc (not on the B or C versions) and 1.5-18mc. The military considered the two versions of the BC-348 to be interchangeable with virtually no difference in performance and operation.

When the receiver was installed on its FT-154 shock mount and installed in the aircraft, an eight pin Jones plug mated with a receptacle and cable that exited from the rear of the mount containing the 28vdc input, the remote stand-by relay function and an audio output line. The BC-348 was generally interconnected with the transmitter to control boxes allowing the transmitter's control relay to provide antenna switching, receiver stand-by and providing side tone monitoring which allowed for full "break-in" keying. Since there are so many variations, military collectors have generally divided the BC-348 into two groups, early types (B, C, H, K, L, O, P & R) with grid cap type tubes (usually called the "Grid Capped Tubes" version, GCT) and the later versions (J, N & Q) with single-ended tubes (called the "Single-Ended Tubes" version, SET.) Most of the very early versions (B and C) were rebuilt into later configurations, especially to add the 200-500kc band. These were re-tagged as "R" versions. Many different contractors built BC-348s but Wells Gardner & Co. probably built the greatest quantity of receivers and is the most commonly seen manufacturer.


photo above: RCA Manufacturing Co. BC-348-C  - earlier "Grid Cap Tubes" version - ca: 1941. This BC-348-C does have the original style DM-28 dynamotor installed and the receiver is operational.

photo above: Wells Gardner & Co., BC-348-Q 1943 - this example is dynamotor operated. This one WG contract alone produced over 50,000 receivers. Note that these "Single Ended Tubes" versions eliminated the "ANT. ALIGN." (antenna trimmer) control that was located above the Antenna and Ground input terminals. Many other cost reductions will be found inside the Q, N and J versions. However, the great  performance is retained and these receivers are very easy to do any rework to.

A History of Hacking - The BC-348 became available as surplus almost immediately following WWII and was easily available for a couple of decades from many surplus dealers. Early purchases offered the receiver NOS in the original box. This allowed hams the ability to purchase a great performing receiver at a reasonable price - still in the crate for about $100. Many surplus dealer offered "AC power supply conversions" for as little as $15, so the buyer didn't have to do the work. The down side is that today it's almost impossible to find a BC-348 that hasn't been modified.

Typically, the dynamotor and filters were removed (and discarded) and an AC power supply was built onto the then vacant dynamotor chassis. Tube heaters were rewired for parallel connections. This mod was fairly simple and usually didn't compromise the receiver's performance too much (other than adding some hum to the audio output if the power supply wasn't designed correctly or wasn't well-filtered - which most weren't.) Unfortunately, most of the dealer power supply mods used a separate AC power toggle switch that resulted in a non-original hole on the front panel. This mod also left the nowadays frequently-seen "just the DM-28" with no chassis or filters available. Purists can't blame the just hams for shoddy mod design and workmanship because the dealers were actually charging the purchasers of the surplus BC-348s for the opportunity to destroy the originality of thousands of receivers. Additionally, all of the dealer-installed power supplies were just Pi-network filtered and as cheaply made as possible. Dual section filtering is necessary for proper hum reduction in the BC-348. Also, not realizing that there's a B- return, many "hamster-modifiers" grounded negative leads on filter caps causing rampant hum in the receiver audio.

Certainly the "hamster" destruction of many of the BC-348 receivers was popularized by CQ magazine and their "Surplus Conversion Handbook" (there were multiple editions) in which page after page of "cutting and hacking" examples were presented. Every page seemed to promote the idea that the BC-348 needed "ham-level engineering." Adding an audio output section was a common mod. After all, what ham owned, or could afford, any sort of impedance matching transformer when they could build the mod using "junk box parts." The audio output mod called for the removal of the original audio output transformer which makes finding an original today somewhat difficult. Removal of the 8-pin Jones plug power input connector or adding a useless S-meter were also common mods.

Additionally, many hams would sometimes drill the case with lots of .25" holes thinking the receiver needed extra cooling because of the added AC power supply rectifier tube. Somewhere along the line of destruction, the receiver base mount (the FT-154) always seemed to have been removed and discarded. Since the receiver's engagement pins presented a problem for "mountless" BC-348s, sometimes hams would drill out all of the pop-rivets and remove the entire receiver base and added rubber feet instead. Of course, this ruined the cabinet for future use if the FT-154 ever turned up. Decades of "hamster mods" have pretty much made an "all original" BC-348 a true rarity. Of the well-over one hundred thousand built during WWII, few examples have made it to the present time without some type of modification. However, with diligent searching it's still possible to find unmolested examples. However, not so rare and usually not so pricy are the receivers that just had the AC power supply carefully added. These examples usually aren't too difficult to restore back to original condition. Both versions, the Grid Cap Tube versions and the Single-ended Tube versions, are great performing receivers if restored to original or when conservatively modified (a properly designed AC PS only.) Accurate IF-RF alignment and repair of the crystal filter is an absolute necessity for top performance (and must be done before any critical comments are to be believed.)


If you're interested in restoring your BC-348 or BC-224 receiver back to its original configuration with the installation of the correct DM-28 or DM-24 dynamotor, go to our web article "Rebuilding the BC-348 Family of Receivers" This article tells you about all of the "pitfalls" that might await you in this seemingly easy project. Also includes information on other aspects of the restoration of these fine performing receivers, such as building (or re-building) a minimally invasive AC power supply that operates the receiver correctly.

photo above: Inside the BC-348 "Single Ended Tubes" version. Note that the shielded cans in the IF section has been reduced from six to only three cans. Many other cost-reduction differences can be found when comparing the construction of the SET version to the GCT versions of the BC-348 receivers.


ARC-8 Airborne Radio Gear
ART-13A Aircraft Transmitter, BC-348-R Aircraft Receiver

When the ART-13A transmitter and the BC-348 receiver were installed in an aircraft, the pair was generally designated as the ARC-8 station. However, to be absolutely correct for the ARC-8 designation, the ART-13 and the BC-348 should be operating in an aircraft from the +28vdc battery-generator buss and each should be powered by their respective dynamotors. That's not the case with the station shown to the left since it operates from a house and is uses AC power supplies that operate from the 120vac line. The ART-13A is a "SAAMA" fugitive that ended up at that facility, the San Antonio Air Materiel Area, in the 1950s. SAAMA was part of Kelly AFB and was given the SAAMA designation in 1946. SAAMA could and did rework almost any type of Air Force equipment. Unfortunately, this particular ART-13A seemed to be a problem for their technicians. Some how it made to the surplus market in "non-working" condition where it was sold. Some time after the sale it was disassembled. Then some parts were "cut out" and other parts lost. I obtained this ART-13A as a "basket-case" about fifteen years ago - actually three boxes of parts along with several pieces of sheet metal. I thought it was just a parts set for quite a while. Eventually, I inventoried the parts and was amazed that I had almost everything to "build" an ART-13A. Lost forever were the three wire wound resistors located in the PA/MOD compartment but Fair Radio Sales supplied original replacements. The metal pocket for the cal book needed to be replicated. The audio module had been "gutted" so a replacement was purchased on eBay. The Plate/Grid/Batt meter needed to be replaced (from Fair Radio.) That pretty much was enough to get the SAAMA fugitive working up to a point. Apparently, at SAAMA, the meter function switch was replaced. Somehow, the SAAMA techs had mis-wired the switch causing the grid drive not to work (two wire positions swapped.) A few other problems were repaired and this ART-13A was pretty much resurrected from the junk pile. Over the years I've replaced the non-matching meters with a "matched set" of meters, added the O-17 LF Oscillator, added the calibration book and many other minor items that have added to the overall completeness of the transmitter.

The Belmont BC-348-R was obtained from a local ham. It was in excellent cosmetic condition and was fairly original only having had an AC supply added to replace the DM-28 dynamotor at sometime in the past. That PS was poorly designed and had excessive hum due to having the input filter capacitor negative terminal connected to chassis. I built a completely new power supply that fit exactly into the dynamotor position and utilized the still present "dynamotor harness." The new PS used dual section filtering, solid state rectification and utilized the AVC-OFF-MVC switch to turn the receiver on and off. I also used the PL-103 rear connector so that the FT-154 shock mount could be used correctly. A full IF/RF alignment was performed. Also, the Micamold brand capacitors were replaced (notorious for shorting,...and two were.) It's a nice example of a minimally modified (AC PS only) "grid cap-type" BC-348-R.

There's no doubt that today the ART-13 is by far the most popular transmitter used by military radio enthusiast/hams in their vintage military radio stations. The ART-13 is easy to rebuild, easy to power up, it provides over 100 watts of carrier power in the AM mode and does this while maintaining a truly excellent stock audio response. The ART-13 started out as the Navy ATC and soon became the T-47/ART-13. It wasn't long before the USAAF wanted a version specifically built for their requirements. This slightly different transmitter was the T-47/ART-13A. The following write-up profiles both types of transmitters,...

Navy Dept. - Collins Radio Company  -  ATC, T-47/ART-13

Signal Corps USAAF - Various Contractors - T-47/ART-13A,
T-412/ART-13B, AN/ART-13 & AN/ART-13A

The Collins T-47/ART-13 is a 100 watt carrier output, AM-CW-MCW transmitter that was initially used in USN aircraft and later used in USAAF/USAF aircraft. The ART-13 could also be found onboard USN ships as the TCZ installation. There were even some vehicular uses for the T-47/ART-13. The T-47/ART-13 was developed from the earlier Collins ATC Navy Aircraft Transmitter that appeared around 1941. By 1943, Collins was building the T-47/ART-13 for installation in Navy aircraft and other applications. The USAAF also wanted to use the T-47/ART-13 and a very slightly different transmitter was produced for their use, designated as the T-47A/ART-13. Many of the T-47A/ART-13 transmitters were built by contractor Stewart-Warner and also some were built by Zenith Radio Corporation (Zenith also built some of the ATC transmitters.) Documentation was changed near the end of WWII to designate the transmitters as AN/ART-13 with the A suffix included. When combined with other equipment the set-up designation is for the entire installation. Hence, the combination ARC-8 describes the ART-13 combined with a BC-348 receiver along with the other necessary auxiliary equipment for a complete aircraft radio installation. The ART-13B used a crystal oscillator the provided 20 HF and 4 LF channels.

The T-47/ART-13 power requirements were supplied by a dynamotor that ran on the aircraft +28vdc battery/charger system. The aircraft battery buss supplied the +28vdc@10Amps necessary for the transmitter's tube filaments and relay operation while the dynamotor provided a dual output of  +400vdc and +750vdc. The dynamotor would have the two B+ levels connected in series for the HV Plate ( +1150vdc) below 20,000 to 25,000 feet altitude but a barometric pressure switch (located inside the dynamotor housing) would separate the outputs at higher altitudes and only allow +750vdc maximum to prevent arc-over. There were at least three types of dynamotors used, the DY-17, the DY-11 and the DY-12 (after WWII an improved DY-17A was produced.) The shipboard TCZ featured two types of power supplies, a 115vac operated power supply (of enormous proportions) that supplied the required +28vdc, +400vdc and +1150vdc directly to the transmitter. Additionally, the 115vac unit had a motor-generator that provided +14vdc and +28vdc (the +14vdc was required for relay operation inside the ac or dc operated TCZ power supply.) The 115vdc operated TCZ power supply used two dynamotors that ran on 115vdc input and provided +14vdc and  +28vdc output on one dynamotor and +400vdc and +1150vdc on the second dynamotor. The USMC had a vehicular set-up that installed an ART-13 transmitter with a BC-348 receiver that operated from the back of a Jeep and ran on the +28vdc battery system with HV provided by a DY-12 dynamotor. The antenna used was a whip.

photo above:  ART-13A "SAAMA Fugitive" with the CU-32 LF Loading Coil. I used this set up on 630M CW working into a 163 ft end-fed wire. This ART-13A is powered by the "DynaSim" AC power supply, a homebrew SS AC supply that uses a +440vdc and a +700vdc supplies in series to create the +LV (+440vdc) and the +HV (+1140vdc.)

photo above: Navy-Collins T-47/ART-13 on top of a vacuum tube-based homebrew AC power supply in an amateur vintage military radio station. Note that this transmitter has the earlier style shock mount rails. Also the LFO is the Navy type O-16. The receiver is a Navy-Wells Gardner & Co. RAO-3. The Mill is WWII Underwood S I I. Although most of this equipment is from the same time period in WWII, it would not have been used together at that time. The RAO-3 was a surveillance receiver used with the S I I Mill at USN shore stations.

The T-47/ART-13 featured an advanced Autotune system that would automatically tune up to 10 preset channels selectable by a front panel switch. The Autotune system would tune the transmitter frequency and output network to mechanical presets that then would match a properly selected antenna. The Autotune cycle took about 25 seconds to complete. Switch position MANUAL would allow manual adjustment of the tuning without disturbing the Autotune presets. The T-47/ART-13 uses an 837 as the variable frequency oscillator, two 1625 tubes are used as multipliers, an 813 as the power amplifier and two 811 tubes as the P-P modulators. There are also two small modules. One provides the audio amplifier and sidetone amplifier using two 6V6 tubes and a 12SJ7 tube and the other module, the MCW/Frequency Calibration Indicator, uses two 12SL7 tubes and a 12SA7 tube. FCI allows the operator to calibrate the frequency of the transmitter by providing a 50kc calibration signal derived from a 200kc crystal oscillator. The transmitter frequency range is from 2.0mc to 18.0mc, however many Navy T-47/ART-13 transmitters were equipped with a plug-in Low Frequency Oscillator (LFO) module that allows the transmitter to operate from 200kc to 600kc or 200kc to 1500kc. Early LFOs (O-16) have a frequency range of 200kc to 1500kc in six ranges while the later LFOs (O-17) cover 200kc to 600kc in three ranges. The LFO module uses a single 1625 tube. There are some indications that the Navy preferred the 200kc to 1500kc LFO while the USAAF used the 200kc to 600kc LFO. Many versions of the T-47/ART-13 will have a blank plate installed where the LFO module was installed (along with a resistive load substitute for the LFO's 1625 filament.) After WWII, the USAAF/USAF didn't use the LFO module but the USN still did. This statement is according to the USAF Extension Course 3012 book on "Radio Mechanics" although this book is from the 1950s and may reflect the uses of the LFO at that time rather than during WWII. LF operation does require an external tuner called a "loading coil." The Navy used the CU-25 or CU-26 while the USAAF used the CU-32. Also most installations on aircraft included a small Remote Control Panel that allowed the pilot to operate the transmitter from the cockpit. There are a couple of different remotes that could be used with the ART-13. Many transmitter installations also used three selectable condensers to allow easier loading into various antenna impedances at lower frequencies.
To the right is a photo showing the chassis of the Collins-built T-47/ART-13. This transmitter has the Navy version LFO installed. Also, this is a fairly early version of the transmitter so there are some differences when compared to the T-47A/ART-13 versions. Of note is the lack of an interlock switch which on the early versions allows you to easily operate the transmitter with the lid off. The module to the lower right is the Audio Amplifier unit and directly behind it is the 837 VFO tube. Behind the VFO tube is the FCI/MCW module (later three tube version) and to the left of it are the two 1625 multiplier tubes. The module in the center of the transmitter is the LFO. In the section at the rear of the transmitter, to the left side is the modulation transformer from which its plate leads connect to the two 811 modulator tubes. To the right of the 811s is the 813 PA tube. The left-center section of the transmitter contains the matching network and the LF relay (next to the LFO module.) On the far left is the vacuum TR switch and behind it is the keying relay. The round ceramic unit in front of the vacuum TR switch is the inductive pickup for the Antenna Current meter.

The somewhat later USAAF T-47A/ART-13 version added some minor improvements to the transmitter with a vernier scale on the VFO Fine Tuning, a top lid interlock switch, a different bottom plate with built-in guides for the shock mount and a white ceramic insulator bell on the antenna connection being among the most apparent changes. There was also a T-412/ART-13B that added a COMCO selectable crystal oscillator in place of the LF module. The COMCO crystal oscillator normally has 4 LF/MF channels and 20 HF channels. All ART-13Bs are retrofitted earlier models and it is possible to find even an early ATC transmitter that has been converted to the ART-13B version.

The T-47/ART-13 and its variations had a very long life. Introduced around 1943-44, actively used during and after WWII and well into the fifties (sometimes found still being used well into the sixties and early seventies.) The USSR also produced a copy of the ART-13 that they used up well into the 1980s (the R-807.) Because of its long useful life, most T-47/ART-13 transmitters found today will have had many scratches and a few dents and paint scrapes. Non-matching modules are common and will be encountered with some parts having MFP applied and others that are bare. A book containing brief instructions and the calibration settings for specific frequencies was usually stored in the metal pocket underneath the transmitter. This book is also usually missing on most transmitters although the same information is in the standard manuals. Luckily, many tens of thousands of T-47/ART-13 were built and spare parts are very easy to find which allows for the fairly easy restoration and maintenance of these durable and potent transmitters.

photo above
: Chassis of the Navy-Collins T-47/ART-13. The MCW/FCI module is the later, three tube version. The Collins chassis are usually painted gray and have a "winged emblem" embossed near the 837 VFO tube.

Nowadays, the T-47/ART-13 has become one of the most popular aircraft transmitters being used in amateur vintage military stations. It can provide plenty of power and excellent audio. It's relatively easy to restore since there are many, many parts sets out there. Though the ART-13 can run on +28vdc and its original dynamotor, most users opt for a homebrew AC power supply. The +HV can be safely increased to around +1400vdc to provide even more output power and some brave users will run the HV up as high as +2000vdc (not for the timid and distortion might be encountered at this level of HV.) 

After years of operating, repairing and restoring ART-13 transmitters, it's my opinion that the transmitter operates very well and quite reliably with +27.5 to +28.0vdc at 12 amps minimum available current, +420vdc to +440vdc for the +LV and +1100vdc to +1200vdc for the +HV. The transmitter will easily and reliably produce 110 to 120 watts output power on 75 meters - 75 meters is where most of the vintage military radio nets are located.

The Audio Module has a "fixed-level" gain setting that was designed to work with specific WWII vintage military microphones. The carbon bias resistor R203 was a 15K value that provided sufficient bias voltage for most "then new" carbon mikes. After WWII, many ART-13 audio modules were modified by changing the carbon mike bias resistor R203 from 15K down to 4.7K. In fact, the ART-13B schematic shows the R203 value as 4.7K. Audio modules with 4.7K bias resistors will have no problem providing plenty of carbon mike response. Check the value of R203 if you're having carbon mike problems. An easy way to achieve proper modulation levels is to use an Astatic TUG-8 stand with a D-104 or 10-D microphone "head" (with "DYNAMIC" selected on the Audio Module.) These mike stands have a built-in, adjustable gain amplifier that provides ample audio output to drive the fairly low input Z of the T-47/ART-13 (~ 250Z ohms.) An oscilloscope should be used to monitor the transmitter output when trying out different mikes as it will be very apparent on the 'scope whether proper modulation is being achieved. Due to the unbalanced, low reactance typical ham antennas used on 80 and 40 meters, the T-47/ART-13 will require an auxiliary capacitor connected to the COND terminal to ground for proper loading of these kinds of antennae, particularly on 80M. These external capacitors should be high voltage rated ceramic types. A fixed value 75pf to 150pf capacitor will work fine on 80M. 40M depends on the antenna used - a 40M dipole won't require an auxiliary capacitor if the C control is at 7 or higher for proper loading and antenna tuning (7 and above becomes a Pi-network.)

For copious amounts of detail and information on the Restoration and Operation of ART-13 Transmitters go to the web-article "ART-13 Transmitter - Restoration to Complete and Operational Condition"  - there are four parts to this write-up - go to Home/Index for navigation.

photo left: Closer to an Airborne set-up, sort of - this is the USAAF ART-13A SN: 2054 (Stewart Warner contract) shown with a Collins R-648/ARR-41 Aircraft Receiver (1957 vintage.) Both the transmitter and the receiver run on their respective dynamotors and are powered by a PP-1104C +28vdc 50A power supply (actually a depot battery charger but it works great for high current voltage requirements such as running transmitter dynamotors.) This ART-13A was infested with "mud dabber" wasp's nests when found. Restoration (in 2016) of this ART-13A is detailed in Part 3 of the "Rebuilding the ART-13" article. Use Home-Index link below for navigation.

photo above:  USAAF T-47A/ART-13, aka AN/ART-13A. Note the blank panel installed to replace the LFO. Also note that the two meters don't match. It's very common to find mismatched meters ART-13 transmitters and was probably a result of depot repair work, in this case from SAAMA (San Antonio Air Materiel Area, these guys worked on a lot of ART-13s.) Photo taken in 2011. It's the same transmitter that's shown above in the ARC-8 photo.

photo above: Chassis of the AN/ART-13A. Note the "gold" appearance from the MFP coating (SAAMA laid it on thick.)

photo above: The latest (Feb. 2021) ART-13 restoration, actually a Navy-Collins ATC version that was MWO'd into the ART-13 configuration by SAAMA post-WWII. Six MWOs were installed but enough of the original ATC parts were still present to identify the transmitter's origins. This Collins ATC/ART-13 was found in a rented storage unit in Carson City, NV. The ATC was at the very bottom of a six foot tall "stacking" of electronics gear. It also had several "stripes" of pink paint applied on the top and front of the transmitter. Restoration is detailed in Part 4 of "Rebuilding the ART-13" write-up - use Home/Index below for navigation.


1952 Collins R-105A/ARR-15

USN, USAAF and USAF - Collins Radio Company

R-105A/ARR-15 Aircraft Radio Receiver

The R-105/ARR-15 was developed late in WWII specifically for the Navy but it wasn't long before the USAAF and later the USAF became major users also. There were some versions built and installed in aircraft in 1945 but most of the receiver's career was post-WWII. Collins originally designed the R-105 for the Navy to be paired with the ART-13 transmitter. Since the ART-13 could be set-up to "autotune" to ten selected channels, it made sense that the new aircraft receiver should also have the same capability. The autotune function would allow either a radioman to change frequency at his location or, via remote controls (C-733/ARR-15,) the pilot or co-pilot (or anyone else setup with a remote) would also be able to select control over the receiver (and transmitter via its remote.) With the possibility of several remotes being in possible control of operation, priority was determined by having the last remote switched "ON" having "control" until another remote would be switched "ON" and that action would pass the control of operation to the remote just switched "ON" by switching "OFF" all other remotes. Each time a remote was activated, the proceeding remote was deactivated which left control only on the "last activated" remote.

The R-105 uses 14 tubes in a single conversion superheterodyne circuit that tunes from 1.5mc up to 18.5mc in six tuning ranges. The receiver is powered by the aircraft battery-generator buss at +24vdc to +28vdc with the rated input voltage at +26.5vdc drawing 8.5 amps when auto tuning and about 3.1 amps in normal operation (dynamotor surge current is about 15 amps.) The B+ is supplied by an onboard dynamotor that provides +220vdc at 100mA. Only one RF amplifier is used along with two IF amplifiers. To maintain better stability, a VFO (PTO type 70E-2) that tunes 2.0mc to 3.0mc operates with a Frequency Multiplier to provide the correct combining and excitation to the Mixer stage. The IF can be variable from 450kc up to 550kc when using the BFO-CALIBRATION control but when set to the detent at "0" the IF operated at 500kc. In CW, the BFO provides a heterodyne for demodulation. However, the BFO control also operates the CAL 100kc CFI oscillator when in the MCW-CAL position so when the BFO is tuned off of "0," it turns on the CFI calibrator. When setting up an autotune channel, if the frequency 7225kc was to be set, then the TUNING and BAND were set to 7225kc and the BFO was set to +25kc. This would have the receiver responding to the 100kc CAL harmonic at 7200kc plus the +25kc offset so that a heterodyne was heard and was fine tuned to zero beat using the TUNING control (not the BFO.) The autotune controls were then locked and another channel selected and the procedure repeated for that channel's desired frequency setup. The differences between the R-105 and the R-105A are slight and involve an improved autotune and a few minor circuit and component changes in the "A" version.

IF Bandwidth in MCW/AM Voice - The IF bandwidth isn't adjustable and is usually considered by most AM operator's to be very broad (specs are 15kc at 6db down.) This really only affects AM Voice reception and was undoubtedly a design decision that must have been impart due to the mechanical nature of the "autotuning" and the repeatable accuracy of the system (although the autotune accuracy was generally within 1kc or so) along with the accuracy of the transmitted signal's channel frequency. If retuning a signal would be necessary that involved "unlocking" the autotune for the manual tuning function which then changed the autotune settings for that channel. These possible autotuning issues made it mandatory that the receiver reliably "come up" on the selected channel frequency with the completed autotune "stop" ultimately having the received signal within the IF passband. For Voice reception, a wide bandwidth could provide enough recovered audio for good copy even if the receiver or the transmitted signal (or both) were several kilocycles "off frequency." This allowed the radio op to leave the R-105 "locked." It's important to adjust the SENS pot when in MCW. If the SENS is adjusted to maximum the increased noise will make weak-signal detection difficult and will also tend to broaden the apparent bandwidth of stronger signals (strong AM signals are fairly broad anyway.) The SENS pot is not used in CW. 

The CW mode seems to have a much narrower bandwidth probably due to the BFO action (being a Navy receiver, CW was the primary mode of reception at the time.) Also, that the AVC is eliminated, the VOLUME control adjusts the RF gain and the audio bandwidth is narrowed rolling off the high-end at about 1200hz helps narrow the apparent bandwidth. When receiving CW, the BFO can be adjusted to compensate for off-frequency reception to optimize the CW signal.

The 24 pin Cannon Connector - The installation onboard the aircraft required an aircraft wiring junction box and audio phone jack patch boxes (as needed) along with the necessary remotes and cabling. The original shock mount also included the mating connector for the R-105's rear mounted input-output connector which was a large rectangular Cannon 24 pin male receptacle. Besides power input, audio output and a remote standby there are ten pins dedicated to the remote channel selection and a few other pins for remote priority sensing for the receiver. Since the mating connector was part of the shock mount which seems to have always been left mounted inside the airplane, most R-105s today don't have the original mating connector or the original type of shock mount. There are other types of shock mounts available that fit correctly but don't have the rear connector mounting bracket (the R-105A shown in the photo is installed on a newer "generic" Barry Mount aviation shock mount.) The 24 pin Cannon connector was only used in one other application that might be encountered, the ARC-2, so finding an original plug can be somewhat difficult. There are several methods to interface power and other required inputs and outputs, so the lack of an original Cannon plug won't present too much of a problem. In the case of the R-105A shown, the former owner had a delrin block machined and bored to accept the proper size Molex female receptacle pins. The holes were counter-bored to provide "non-movement" of the Molex pins (once inserted) when either pushing in the plug or removing it. Though all the parts for the plug were included in the purchase, I've never completed assembly of the plug since most of the R-105A pin outs are for using the C-733A/ARR-15A remote box (examples of which seem to be "unobtainium.")

Actually, about the only rear connector-plug wiring absolutely necessary for local operation of the receiver are the +26.5vdc and chassis ground/-26.5vdc connections along with disabling the remote standby control line and maybe using the audio output line (instead of the front panel PHONES jack.) All other pins are for remote channel select and remote priority operations. Powering up for check-out, only pin 9 (Chassis, -26.5vdc,) pin 17 (+26.5vdc) and a jump from pin 3 to pin 22 are required for receiver-only operation. For a station setup, Pin 3 and pin 22 would be used for remote standby function with NC for receive and NO for transmit. Antenna isolation has to be provided (the ART-13 has an internal vacuum T-R switch for receiver connection to the antenna.) I had lots of spare Molex pins that were the correct size, so I just made up a test cable with two 14 gauge wires with Molex pins on one end for power and a 22 gauge jumper about 3" long with Molex pin ends to jump pin 3 and pin 22. For testing, this works fine (actually, unless a remote turns up, this setup works fine for operation in a vintage mil-ham station too except that the jumper from pin 3 to pin 22 has to be removed and two wires substituted and then routed to the ART-13 sending relay.) The proper size Molex pin is "HCS 125 Socket" with a Mfg PN: 18-12-1602 (or a Mouser PN of 538-18-12-1602.)

Dynamotor Servicing - After test operating the R-105A a few times, I noticed that the dynamotor seemed pretty noisy (the receiver was out of the case) and the dynamotor was running very hot. Normally, you always have to service any dynamotor since most of them have never been serviced in their entire existence. The old style grease used in the ball bearings at that time will harden with age and inactivity, resulting in the bearings running with a minimal amount of lubrication. Also, internally, most dynamotors will have a lot of brush carbon residue all over the inside and that carbon can be somewhat conductive if there's enough residue. Almost always the commutators will need thorough cleaning along with de-glazing and leveling their surfaces. The brushes will also need inspection for length and proper seating. This dynamotor's condition was as expected,...probably never properly serviced and with lots of carbon residue all over everything internally. Servicing requires some disassembly and consists of pulling both end bells and removing the brushes (mark for correct reassembly.) At this point, I tested how freely the armature turned to. It didn't move freely at all and was actually fairly resistant to any rotation. With the brushes removed the armature should easily rotate with virtually no resistance to movement. This "resistance to rotation" was probably what was causing the over-heating of the dynamotor (more current required for rotation resulted in more watts dissipation in the form of heat.)

I proceeded with further disassembly by disconnecting the wires going to the brush barrel terminals, removing the two long screws that hold the bearing housings to the dynamotor body and dismounting each bearing housing piece. Now the armature could be taken out. The armature will have commutators on each end. I used 600 grit AlOx paper to clean, smooth and level the surface of the commutators and then washed with Isopropyl Alcohol. I then used a wooden tooth pick that was sharpened to clean out any residue between each segment of the commutators and then cleaned them again with alcohol. The ball bearings are a press-fit on the armature shaft but the bearings can easily be cleaned and flushed with WD-40 and then repacked with modern wheel bearing grease. Pushing the new grease into the bearing will force any remaining "old" grease out the back of the bearing and the old grease will have to be carefully cleaned off making sure to keep any grease residue off of the commutators. The dynamotor body was thoroughly filled with carbon deposits that all needed to be cleaned out. I used alcohol and a combination of small acid brushes and Q-tips to remove all the carbon residue. I cleaned the brush barrels, the brushes and brush connector terminals with alcohol.

I carefully reassembled the dynamotor except for the end bells. This was so I could see the brush to commutator contact and make sure there wasn't going to be any "sparking" which is an indication of a rough commutator surface or poorly fitted brushes. I didn't see any sparking during this test operation, so the end bells were installed and the dynamotor installed into its receptacle with the slide-clips used to "lock" the dynamotor in place inside the R-105A.

The newly serviced dynamotor ran much quieter, much smoother and a lot cooler. Not for long, however. The vibration noise progressively worsened. When out of the receiver the dynamotor ran quietly although some noticeable vibration could be felt. I added extra rubber cushioning to the mount and reinstalled the dynamotor. This seemed to reduce the noise and the imparted vibration significantly. I ordered new rubber mounts. These are small vibration isolation mounts that are 0.375" square and 0.50" tall with 8-32 studs for mounting.

photo right: R-105A chassis (top) showing the Autotune motor at the left (front of receiver) and the dynamotor at the left (rear of the receiver.) This particular receiver has been given the MFP treatment hence the gold color of most of the metal pieces. Also, note the main PTO 70E-2 (lower PTO) and the PTO for the BFO (upper PTO.) Between the two PTOs is the crystal controlled CFI module. There are two tubes inside the CFI module, also one tube under the shield in each PTO. The lower three tubes are the RF/Mixer section and the upper six tubes are the IF/AVC/Audio section.


Signal Corps - U.S. Army Air Forces - General Electric

BC-375-E - Aircraft Transmitter

Though the BC-375 was initially designed in the early thirties (AA-191 transmitter) and utilized parts and technology from a decade earlier, it found a long-term usage with the Signal Corps due to its ease of operation and reliability. Its earlier kin, the BC-191, was the first version built from the mid-thirties on up to mid-WWII. Around the beginning of WWII, an airborne +24vdc version was necessary and the BC-375 became the designation for a slightly different transmitter for use in larger aircraft. GE got a manufacturing contract for close to 100,000 BC-375 that were built through the first half of WWII. Commonly used on B-17s, the SCR-287 was found on thousands and thousands of those bombers. Towards the middle of WWII, the ARC series of transmitters-receivers were introduced, along with the Collins ART-13A, to replace the BC-375. At the end of the war, thousands of BC-375-E transmitters remained unopened in their original crates (ready to flood the post-war surplus market.)

The BC-375 uses four VT-4-C triode tubes (type 211E) and a single VT-25 (10Y) triode with one VT-4 used as the Master Oscillator, another VT-4 as the Power Amplifier and two VT-4 tubes for the P/P Modulator. The VT-25 serves as the speech amplifier in the Voice mode (AM,) as a 1000hz oscillator in the Tone mode (MCW) and as a sidetone oscillator in the CW mode. Power is provided by the aircraft battery/charger system (+24 to +28vdc) and by a high voltage (+1000vdc) dynamotor (PE-73.) The BC-375 provides full break-in keying by allowing the elaborate internal antenna relay to control the receiver antenna and the receiver standby circuit. Additionally, external inputs via the PL-64 cable allow remote microphone and key operation along with remote power control. The aircraft was usually set-up to allow the pilot to also access the transmitter/receiver for various reasons. 

BC-375-E Transmitter with the BC-306-A LF Antenna Tuner

SCR-287 display at WHRM in 2009

After WWII, the BC-375 was available surplus, initially for about $125 (in the crate price,) but soon prices plummeted down to as little as $15. The various TUs were also available at "give-away" prices. This lead to many hams buying  the BC-375 for an economical way to get "on the air." Unfortunately, most hams tried to run the transmitter at full power on automotive batteries or tried to rebuild the transmitter into something that it was NEVER intended to be - a ham transmitter. The end result was a bad reputation that the BC-375 was unstable, sounded awful, created horrible TVI and was only useful as a resource for parts to build other ham projects. Though TVI was a major issue in the fifties, today's HD digital, uW signals routed though strong TV or satellite dish systems are not affected by the BC-375 operation. The transmitter can produce excellent "military" audio if it is carefully operated after a thorough checkout that includes a dynamic adjustment of the neutralization, using a good quality carbon microphone and, probably the most important,...not running the PE-73 dynamotor on batteries but running it from a high current, +28.5vdc power supply, e.g., the PP-1104.

The SCR-287 comprised a complete liaison radio station installed onboard various bombers and transports during WWII. The transmitter used was the BC-375. The other components shown are the BC-348-Q receiver which does run on its original dynamotor from the battery supply, the Lionel J-47 telegraph key and the Shure Bros. T-17 carbon microphone. The speaker is an LS-3, although these were never used in the SCR-287 or onboard the aircraft. Four BC-375 Tuning Units are mounted in their CS-48 containers on the wall. The olive-drab console is not a WWII vintage item - it's homebrew. I designed and built the desk console as an easy way to display the BC-375/BC-348 and to have all interconnections neat and to have the equipment easily accessible. The console wasn't equipped with casters in Virginia City but I did install them with the move to Dayton. The panel to the left of the BC-348 has all of the remote connections for receiver audio output, receiver stand-by, xmtr CW sidetone select, xmtr microphone input and xmtr key input. The console features a fold-down desk, a sound-proof (almost) compartment for the PE-73 dynamotor and a bottom shelf for the four storage batteries - the four 12v batteries in series-parallel = 24vdc which I don't use as a power source anymore. Also, the PE-73 was moved to the bottom shelf to make the console less "top heavy" after the batteries were eliminated. The photo shows the station as it was set-up in the Western Historic Radio Museum in Virginia City, Nevada around 2009. The station is now set up in Dayton, Nevada where it runs on a PP-1104 high current +28vdc 50+amp power source. BTW, the +24vdc worth the batteries never worked correctly and were the major source of problems when operating the BC-375. The PP-1104 solves almost all of the operational issues with the transmitter - that and dynamic neutralization will cure 95% of the BC-375 operational problems.


I've operated this BC-375 on 630M CW using the TU-26 LF tuning unit. The antenna was a 200 ft end-fed wire. CW note was very good and didn't "bloop" using PP-1104 for +28.5vdc 50A. The TU-26 has the "matching serial number" to the transmitter (it's the only original TU that I have for the BC-375.)


For the ultimate information source on how to successfully operate the BC-375 "on the air" without enduring endless criticism of your signal's lack of audio quality, go to our web-article "Successfully Operating the BC-375 on the Ham Bands Today" - Use Home/Index below for navigation.

The End of Operational BC-375 Transmitters?

Shown to the right is the interior of the BC-375 showing the four VT-4/211E tubes along with the 10Y/VT-25 tube to the far left. The demand by audiophiles (actually by "vacuum tube investors") for the 211E has had a catastrophic effect on many restorations or rebuild attempts of the BC-375 transmitters today. It's not at all uncommon for a single 211E in good usable condition to sell for $250 (that's Fair Radio's 2021 price) and, if the tube is NOS in the original box, it would sell for double that. Audio tubes always sell for more in a set, so a good condition "quad" of 211E (necessary for a "tubeless" BC-375) could easily sell for $1200 to $2000. Luckily, these insane prices only are found on eBay and only when dealing with tube investors from Asia (or sellers that ONLY want to sell to tube dealer/investors in Asia.) Between military radio enthusiasts a more common price is about $125 a piece for a good usable 211E but that's still $500 for a quad if you have a "tubeless" BC-375. The 10Y is still not expensive unless you want the WE VT-25 version.

There is a fairly common VT-4 substitute tube, the 805. However, although the 805 is an identical tube to the 211E, the external structure is different in that the 805 employs a plate cap where the 211E uses a base pin connection. These's ample room for plate leads without drilling holes so the incorporation of 805s into a BC-375 can be accomplished fairly easily. The only problem is that the tube dealers have now discovered the 805 also and the price of that tube has started to climb. 805 tubes are still cheaper than the 211E,...but for how long?

The high prices of power triodes like the 211E or the 805 have certainly halted or, at least, slowed down most BC-375 rebuilds. Today, the "standard" is to find most BC-375 that are for sale don't have their tubes. Unfortunately, the high cost to "retube" a BC-375 has relegated "tubeless" transmitters to a "parts set" status. And, if a complete "tubed" BC-375 is for sale, the "tube investor" price will certainly be factored in.


Setchell Carlson, Inc.  -  Model 524 Beacon Receiver

U.S. Army Signal Corps Designation:  BC-1206-CM

The Model 524 Beacon Receiver is a small size, light weight aircraft receiver that covers 195kc up to 420kc. The circuit is a five tube superheterodyne utilizing loctal type tubes. The receiver is entirely powered by the +28vdc aircraft battery-charger buss. No higher voltages are required to operate the 524. The "PHONES" output is 300 ohms Z although internally the output Z can be switched to 4000 ohms Z, if desired. The IF is 135kc. The tubes used are RF Amp 14H7, Mixer 14J7, IF Amp 14H7, Det-1st AF 14R7 and AF Output 28D7.

The 524 had a rather interesting use during WWII. These small receivers were installed into the instrument panels of airplanes that were going to be flown to specific destinations by WASPs (Women's Airforce Service Pilots.) This would generally be smaller fighter types of aircraft but did include larger aircraft as well. Some aircraft manufacturers had access to adjacent runways or the airplanes could be ferried from the manufacturer to either an airport or an export facility. Since the destination was known, the 524 provided a way to navigate to a specific airport via the Airway beacon "beam." At that time, Airway beacon transmitters also provided weather reports and other information necessary for piloting aircraft. Once the airplane was delivered, the 524 was usually removed from the instrument panel and returned to the aircraft factory where it was eventually recycled into another aircraft destined for delivery.

The 524 is very sensitive with a specification of 3uv for 10mw output. The receiver shown in the photo to the left does function quite well and receives many NDBs and other signals in the 195kc to 420kc range. However, the lack of a BFO does limit the reception to only fairly strong NDB signals. It is very small weighing only about 4 lbs and measuring 4" x 4" x 6.625". The four holes in the front panel surrounding the dial plate are tapped and are provided to allow mounting the 524 into a standard instrument panel opening (3.125".) 


the Hallicrafters, Inc. -  R-45/ARR-7

The R-45 was an airborne search and surveillance MF and HF (.55 to 43mc) receiver that was primarily used for visual analysis of enemy radar and other types of signals. The Panadaptor and Video Outputs were designed to feed into specific airborne versions of typical panoramic adapters and oscilloscopes. The oscilloscopes normally utilized external oscillators to create lissajous patterns for audio analysis of incoming signals (Video output is from the 6V6 audio stage of the receiver.) The panoramic adapters monitored the output of the Mixer stage of the receiver and provided a visual representation of the spectrum surrounding the receiver's IF passband. This allowed the operator to "see" signals that were outside the receiver's passband and couldn't be heard - but they could be seen on the panadapter, allowing the operator to tune to the signal for investigation. The motor drive tuning could be set to scan just small segments of a particular band. The motor tuning automatically reverses at each end stop so when "programmed" the receiver would automatically keep scanning the selected frequency segment until the motor was turned off. The R-45's circuit has some vague similarities to the SX-28A, although considerably "stripped down" to the essentials and lightened for aircraft use. 12 tubes are used (not including the rectifier that is located in the PP-32 power pack.) Some of the similarities to the SX-28A are the use of the same Micro-set coils in the front end, six selectivity steps with three utilizing the crystal filter. The differences from the SX-28A are a Noise Limiter which is just a clipper circuit, use of a VR tube, no bandspread and the "militarily basic" audio output system which is just a capacitive coupling from the 6V6 plate to drive the headphones. An audio output for 'phones was only necessary to aid in finding signals for visual analysis and, even then, the panadapter was probably more a more informative device for finding signals. 

Receivers that weren't well-shielded could allow Local Oscillator leakage into the antenna which then could interfere with other surveillance equipment. Rather than add sufficient shielding that would add weight, Hallicrafters added a "re-radiation tube" into the antenna input to block the LO radiation from leaking back into the antenna. Unfortunately, the re-radiation tube also does a pretty good job of blocking signals from getting into the receiver's front end. It's normal for the R-45's S-meter to never exceed S-9, even with a RF signal generator connected directly to the antenna input "pumping in" a 1.0 vrms sine wave. Removal of the re-radiation tube and a simple capacitive coupling between the grid and plate tube socket pins (making a plug-in adapter, of course) can perk-up most R-45 reception problems. Up to three combinations of the R-44 or the R-45 receivers could be powered by the PP-32 power pack that provided the heater voltage (6.3vac) and the B+ (+275vdc) but the scanning motor drive was powered by the aircraft battery system (+24vdc.) Additionally, the PP-32 operated off of 115vac 400 cycle provided by the aircraft's ac system.

the Hallicrafters, Inc. - R-44/ARR-5

This VHF receiver was generally paired with the R-45 HF receiver and provided similar airborne search capabilities. The design intent was to allow airborne search and analysis of enemy radar or other signals. The R-44 is a 14 tube superhet tuning from 27.8 to 143 Mc in three bands and receiving AM, CW or FM signals. Motor-drive tuning provided a "search" scanning mode. Like the R-45, outputs for visual indicators were provided. A special "stub" antenna was used (AT-38) and a separate power pack (PP-32) provided the power for the receiver (and added three more tubes, although these were to provide B+ for three individual receivers.) Sometimes this receiver is called the "Airborne S-36" based on its vague similarity to Hallicrafters' VHF receiver, the S-36.

r-44.jpg (22294 bytes)

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