Radio Boulevard
Western Historic Radio Museum


Pre-WWII and Post-WWII
(Airport, Shipboard, General Purpose & Military Gear)

1932 - 1942  &  1946 - 1960s

in Two Parts

< For WWII gear go to "WWII Communications Equipment - Parts 1, 2 or 3">
Use Home Index at the bottom of this page to Navigate



photo: Radio Room of the USCG Cutter Taney ca: 1940

Pre-WWII and Post-WWII
Commercial & Military Communications Gear - 1932-1942 & 1946-1960s
Airport, Shipboard, General Purpose and Military Gear




Pre-WWII Gear - 1932 - 1942


Airport and Airways Communication Receivers

National Company, Inc. - 1932 to 1934 Airport Receivers 

By the early thirties, National had grown from a company that produced radio parts and regenerative TRF receivers into one of the top shortwave receiver producers in the country. National's chief engineer and general manager, James Millen, had guided the company from its early radio designs (that usually had National as a parts supplier) into the new shortwave receiver market that was becoming popular by 1930.

In late-1931, National Company was selected by the Department of Commerce (who was in charge of airports and airways through the Aeronautical Branch) to build new superheterodyne receivers to replace the old regenerative receivers then being used at airports around the country. The entire system upgrade of airport and airway communication equipment included General Electric, who got the contract for the new transmitters and Aircraft Radio Corporation, who got the contract for the  new airborne gear. National got the contract for the ground-based airport receivers. It seems likely that Herbert Hoover Jr. and his West Coast design team were involved in some of the electronic engineering work of the new receiver that was designated RHM. The contact was identified as 32-15305 and was dated May 12, 1932. The RHM was National's first superhet and it had some of the features that were to become National's trade-mark. Plug-in coils to select the tuning ranges, a separate power supply and a micrometer-type tuning dial are foremost in the design and were to become standard features for National receivers over the next several years.

Since the RHM was a commercial airport receiver it had to be built with the best material and best parts available to assure top reliability and performance. Each receiver was hand tested and aligned by engineers at National resulting in a very modern receiver that provided excellent sensitivity and selectivity (along with top-notch image rejection due to its TRF amplifier stage. Frequency coverage of the RHM was 2.3mc up to 15.0mc using a set of 15 coils. Each band required three coils, RF Amp, Mixer and Local Oscillator which gave the user five tuning ranges. The IF was shown as 600kc in the operating instructions but it seems likely the 500kc was the actual IF.*. It's estimated that only around 100 RHM receivers were built and just a few survive today since most of the airport equipment was scrapped when it became obsolete.

photo left: 1932 RHM Receiver

The AGS-X "Single Signal" Receiver - To take advantage of the prestige the Department of Commerce contract had given them (and to profit through additional sales to the general public,) National adapted the RHM for commercial and ham use dubbing it the AGS. National's advertising implies that AGS receivers continued to be provided for airport use after the initial RHM contract. These receivers had many upgrades from the earlier RHM with more frequency coverage with better calibration procedures and some later-style tubes.

The major AGS-upgrade was with the introduction of the "Single Signal" AGS-X in March, 1933. The AGS-X was tailored for ham needs in that a front panel BFO control and a James Lamb crystal filter were added to the receiver. In late-1934, optional 10 meter coils were added as the AGS frequency coverage was increased to reflect the needs of a "ham receiver" (although at $265 with all accessories, not many hams could afford it.) The AGS-X shown in the photo to the right is a very late production version with serial number F-151 dating from mid-to-late-1934. This particular receiver was originally purchased with all 27 coils that were available at the time. That would have been the standard general coverage coils for the five tuning ranges from 1.5mc up to 20mc, totaling fifteen coils. Also, the amateur bandspread coils that covered 160M, 80M, 40M and 20M (12 coils.) Each ham band was tuned within 100 divisions of the the Type-N micrometer dial (20 to 120 on the scale that was 0 to 150 divisions in a 270º rotation.) Two coil holder rack panels were necessary to store all of the coils which would be 12 coils in each rack and 3 in the receiver. These rack mount coil holders differ from the earlier RHM coil holder that was a combination of a wooden holder with a metal front panel in that these later coil holders are a metal panel with four flat spring retainers per coil. The power supply is the GRSPU for powering a single AGS receiver. I built the table rack that this AGS-X is mounted in based on a ham station photograph that appeared in a 1934 QST magazine. The rack is built from 3/4" square steel tubing with welded joint seams.

Although the initial high selling price may have limited the sales of the AGS and AGS-X receivers to the ham market, as the receiver design began to show its age the prices dropped significantly. By 1935, Leeds was selling the AGS-X for $123. Since National had introduced the HRO by that time, the AGS was considered obsolete technology and priced accordingly.

Other RHM/AGS Family Receivers - In addition to the RHM and AGS receivers, National also produced the RHP and RHQ receivers that were very similar to the RHM within their design circuitry but ganged the three coils together behind a small panel that created a plug-in "coil set" for each tuning range covered. Additionally, the National Type-N micrometer dial was replaced with a National Type-BX illuminated "Velvet Vernier" dial similar to those used on the SW-3 receiver. The RHQ receivers were specifically designed for airport communication and therefore were only supplied with two coil sets that allowed tuning from 2500kc up to 6500kc. The RHQ was also identified as the AGU receiver in some uses (photo below.) Also, National produced a long wave receiver built along the same lines as these early airport receivers, the RIO (see below.) The RIO was also produced as the RIP and was also identified as the AGL. There was a slight difference in the RIP/AGL frequency coverage in that the RIO tunes 175kc to 650kc while the RIP/AGL tunes 175kc to 750kc.

photo above: AGS-X SN: F-151

photo above: 1933 RIO Medium Wave Receiver

The RIO Receiver - Many of the navigational and communications requirements for some airports and various airways stations were not on HF but were on lower frequencies. In 1933, National supplied the RIO receiver for tuning those lower frequencies, 650kc down to 175kc. The circuit is not a superhet. The RIO uses three TRF amplifiers followed by a detector and audio output stage. Additionally, an AVC circuit and a "tracking" BFO circuit are provided. Use of a TRF circuit was to allow complete tuning within the 400kc region of the spectrum. Any of the common IFs used at the time would have interrupted the tuning coverage somewhere within the tuning ranges provided. Also, at the lower frequencies, noise was always a problem and the TRF circuitry was considered to be one of the "quietest" tuners. Seven tubes are used. The RIO is powered by the same type of power supplies as the AGS used. The dial should be a National BX although the dial on the the receiver above appears to be a replacement from an SW-3 receiver. This particular RIO shown above is serial number 3. It's functional and is a very sensitive low frequency receiver.

photo above left: The AGS Airport or Airway station setup (bottom to top) dual power supply GRDPU-26, 58C monitor receiver, AGS receiver, spare coil holder and loud speaker. Note that the AGS receiver has the short data plate indicating it's an AGS (the RHM used a long data plate.)
Photo from:  Radio News - January 1933

photo above right: Airways Radio Range Station showing the RHM receiver and below it, the RIO receiver. This photo is from Aeronautic Radio Bulletin No. 27 published in July, 1937 by the Dept. of Commerce-Bureau of Air Commerce.

The RHQ Receiver - Shown to the right is a B&W of the RHQ receiver I owned back in 1990. I had both coil sets plus one spare. The receiver did function on all original parts. This photo is from the article I wrote for Electric Radio (issue #27) on the AGS Receiver in 1991. Unfortunately, I sold the RHQ around 2005. It's possible that since this receiver was missing its ID plate that it could have been either the AGU version or the RHP. All three versions are very similar in external appearances.

RHM in Top Photo - I've owned this particular RHM receiver since early 1991. It came with all fifteen original coils and the original wood and metal, rack mount spare coil holder. This receiver was missing its original ID plate but the dust cover was marked "RHM #4" in black grease pencil. The ID plate shown in the photo is a reproduction (not a very good one - made at a "trophy shop.") This RHM is functional on "all original parts" but this only means that the receiver barely functions. It does receive quite a few stations and it seems to work okay but a comparison to RHM SN:8 revealed the true RHM performance potential (explained in the next paragraph.)

I've recently obtained RHM SN: 8. This receiver was rebuilt (by me) in the late-1990s for NU6AM from whom I purchased it recently (May 2018.) Back in the late-1990s I had to build replica capacitors and replica resistors to not only have the circuitry function correctly but to also look authentic. RHM SN:8 is a phenomenal performer with excellent sensitivity and stability. This shows that, when new, the RHM receivers could certainly have met the demands of airport and airway communications service.   >>>

photo above: 1933 RHQ Airport Receiver

>>>  Today however, the performance of any of the RHM/AGS family of receivers, even rebuilt ones, will seem somewhat antiquated and crude. In 1932 they were "state-of -the-art" but, within just a few years, National had completely surpassed these receivers in design and performance with their HRO receiver. Although the RHM/AGS performance might be considered "dated," the fact that many of these receivers are still operational and are still fairly accurate in their dial readout is testament to National's build quality and Herbert Hoover Jr. and James Millen's design capabilities. This same design team again worked together in 1934, producing the famous HRO receiver.

* 600kc IF for the RHM? - I have two manuals for the RHM. The first one had 500kc IF penciled in. The second manual didn't have the correction and shows that the RHM manual has a significant error in that it has the IF listed as 600kc. I tried aligning the RHM to 600kc and the results made it obvious that 500kc is the correct IF. The first manual was "hand corrected" but the second manual never was. Interesting.


RCA Manufacturing Company, Inc.  -  AVR-11A Airport Receiver

RCA supplied some airports with this 16 tube superheterodyne receiver beginning around 1937. The receiver used many of RCA's developments, including the "Magic Brain" which was a modular receiver "front end" and the innovative "band-in-use" dial mask. Many other components are recognizable as exclusive-RCA parts and included the "Magic Eye" cathode ray tuning indicator. RCA included a BFO, a sensitivity control, a noise suppressor circuit and a headset output jack - all necessities for a communication receiver. Additionally, an optional Crystal Filter assembly was available on special order. There were three models available, the AVR-11 that was installed in a metal cabinet with matching speaker. The AVR-11A was a rack mount receiver with gray painted panel but without a chassis dust cover and with matching rack mount speaker. The AVR-11B was a rack mount with dust cover with black painted panel and rack mount speaker. 

The AVR-11 receivers provided frequency coverage from 140kc up to 23mc which was more or less standard for the largest "All-wave" receivers. The airports favored the longer wavelengths used in air navigation, weather reports and for some airport communications in the 1930s. Higher frequencies were also starting to be used at the time, so the coverage up to 23mc was also an advantage. The dial also provided a logging scale for an accurate frequency reset function. The "RCA Magic Eye" (RCA's 1936 selling moniker for their cathode ray tuning indicator) was installed and is on the left side of the panel. The "eye" on the right side of the panel is a "Stand-By" indicator that glows green (looking similar to the actual "eye tube") when the receiver is in stand-by. The speaker system is fairly elaborate with connections for the speaker field coil to double as a power supply filter choke, connections for a "hum bucking coil"  besides the regular push-pull audio output transformer. RCA also offered a 15 tube ham version of this receiver in the ACR-111. RCA Manufacturing Company, Inc. was a division of RCA that built communication and broadcast equipment.

The AVR-11A shown in the photo above is awaiting restoration. I've had it for several years but have yet to get started on the work. The plastic dial cover is missing from the bezel. Much of the front end wiring used rubber insulated wire which has hardened over the years and is now falling off. Very few AVR-11 receivers were produced and they are quite rare in any condition.

To show what the AVR-11 should look like (although the side panels that cover the rack screws are missing,) see the photograph to the left which is the complete AVR-11 owned by Joe Connor, who has supplied this photo. Note that the speaker panel includes a fabulous deco "winged" RCA emblem.


National RCE, ca: 1937

National Co., Inc. - Airport and Airway Communication Receivers

National Co., Inc. had been supplying receivers for airport and airway communications since the RHM receiver in 1932. The mid-thirties HRO also was used in some airports. The most popular airport receiver by far was National's Airport and Airway Communication Receivers that were based on their NC-100 receiver. The continuing upgrading of communication and navigation equipment was initially the responsibility of the Department of Commerce and the Bureau of Air Commerce with its various branches in charge of airports and navigation. The new and developing radio navigation systems provided a pilot the means to fly an airplane to an airport using a radio range beacon called a "beam." The navigational radio beam allowed that pilot to follow a predetermined altitude and route called an "Airway." The navigational beam and various beacon stations also provided two-way communications with pilots to report weather conditions and other information that might be needed by pilots. National's history of providing top quality receivers for airport use (RHM, the fore-runner to the AGS, and the HRO) practically assured them of continuing contracts for airport-specific receivers. The first "moving coil" receivers supplied to airports were standard NC-100X receivers. Some of these early receiver used the "art deco" overlay panel while others used a black wrinkle finish rack mount panel. In 1937, National began supplying the Department of Commerce, Bureau of Air Commerce, Dept. of Navigation (still in charge of airport communications at that time) with the RCD receiver, a slightly modified NC-100X receiver that had the AM BC coils replaced with 200kc to 400kc coils. At the end of 1937, the RCE was introduced and it became the "standard" Airport-Airway Communication receiver from National. When the U.S. Civil Aeronautics Authority (CAA) was created in 1938, National then supplied the CAA with airport receivers. These new CAA receivers were continually being upgraded as new contracts were issued. Some receivers were even produced during WWII for use at both military and civilian airports. The RCK tuned 200kc to 800kc in the two lowest frequency bands and 2.5mc to 23.5mc in the three highest frequency bands. The RCK was supplied to the U.S. Navy during WWII. Also during WWII, the RCL was introduced and it featured a two position bandwidth switch. After WWII, early versions of these receivers (RCK and RCL receivers) were upgraded into the last versions, the RCP and the RCQ. Most of the upgrades were professionally installed by well-known companies such as Schutigg & Company or National Electrical Machine Shops (NEMS.) Even National managed to upgrade a few of their earlier models. These later versions were used up into the early fifties when more modern equipment was becoming a necessity.

Although the Airport-Airway Communication Receivers are based on the NC-100 chassis, there are some significant additions to the circuit. The most obvious is that the receiver is rack mounted and has a very different front panel when compared to the striking "art deco" panel of the standard NC-100 receiver. These receivers use a 3/16" thick aluminum front panel that is black wrinkle finished. All early Airport-Airway receivers were equipped with an I.N.S. control. I.N.S stood for "Interchannel Noise Suppressor," which was actually a "squelch" control. 

Later versions have a C.O.N.S. (Carrier Operated Noise Suppression) control that uses a relay to mute the receiver if no carrier is being received. Additionally, the push-pull audio was changed to single-ended and the output transformer was internal to the receiver. The audio output impedance was 600 Z ohms and 20K Z ohms. Another addition was a relay that could remotely quite the speaker without affecting headset reception. Most Airport-Airway Communication Receivers came with two speakers, a single table top speaker box and a rack mount dual speaker. No carrier level device (meter or eye-tube) was used on the majority of the receivers. The chassis is covered top and bottom with a "slide on" dust cover although early versions have separate top and bottom covers. 12 tubes are usually used in the circuit which utilizes a 457kc IF. Post-WWII receivers RCP and RCQ will have a selectable crystal-controlled fixed-frequency function installed operated by a front panel toggle switch. Shown in the photo left is a 1940 Airway Communication Receiver Type RCF-2 sn:13. The RCF-2 is the only version that is specifically identified as an "Airway Communications Receiver." All others are "Communication Receivers." The Airport Communication Receiver shown in the top photo is a 1937 RCE sn:302 which happens to still have its original dust cover.

Shipboard or Coastal Station Radio Equipment (Pre-WWII)

U.S. Navy  -  RAD-2 (CNA-46013)
Contractor: National Company, Inc.

In 1932, the U.S. Navy contracted with National Co. to provide the RAD receiver. The RAD was based on National Company's SW-5 "Thrill Box" but was provided to the Navy with a "complete" set of coils to cover 250kc up to 33mc for the frequency coverage. The coil sets were contained in a box and were assigned a USN model number (CNA-47040) The separate power supply was also part of the RAD equipment supplied (CNA-20002.) The RAD receiver appears to be a stock SW-5 with the exception that it was painted black wrinkle finish and had a large data plate installed on the top lid. Also, each control had an index plate mounted behind the knob.

Shown in the photos is a RAD-2 receiver. The USN model number is CNA 46013. The "project date" is 1932 and the contract is also dated 1932. The photos show that the RAD-2 is almost identical to the standard National SW-5 with the exception of the black wrinkle paint and the index plates behind the two control knobs. Internally, the RAD-2 is identical to the SW-5 except for the black paint. The RAD-2 shown is serial number 13. The screen grid tubes are 24-As and the audio tubes are 27s. Knobs are probably not original.

photos from: WA6OPE


Type 105-A - Sepia Photo from Original Brochure

Mackay Radio Type 105-A

Mackay Radio & Telegraph Co.  -  Type 105-A
Contractor: Federal Telegraph Company

Mackay Radio & Telegraph Company was founded by Clarence Mackay, son of John W. Mackay, one of the "Big Four of the Comstock" fame in Virginia City, Nevada. John Mackay initially made his fortune in Comstock silver but he later (1883) moved into telegraphic communications. Mackay, along with newspaper publisher James Gordon Bennett Jr., formed several telegraph communications companies to compete with Jay Gould's Western Union. Postal Telegraph Company (1886) was the best known, along with Commercial Cable Company (1884). Eventually, these companies, along with other Mackay-Bennett telegraph companies, had transoceanic cables across both major oceans. When John Mackay died in 1902, Clarence inherited the businesses. Clarence Mackay saw to the completion of the transpacific cable in 1904. Radio was added to the business end of things in 1925 to provide "radiogram" service to every area of the world. Mackay Radio was mainly interested in maritime communications which went along with the maritime radio-telegraph business. By 1928, ITT had merged with most of Mackay's business interests but the Mackay name continued on for several decades. Today, Mackay Communications is still doing business, located in North Carolina.

Federal Telegraph Company started out in Palo Alto, California mainly dealing in arc transmitters. At one time, Lee DeForest worked for the company but Frederick Kolster was the head engineer for most of FTC's history. FTC bought Brandes and created a division called Kolster Radio Company for selling consumer radios in the mid-twenties. FTC became involved with Mackay Radio in 1926 when Mackay bought a radio station that had belonged to FTC. When Mackay sold his interests to ITT, then Federal Telegraph was contracted to do most of the Mackay Radio work. Federal Telegraph moved to New Jersey in 1931 when it was purchased by ITT (International Telephone and Telegraph Corp.) For awhile ITT tried the consumer radio market with Kolster International but it was a short-lived venture. The name of Federal Telegraph Co. was changed to Federal Telephone and Radio Company around 1940.

The Type 105-A is actually a pre-WWII commercial shipboard receiver that dates from 1932 (SN: 32081 - the first two digits of early Mackay equipment serial numbers indicate the year built) which is sometime after the Federal Telegraph move to New Jersey since the ID tag lists Newark, N.J. as FTC's location. It is a four tube receiver that originally used either 201-A tubes or type-30 tubes but now uses five-pin cathode-type tubes. The level of workmanship involved with the upgrade suggests that the work was done either at the factory (unlikely) or by a professional shipboard radio work shop. Several companies did exist that supplied marine radio gear and also repaired and upgraded older marine gear. This is the most probable source of the modification upgrades. The patina of the solder joints leads one to believe the rework was done just before or during WWII. It is possible to use type 27 or type 56 tubes and with an increase in the filament voltage, type 76 tubes could also be used. The frequency coverage is 1500kc down to 15kc in seven tuning ranges. Power is supplied by batteries. Like earlier designs for shipboard receivers, e.g. the IP-501-A, the Mackay 105-A utilizes an LC Antenna tuner ahead of the regenerative detector to increase gain and selectivity. An Antenna Series Condenser switch selects various value capacitors to match the ship antenna to the receiver input and a stepped Tone control provides some relief from static. The panel meter is a dual meter that normally reads filament voltage but B+ voltage can also be monitored by activating a panel switch. The left large tuning knob tunes the Antenna Condenser, the middle large knob controls the Regeneration Condenser and the right large knob tunes the Detector Condenser. The Mackay 105-A is built for shipboard use being physically stout and very heavy. The chassis, the cabinet (if I had one,) the panel and most of the shielding is cadmium-plated brass with the cabinet and panel painted Mackay Dark Gray. Originally the receiver was probably mounted in a cabinet however, according to the original Mackay Type 105-A brochure it was possible to order the receiver without the cabinet for installation in a shipboard console. It's also possible that sometime later the receiver's use it was removed from the cabinet and mounted in one of the Mackay Marine Radio Units that housed the majority of the radio gear for the ship. This may have coincided with the tube upgrade modifications to the receiver. (See our "Vintage Longwave Receivers" webpage for an in depth article about this receiver.)

Mackay Radio & Telegraph Co. - Type 101-A Monitor Receiver
Contractor: Federal Telegraph Company

Mackay Radio Coastal Station WSE - Amagansett, Montauk Point, L.I., NY


The Type 101-A Monitor Receiver was used to monitor near-field radio transmitter signals. The receiver uses three type-30 triode tubes that perform as the Regenerator, the Detector and the Audio Output. A set of six plug-in coils provide a frequency coverage of 350kc to 550kc and 3.0mc to 25mc in six tuning ranges. The manual contains a graph showing the dial readout to frequency correlation for each coil. The Monitor Receiver was designed to run on batteries with storage for dry cells provided within the cabinet. Voltages required are +3.0vdc for A voltage and +45vdc for B voltage. An external +6vdc storage battery could be used if desired and its connection is via the phone plug marked "EX. BATT" (phone tip +) The front panel switch marked IN-OFF-EX selects the filament voltage source and operation of the receiver. The panel meter monitors filament voltage (2.0vdc for Type-30 tubes.) Unused coil storage is provided by way of five "dummy" sockets on the receiver chassis (with the sixth coil installed in the active coil socket located under the coil shield.) An antenna is normally not required but it does depend on the transmitter-antenna location in relation to the monitor receiver location. If the signal is too weak an additional short wire can be added to the antenna input terminal connection. If the signal is too strong, the antenna input has an adjustable rod that electrostatically couples the antenna input level to the receiver input coil by the length of the rod within the receiver cabinet. Normally, the cabinet was kept closed to provide shielding for strong signals but if a weak signal was difficult to find, then the cabinet lid could be opened (though this was considered a temporary measure - as soon as the signal was located the lid should be closed.) The audio output was to Hi-Z 'phones. Normal operations had the Type 101-A monitoring a CW transmitter so the Regenerator control was advanced until the receiver was oscillating to provide an autodyne signal. Noting the frequency coverage, the AM-BC band was not tuned, therefore the intended use of the Type 101-A was for ship-to-shore stations or other types of uses where  the station's CW transmitter signal needed to be monitored. Stout construction with cabinet, panel and chassis entirely made out of cadmium-plated brass sheet metal with the cabinet and panel painted the standard "Mackay Dark Gray."

The photo upper left shows the Type 101-A Monitor Receiver. The lower photo shows the chassis and compartments inside the cabinet. The three tubes are type-30 tubes. The five HF coils (3.0mc up to 25.0mc) are stowed in their spare coil holder sockets. The medium wave coil (350kc up to 550kc) is installed in the active coil socket located under the removable can-type shield. The compartment to the left side is where the two 1.5vdc dry cells were mounted with the aid of the special clamp. The compartment to the right side was for the 45vdc B battery which actually was two 22.5vdc B batteries connected in series. The two B batteries were held in place with the bakelite strip that also had connection terminals for the batteries. Note the rod protruding into the cabinet from the Antenna Coupling mount on the front panel. The length of this rod inside the cabinet determines the level of RF energy electrostatically coupled into the Regenerator and Detector circuits.

This Type 101-A SN: 32191 ("32" is the year built, SN is 191) has written under the lid of the receiver and on the front cover of the original manual "W.S.E. Montauk Station." WSE was located at Amagansett, Montauk Point, Long Island, New York and was operated by Mackay Radio and Telegraph Company. The powerful CW transmitter was operated by remote control from a short distance away in Southamton, LI, NY and was also connected by wire to the Mackay offices at the ITT building in New York City.

Mackay Radio & Telegraph Co. - Type 102-B Frequency Monitor
Contractor: Federal Telegraph Company

Shown in the photo to the right is an accessory piece of test-monitoring equipment for shipboard operation, the Type 102-B Frequency Monitor. This device works something like an "uncalibrated" heterodyne frequency meter. A small antenna would be connected to the binding post terminal on the front panel (top-center.) This antenna can act as a pickup or a radiator depending on the operator's intentions. Internally, the 102-B has a type 76 oscillator feeding a type 6C6 buffer. These stages then capacitively feed the tank circuit of a type 76 broadly tuned RF amplifier whose input grid is connected to the antenna post and whose plate output is capacitively coupled to the telephone jack on the front panel. When it is desired to monitor or test a transmitted signal, the transmitter's signal could be received as a heterodyne beating with the internal oscillator heard on 'phones plugged into the phone jack. Since the dial is uncalibrated except for a general logging scale, frequency would have been determined using a true frequency meter. Most shipboard transmitters in the thirties operated CW or MCW, so the operator could monitor the transmitted signal for other characteristics or suspected problems (such as drift or other instability,) hence the term "Frequency Monitor." When it was desired to test a receiver, the internal oscillator-buffer of the 102-B emits a small amplitude signal from the small antenna which can be received and beat with the receiver's tuned frequency to determine if the receiver was operational. Actual receiver frequency would have to be determined with a frequency meter. The Type 102-B tunes from 5.5MC up to 16.5MC. Built by Federal Telegraph Company, the Type102-B is stoutly built into a steel cabinet. The airplane-type dial is not illuminated since the entire device runs on batteries. There is a power cable access hole on the right side of the cabinet. Dates from 1938.


Navy Department - Hygrade Sylvania Corp.
Model RAG-1  CHS-46042
Long Wave Receiver - TRF with Tracking BFO - 1933

The RAG-1 Type CHS-46042 was a USN shipboard longwave receiver that was usually paired with its high frequency matching receiver, the RAH-1. Contact was July 13, 1933 and Hygrade Sylvania Corp. was the contractor. The RAG-1 tunes from 600kc down to 15kc and uses an eight tube, TRF circuit that also employs a tracking BFO that is adjusted to always be 1kc higher than the receiver's tuned frequency. Additionally, the RAG-1 uses three grid bias-controlled RF amplifiers (SENSITIVITY control.) The triode detector is interstage transformer coupled to a tuned audio frequency bandpass filter is selectable for wide-band (OFF position) or 450hz to 750hz or from 750hz to 1300hz. The audio signal is then routed through a passive Audio Bandpass Filter that limits the frequency response to 450hz to 1300hz with the peak response being 800hz (ideal for CW.) The output of the BP Filter is interstage transformer coupled to the first audio amplifier and that output is RC coupled to the Type-41 audio output tube. An audio AVC that acts like a limiter to keep the receiver output from over-driving the operator's ear with unexpected strong signals or static bursts can be switch in during noisy conditions. The tuning ranges are 15kc to 38kc for Range 1, 38kc to 95kc for Range 2, 95kc to 240kc for Range 3 and 240kc to 600kc for Range 4. The tuning dials are a 0-100 lower dial and a 0-10 upper dial with ten revolutions of the 0-100 lower dial showing from 0 to 10 on the upper dial. 

Tubes used are (3) 6D6 - RF Amplifers, (1) 76 - Detector, (1) 6D6 - BFO, (1) 76 - 1st Audio Amplifier, (1) 41 - Audio Output Amplifier, (1) 84 - AVC Limiter, (1) 80 - PS Rectifier. Voltage required is 6.3vac for the tube heaters,+180vdc for B+ and -55vdc bias. 5.0 vac was required for the PS rectifier and was supplied by the PS power transformer. The negative bias was obtained by elevating the PS transformer CT above chassis ground using a WW resistor network. The audio output impedance at the TEL jack is 600Z ohms. The audio output power is only 250mW implying that earphones were the intended reproducers to be used. Sensitivity is rated at an impressive 1uv to 4uv. Power Supply was identified as CHS-20032. The power supply could operate both the RAG-1 and its companion MW-SW receiver, the RAH-1. Both receivers were controlled by the CHS-23067 Control Unit that routed power to each receiver and provided switched audio outputs from each receiver. ON-OFF switch on receiver is only for battery operation. The battery power only version of these receivers were the RAG and the RAH, no numeral suffix.

The OUTPUT meter measures the audio output level and has a scaling switch that allows changing the meter full scale or turning the meter off. The FILAMENT meter acts as an ON-OFF indicator and also measures the tube heater voltage. The OSC. TEST disables the BFO. If the OSC. TEST button was pushed and a resulting "click" was heard in the 'phones which confirmed that the RAG-1 was operating.

For restoration details on this rare Navy receiver, go to "RAG-1 LW Receiver" Use Home/Index for navigation.


Navy Department - RCA
RAA-3 Long Wave Superheterodyne - 1935

The initial contract for the mammoth long wave receiver, the USN RAA-1, dates from 1931. At 465 pounds (when fully assembled in cabinets) and with its gargantuan physical size, the RAA was a "cost absolutely no object" receiver that also had a matching high frequency version, the RAB. Both receivers were initially built by RCA-Victor Corp. but by the RAA-3 versions, RCA Manufacturing Co., Inc. was building the receivers. The RAA-3 tunes from 1000kc down to 10kc in five tuning ranges. The receiver has four different frequency, two-stage IF amplifiers with the proper IF selected by the band switching set-up. The four different IFs required four different frequency BFO circuits also selected by the band switching function. A separate power unit supplied all required voltages. The chassis of all three units are made of nickel-plated brass. The cabinets are also made of black wrinkle finished sheet brass. The front panels are black wrinkle painted aluminum.

This RAA-3 has been undergoing restoration since 2017. Unfortunately, the receiver units and the power supply were stored outside for decades (wrapped up in a tarp) in central Oregon. Most of the extensive mechanical and cosmetic restoration has been completed. What remains is repairing the significant electronic damage caused by corrosion and mice infestation. The power supply is 90% finished. The Tuner and the IF/AF unit have only been partially electronically rebuilt. The photo shows how the RAA-3 currently looks with most of the cosmetics finished.

This is a lengthy project that is being fully documented in the "RAA-3 LW Receiver" article. Use Home/Index for navigation.


CGR-32-1 - Coast Guard AR-60 - 1939

RCA Manufacturing Company, Inc. - CGR-32-1 Coast Guard Receiver (AR-60R)

In 1935, RCA offered what must have seemed like the ultimate receiver. So over-built and so expensive that it was obviously not for any Depression-era ham. The AR-60 was priced at an astounding $495 at a time when this amount of cash could easily buy a new car. To those familiar with RCA's equipment built for the U.S. Navy where cost was not even considered as a "limiting factor" the AR-60 was normal "Navy shipboard construction" but the AR-60 wasn't built for the Navy. Though RCA's intended market was the commercial and military users, RCA did advertise the AR-60 in QST one time. RCA obviously didn't expect many sales to hams since the AR-60 was only available through RCA dealers (rather than discount dealers like Leeds and others.) The AR-60 was intended as a robustly-built, extremely reliable, commercial-military receiver that featured performance that was at the limits of the designs of the time. It was a receiver that could endure and survive the rigors of shipboard use and function superbly while doing so. RCA built the AR-60 through their subsidiary, RCA Manufacturing Company, Inc., the division that generally handled all of the commercial manufacturing. RCA also advertised the AR-60 in their Broadcast Equipment catalog. Since the AR-60 was a "limited production" and was a "built-to-order" receiver, the total quantity of AR-60s built from 1935 until 1940 was around 300. Most of these went to the U.S. Coast Guard, one of the major users of the AR-60 (USCG designation CGR-32-1 and CGR-32-2) along with the Signal Corps, where they were used in Triple Diversity receivers. RCA used the AR-60 in some of their Coastal Stations and Pan-American Airlines used the AR-60 in their HF direction finders in the Pacific.

The AR-60 was built on a heavy-duty nickel-plated brass chassis with three nickel-plated brass bottom covers, unheard of, in 1935, for civilian equipment but a construction method RCA (and others) used to reduce corrosion in shipboard equipment. The receiver tuned from 1.5mc up to 25mc in six tuning ranges. The bandspread range gave great vernier effect because its span was limited to an average of about 100kc for the entire bandspread range (although its exact span depends on the tuning range selected and where you are tuned with the main dial in that range.) The AR-60 front-end used double pre-selection or two TRF amplifier stages, although the double pre-selection is only used on the top three frequency ranges (5.6mc to 25mc.) Radio engineers generally believed that double preselection was only for image rejection and not really necessary below around 7 mc where the receiver circuitry was more efficient. The AR-60 receiver featured an elaborate antenna input system with selectable links for doublets or end-fed wire antennas and then variable antenna primary coupling allowed the operator to adjust how much signal level was going to be needed for low-noise reception. All of the RF and IF coils were wound on ceramic forms. When the three bottom covers are installed the chassis is compartmentalized and fully shielded. Nearly all of the tube sockets are Isolantite (ceramic.) Ten tubes (along with a 991 neon bulb voltage regulator) are used in the circuit and a heavy-duty sectional bandswitch was used. The audio output is from a single 41 tube that uses a 600 Z ohm output transformer. The B+ levels are fairly low in the AR-60 so only about 1/2 watt of audio power is available and since the receiver was designed for commercial use, headsets were the intended audio  reproducers. The AR-60 can be operated on batteries although it requires moving some wires on the terminal boards in the power supply section. The AR-60 was available as a black finished table model (suffix T), as a rack mounted unit with full dust cover (suffix R) or in a deluxe two-tone gray table cabinet (suffix S.)

 Perhaps the most famous use of the AR-60 receiver is aboard the USCG Cutter ITASCA in its assistance to Amelia Earhart's ill-fated flight in July 1937. The ITASCA was equipped with two CGR-32-1 receivers. During the late thirties, many USCG Cutters were equipped with CGR-32-1 versions of the AR-60-R that were specifically built for the Coast Guard. In Nov.1939, a contact was issued for approximately 30 CGR-32-1 receivers for U.S. Coast Guard installation on the ten Lake Class Cutters that were being rebuilt and refitted at the time. This was probably the last contract for the CGR-32-1 receivers. Soon after that, the ten Lake Class Cutters were loaned to England as part of Lend-Lease for the duration of WWII. However, according to the USCG website much of the sensitive equipment was removed prior to delivery of the Cutters. It's likely that the CGR-32-1 receivers were used elsewhere during WWII, either in other USCG facilities or other military uses.

photo left: Radio Room on the USCG Cutter TANEY ca: 1938 showing the two CGR-32-1 (or CGR-32-2) receivers. USCGC TANEY was a Treasury Class Cutter with a spacious radio room when compared to the cramped quarters of a Lake Class Cutter's radio room.                            photo from:

The AR-60-R shown in the top photo is the 1939 Coast Guard version, the CGR-32-1 bearing the serial number of 25. This receiver was built on contract Tcg-31919, dated November 16, 1939, which was probably the last contract for CGR-32-1 receivers. This CGR-32-1 is a functional example and its performance is impressive. Very similar to the Hammarlund Super-Pro SP-100 in overall front end noise and sensitivity. Very similar to the 1940 Navy RBB and RBC in audio output capabilities. Coupling and Antenna Trim controls will interact somewhat but it's best to set the Coupling to the minimum amount that gives the desired signal level. Although maximum Coupling will appear to result in stronger signals it will also produce higher noise levels that will interfere with signal copy. The two 0-100 dials necessitate using a frequency meter or signal generator as a calibrated signal source unless you have the frequency chart from the manual. Weighing in at 75 lbs, the AR-60 is a durable, robustly-built, almost indestructible receiver that can still perform in an impressive manner.

The AR-60 was available from RCA Manufacturing Company, Inc. (subsidiary of Radio Corporation of America) up until around 1940. RCA was designing the AR-60's successor, the AR-88, in 1940 and that receiver continued the line of robustly-built, durable, hard-working and reliable receivers. More on the AR-88 below,...

For the ultimate information source on the AR-60 receiver, including history, performance comparisons, restoration information, serial number analysis and more, including the frequency to dial readout chart, go to our web-article "RCA's Legendary AR-60 Receiver." Use the Home/Index for navigation.


General Purpose Communication Receivers

photo above: SP-100-LX sn: 2730 from 1938

Hammarlund Mfg. Co., Inc.

Series 100 Super-Pro  SP-100-LX

Hammarlund's relationship with the U.S. Army Signal Corps dates back to the Army purchases of a few Comet Pro receivers in the early thirties. The first official Super-Pro contract that Hammarlund had with the Signal Corps was from June 1935 for the SPA receiver. The SPA was, without a doubt, the first version of the SP-10 Super-Pro that was produced. The official Super-Pro announcement for the civilian market came nine months later, in March 1936, with a two-page advertisement in that month's QST magazine. The SP-10 was only produced for nine months before it was replaced with the updated Series 100 Super-Pro receiver.

With the Series 100, Hammarlund began to offer a LF/MW coverage options. The standard coverage was .54mc to 20mc but also offered was the SP-100-SX tuning from 1.25mc to 40mc had been offered with the SP-10. The new LF/MW Super-Pro tuned 100kc to 400kc and 2.5mc to 20mc. The "SP-100-LX" was considered the military/commercial version because of its LF and MW coverage. Certainly the 100kc to 400kc range was a tuning range specifically for aircraft use for the Signal Corps. Since the maritime 400kc to 500kc band wasn't included in the tuning range, the early Super-Pro receiver were never used onboard ships by the Navy.

Most SP-100-LX receivers went to the Signal Corps although there certainly were some commercial users also. The receiver's frequency coverage appealed to airport radio operations since LF-MW Navigation or HF Communication could be tuned in. The 6mc part of the spectrum was popular for airport communications. Certainly, Army aviation use must have been popular since most of the LX receivers that turn up today seem to have Signal Corps stamps on them. In addition to airport use, the Super-Pro receivers were used for surveillance by both military and civilian government users, like the FBI. Some commercial broadcasters used the Super-Pro as the receiver-link in a rebroadcasting system. About 1200 SP-100 Series receivers were produced from 1937 to 1939.   


Post-WWII Gear - 1946 - 1960s


Airport Communication Receivers

National Company, Inc. - RCQ Airport Communications Receiver


RCQ receivers were modified from earlier RCL or RCK WWII version receivers in 1948 to add a crystal-controlled fixed-frequency function. Note the additional toggle switch that is identified as TUNE-ABLE and XTAL. The XTAL position selects the fixed-frequency mode. Shown to the left is the RCQ receiver SN: 288. National Electrical Machine Shops, Inc. (NEMS) actually performed the rework and modifications to build the RCQ receivers.

After WWII, fixed-frequency became more and more necessary because it helped to eliminate tuning errors during operation and eliminated the possibility that the receiver might "drift" off-frequency.  Since many of the receivers were operated with a squelch circuit, frequency drift would go unnoticed unless the receiver was in fairly constant use. The crystal-control fixed frequency kept the receiver "on frequency" over long periods of inactivity. Even though the LO is crystal-controlled, the tuning dial must be set to the correct receive frequency so the RF amp and Mixer stages are "in tune." A mechanical dial-lock was added to keep the RF and Mixer correctly tuned for fixed-frequency operation (missing on this example.)

Note the "vented" top cover on the power transformer. This is commonly found on Airway receivers. Also, almost every National Airways or Airport receiver encountered today will be missing its dust cover. This RCQ is even missing the side panels. Airport technicians generally removed the covers thinking it lowered heat buildup and increased reliability.


National Company, Inc.  - RCR Airport Receiver

National had been supplying "Moving Coil" receivers to airports since 1937. All of those receivers were based on the NC-100 receiver with some special modifications for airport communications. After WWII ended, National RCK and RCL receivers were upgraded by NEMS or Schuttig and designating as either RCP or RCQ receivers. In 1947 or '48, National was given a contract for an Airport Receiver that would be based on their then current production commercial communication receiver, the NC-240CS. The receiver was designated RCR and it represents the last of the "Moving Coil" receivers that National produced for airport communication use. The RCR was built for contract number Cca 26391.

The RCR is very much a NC-240CS with only a few changes. The NC-240CS dial scale is the most obvious with its light amber color - quite different from the cream color dial of the standard NC-240D. The other change is the data plate that identifies the receiver. The RCR and the NC-240CS provide a 500Z ohm and an 8Z ohm audio output along with an option to allow a 10K Z ohm input the essentially parallels the audio output transformer in the receiver with another audio output transformer mounted on the speaker. This allowed using the regular National table speaker if desired. Also, one could opt for the rack mount version of the speaker. It was also acceptable to just use the 500Z or 8Z ohm outputs for audio output connections. The audio output uses push-pull 6V6 tubes which differs from previous National Airport Receivers that used single audio output tubes. Since the RCR is a commercial receiver it doesn't have amateur bandspread coils and its six bands covers 200kc to 400kc (Band F) and then 1.0mc to 30.0mc (Bands E to A.) The receiver uses 12 tubes and is single preselection with two IF amplifiers and provides a crystal filter for selectivity control. A Noise Limiter and Tone control are also provided. The RCR was rack mounted and features a slide-on dust cover that entirely encloses the receiver chassis. All cables can exit from the sides of the dust cover with only the mounting screws and the removal handles on the back of the dust cover (although there is a square cut out for the power cord to exit directly to the rear if necessary.)

The RCR shown was installed at a CAA facility located in Honduras, Central America. Its serial number is 17.


Shipboard Communication Receivers

Mackay Radio - Type 3001-A from 1952

Mackay Radio & Telegraph Company  -  Type  3001-A

The Mackay Radio & Telegraph Co. Type 3001-A is a Longwave regenerative receiver covering 15kc to 640kc in four bands and dates from as early as 1948 but with most manufacturing dating much later. The Type 3001-A is based on the earlier version of the receiver, the Type 128-A Series. The Type 3001-A receiver shown was built in 1952. Mackay receiver serial numbers generally incorporate the last two digits of the year of manufacture as the first two digits of the serial number. The 3001-A was mainly for commercial shipboard (non-military) use where it could be set up as the main receiver or as the emergency receiver. The receivers were sometimes installed in the Mackay "Marine Radio Units," like the MRU-19/20, a shipboard radio console which contained two 3001-A receivers along with transmitters and other auxiliary equipment (the MRU receivers were panel mounted.) The 3001-A uses an AC-DC circuit and can operate on 115vac or on the ship battery system. Various filament battery options were available with 6vdc, 12vdc and 24vdc being the most popular. When BATT. is selected, the tube heaters are in parallel requiring 6vdc and a WW dropping resistor was plugged in to drop the voltage as needed to operate the tube heaters with whatever storage battery voltage was used on the ship. B+ was supplied by standard dry cell B batteries when used. When LINE is selected, the receiver connects the tube heaters in series along with a four pin Amperite ballast tube and two lamps behind the dial. These two lamps are only for a series load because the white dial material is opaque. A small built-in speaker provides for radio room monitoring but earphones would normally have been used by the shipboard radio operator. Selectivity is controlled by a combination of the RF Gain setting and the setting of the Regeneration. The seven tube used are RF AMP - 6SK7, REGENERATION TUBE - 6J5, DETECTOR - 6SJ7, FIRST AF AMP - 6SJ7, AF OUTPUT - 6G6G, RECTIFIER - 35Z5 and a Ballast Tube. The 3001-A is very sensitive and capable of receiving any of the NDBs and other LW stations found in the spectrum below 500kc. These type of Mackay receivers were in use for several decades and were commonly found still operating on commercial ships as late the 1990s. These types of Mackay receivers date from the late forties and were manufactured through the fifties. Note that the panel states "manufactured by Mackay Radio & Telegraph Co." rather than "Federal Radio & Telephone" or the earlier "Federal Telegraph Company." Since ITT owned both Federal Radio and Mackay, listing both companies must have seemed superfluous.   (See our "Vintage Longwave Receivers" webpage for an in depth article about this receiver.)


RMCA AR-8506-B from 1953

Radiomarine Corporation of America  -  AR-8506-B  aka: R-203/SR

The origins of Radiomarine Corporation of  America date back to the 1920s,...a time when RCA was controlled by General Electric and Westinghouse. When RCA was created in October, 1919, the US Navy wanted a radio company that could handle operation of the coastal wireless stations, service all the equipment and provide sales of new wireless gear. General Electric created RCA out of some of their own assets but it didn't seem to fulfill the requirements needed for a company as requested by the Navy. The next month, in November 1919, GE purchased American Marconi and added its assets to RCA at which time GE announced the "creation of a new wireless company." GE wanted to keep RCA under its control so the American Marconi manufacturing plant in New Jersey was not included in RCA's assets, therefore RCA had no ability to manufacture radios at that time. As commercial radio developed, by about 1920, GE directed what was to become known as the "Radio Group" - a patent-sharing association of five companies that dominated radio in the early twenties. General Electric, Westinghouse, AT&T, RCA and United Fruit Company/Wireless Specialty Apparatus owned all of the most important radio patents. As radio broadcasting became more and more popular, GE and Westinghouse somewhat controlled that end of business while United Fruit Company's Wireless Specialty Apparatus was more involved with the maritime radio business. Eventually, United Fruit Company sold Wireless Specialty Apparatus to RCA (around 1924) and, in 1927, RCA combined WSA with another acquisition, Independent Wireless Company, to create Radiomarine Corporation of America. The virtual domination of radio by GE, Westinghouse, AT&T and RCA ended in 1930 with a settlement of an anti-trust suit by the government (which actually gave the Navy what they had wanted back in 1919.) The settlement took patents from GE and Westinghouse (and others) and gave them to RCA. Westinghouse and GE couldn't compete with RCA for two years (until 1933) and RCA was freed of any debt to Westinghouse or GE. This meant that the millions of dollars that RCA owed Westinghouse and GE for the loans necessary to buy the Victor Talking Machine Company didn't have to be repaid. In 1930, RCA Victor became the division of RCA that handled consumer entertainment radio along with broadcasting through NBC, recordings and vacuum tube manufacture. RCA Manufacturing Company, Inc. (created about 1934) built all commercial and military equipment and Radiomarine Corporation (created in 1927) became the division that handled all shipboard radio equipment and operated all of the RCA Coastal Ship to Shore stations (providing RCA Radiogram service, among other duties.)

The RMCA AR-8506-B was introduced during WWII with schematics dated November, 1942 and with the FCC approval for shipboard use dating from February, 1943. The AR-8506-B is a five band receiver capable of reception of LF signals from 85kc up to 550kc and medium/shortwave signals from 1.9mc up to 25mc. The circuit is superheterodyne and uses 10 tubes along with a NE-32 (G-10) neon lamp for voltage regulation (LO.) The IF is 1700kc in order to allow the receiver to cover the entire 400kc range without interruption. Much of the ship's communications were in the frequency range of 400kc to 500kc and a standard IF of 455kc would have a gap in frequency coverage from about 430kc up to 475kc due to the IF operating at 455kc. Usually, shipboard superheterodynes will have IFs that are in the AM BC band area since this region of the spectrum wasn't normally tuned by the ship's communication receiver. The receiver can be powered by 115vdc or 115vac and can also be powered at 230v ac or dc using an external resistor unit, the RM-9. Tuning uses a 30 to 1 reduction vernier drive (counter-weighted) and there is an additional "band spread" function using a separate control. A built-in loudspeaker is front panel mounted and can be switched off by the operator if necessary. These receivers were usually integrated within a shipboard communications console that contained a transmitter, another receiver capable of VLF reception (AR-8510,) an emergency receiver (crystal detector receiver,) a power control switching system that allowed battery operation or ship's power operation and other equipment necessary for radio communication at sea. Most of the RMCA radio equipment was usually installed on Victory ships and other merchant ships during WWII. The FCC approval for shipboard use indicated that the AR-8506-B's LO leakage to the antenna was <400pW and thus would not interfere with other shipboard radio equipment and would not radiate a signal of sufficient strength for enemy DF or detection. The U.S. Army Signal Corps issued a manual, TM11-875, giving the AR-8506-B the designation R-203/SR.

After WWII, the AR-8506-B continued to be offered by RMCA for maritime use on various types of ships. The post-war versions are somewhat different in appearance in that the individual celluloid control identification plates are replaced with a "raised letter" type of panel nomenclature. Additionally, the data plate was removed and the manufacturing information became part of the front panel nomenclature. The AR-8506-B shown in the photo above is from 1953 and shows how the later versions looked when installed in the table top cabinet (with shock mounts.) It's interesting that RMCA was still building and selling a ship's receiver design that was over ten years old. Ship owner's reluctance to replace radio gear was probably why RMCA could still find buyers for obviously dated designs. Since the ship owners didn't want to replace their old RMCA consoles anyway, as a consequence, the associated radio equipment was in-use well beyond the normal life-span with examples of the AR-8506-B and AR-8510 still in use as late as the 1980s. These later post-war AR-8506-B receivers were also used at the RMCA coastal station KPH for various purposes.   

The AR-8506-B has an internal 1700kc wavetrap. The wiring and adjustment of the wavetrap should be checked if BC signal leakage is encountered. There are six AM BC stations in the USA that operate on 1700kc located in Texas (2 stations,) Iowa, Alabama, Florida and New York (all are 10KW day and 1KW or less at night.) There's another 1700kc AM BC station located in Baja California, Mexico. Since 1700kc is so sparsely utilized, the chances of encountering 1700kc AM BC leakage is slight. However, things can always change so proper adjustment of the wave trap should be performed. The wave trap should be adjusted on Band 3 for minimum response with a 1700kc RF signal input to A1 on the antenna input of the receiver. If you're unlucky enough to be in an area with a strong 1700kc station and you have the wave trap adjusted correctly and there's still BC signal leakage then using an antenna that is "tuned" for the specific frequency desired should be tried. This could be a resonant antenna cut for the specific frequency desired or an antenna with an antenna tuner. The "tuned" antenna will be very selective and should reduce the BC interference. It's also possible to install an external wave trap between the antenna feed line and the receiver antenna input for severe interference.

Like a lot of shipboard receivers, the AR-8506-B doesn't have a standby switch (either remote or panel.) To use as a station receiver requires either an antenna relay with good isolation for the receiver in "transmit" or you can also use an electronic TR switch. Separate T-R antennas would also work. Since the AR-8506-B is a "transformerless" AC-DC circuit, always operate the receiver using a 1:1 isolation transformer when powered by the house AC line voltage. It's also possible to build a combination isolation transformer and a rectifier-filter circuit to operate the AR-8506-B on +115 vdc (see RMCA AR-8516 below and the details on the MM-555140-B combination isolation transformer and DC filter unit that provides +115vdc to operate that receiver.)

The WWII version of the AR-8506-B is profiled in the "WWII Communications Equipment - Part 2 - Radiomarine Corp" - use Return to Home/Index for navigation.


The post-war AR-8510

Radiomarine Corporation of America  -  Model AR-8510

The AR-8510 is a five tube regenerative receiver that tunes from 15kc up to 650kc in four tuning ranges. Two TRF amplifiers are used with a Regenerative Detector and two stages of audio amplification. The RF amplifiers use a combination of tuned grid and tuned plate with a three-section ganged condenser for tuning. The audio output can drive the panel mounted loud speaker or headsets. The panel speaker can be switched off if only a headset is desired for reception. The receiver requires a separate power source of which many types were available. Various types of battery combinations could be utilized with either the RM-2 or the RM-4 Battery Control panels. These functioned on ships that provided 115vdc or 230vdc power. If 115vac was to be used then the RM-23 Rectifier Power Unit (power supply) was used. There was also an RM-37A Receiver B+ Supply Unit that provided 90vdc output from the ship's 115vdc power. This was to be used if it was necessary to conserve the B batteries that normally provided the +90vdc for the B+. The AR-8510 requires 6.3 volts at 1.8A (AC or DC) and 90vdc at 15mA. The vacuum tubes needed are four 6SK7 tubes and one 6V6G or GT.

The AR-8510 was provided with a cabinet and shock mounts if it was to be used as a "stand alone" receiver. However, if it was going to be installed into a shipboard communications console (as most were) then the cabinet and shock mounts were not provided. Many AR-8510 receivers were part of the shipboard 3U transmitter console that included a 200W transmitter, an emergency crystal receiver, a battery charger switching panel and an automatic emergency alarm receiver. 4U consoles used the RMCA AR-8506 (a MW and SW superhet) and a 500W transmitter. The 5U console had both the AR-8506 and the AR-8510 installed along with all of the other auxiliary equipment. Mackay Radio supplied MRU-19 or MRU-20 consoles with their equipment installed.

The AR-8510 was approved by the FCC for shipboard use in 1942. The schematic drawings are dated 1943. It's likely that it was 1943 at the earliest before any AR-8510s were in use. The AR-8510 shown in the photo above is the post-WWII version and the photo is from the manual. Like the AR-8506-B, the AR-8510 replaced the round celluloid nomenclature plates and went to an embossed nomenclature panel. Unbelievably, the production of the AR-8510 continued into the 1960s and actual use of the AR-8510 lasted quite a bit longer. It wasn't uncommon to find AR-8510 receivers still being used on old oil tankers as late as the 1990s.

The WWII version of the AR-8510 is profiled in the "WWII Communications Equipment - Part 2 - Radiomarine Corp."  - use Return to Home/Index at the bottom of this page


1958 Radiomarine Corp. - Model AR-8516  SN:5830

Radiomarine Corporation of America - Model AR-8516

Circuit Description - RMCA introduced the AR-8516 around 1958 and it was available up into the mid-1960s. It was a tremendous upgrade from the proceeding commercial shipboard receivers that RMCA had been providing (those designs dated from the early part of WWII.) The AR-8516 was a thoroughly modern, complex communications receiver that used 18 tubes in a single, double or triple conversion superheterodyne circuit. The receiver used a variably-tuned IF covering 1.09 to 3.09mc or 2 to 4mc. The fixed-frequency IFs were at 455kc and 45kc. Single conversion was used in the first five bands covering 80kc up to 4mc. Bands 1-4 span different widths of the LF and MF spectrum and, along with Band 5, these single conversion bands tune from left to right being from high to low in frequency. From 2mc up to 30mc is covered in fourteen 2mc-wide bands. Band 5 (2-4mc) is single conversion but Bands 6-18 are double conversion. The double conversion bands (Bands 6-18) tune left to right being low to high in frequency. The total frequency coverage is from 80kc up to 30mc in 18 tuning ranges. Seven crystals are utilized in the Crystal Oscillator with only two crystals used uniquely. The other five crystals are used in various fundamentals and harmonics for a total of 13 crystal oscillator frequencies. The RF amplifier, Crystal Oscillator and Mixer 1 circuit function as the RF input for bands 6 to 18 with the output going to the Variable IF tuning that then provides a fixed 455kc output. The RF input section is bypassed when using bands 1 to 5 and the incoming signal goes to the Variable IF which combines with Mixer 2 and the selected VFO to provide a 455kc output. Two separate VFOs are used with one used for Bands 1, 2 and 3 and the second VFO used for Bands 4 to 18. Main tuning gear reduction is 41.7 to 1 and the SECTOR gear reduction is another 3.8 to 1. The resulting tuning is a constant "bandspread" type at almost a vernier tuning rate.

The dial readout is "band-in-use" and employs a rotating dial drum that is ball chain-coupled to the band switch. The smaller dial to the right is the kilocycle-logging scale which for Bands 1-4 is a logging scale, but, since the Bands 5-18 are 2mc linear scales for each band, if the receiver is well-aligned, then the this dial will readout almost directly "to the kilocycle" for these bands. For example, if the slide-rule dial pointer is set in between 14.1 and 14.2 and the logging dial reads 67, then the tuned frequency is 14.167mc. The kilocycle dial index is not adjustable. The accuracy of alignment will determine just how usable the kilocycle dial ends up being. Spec from 4-30mc is "within 10kc." I was able to achieve an average tracking accuracy of between "dead on" up to about 5kc off. Below 4mc the accuracy spec is 0.5% which is easily attained during alignment (at 4mc the spec of 0.5% is 20kc.)

Bandwidth is selectable from 6kc down to 100hz (five steps) in Bands 3 to 18 but in Bands 1 and 2 the bandwidth is selectable only from 1.5kc down to 100hz in three steps (6kc and 3.1kc, if selected, won't change from 1.5kc bandwidth.) The selectable bandwidths provided are 6kc, 3kc, 1.5kc, 800hz and 100hz. The 3.0kc bandwidth utilizes a Collins 3.1kc mechanical filter. The 45kc IF is utilized for two purposes. In Bands 1 and 2, 45kc is the IF. In Bands 3 to 18, the 45kc IF is utilized as part of the selectivity bandwidth operation and results in triple conversion in the IF section if 1.5kc, 800hz or 100hz bandwidths are selected. Two types of detectors are used with a standard vacuum tube diode detector used in Bands 3 to 18 and a crystal diode detector (heavy duty 1N34 germanium diode) utilized in Bands 1 and 2. Two BFOs are also used with the 455kc BFO being adjustable +/- 2kc but the BFO for 45kc (which is used just for bands 1 and 2) utilizes a fixed-frequency (adjusted for a 1kc heterodyne or 44kc.) When SSB is selected the 455kc BFO output is increased (compared to the output in CW) for better demodulation of suppressed-carrier signals. AGC has selectable fast and slow time constants. The audio output frequency response is 200hz to 3000hz (4db down) and the audio power available is 250mW with 3.2Z or 600Z impedance outputs provided. A 500kc Calibration marker signal is available that functions from the 500kc Crystal Oscillator that also provides mixing with 455kc IF for the 45kc IF. A carrier level meter (showing db above 1uv) is provided. The ANTENNA (trimmer) control is connected to RF/Mixer 1 and only functions when the receiver is on Bands 6-18 (4-30mc.)

Power Requirements - Being a commercial shipboard receiver, the AR-8516 is designed to be powered by various types of ship's power systems. It was expected that the receiver would be powered by +115vdc. Using accessories available from RMCA, operation on +230vdc was also possible. It was also possible to operate the receiver direct on 115vac but the receiver's power input section only provides a half-wave rectifier if AC operation is used. RMCA recommended that the MM-555140-B Isolation Transformer/DC supply be used to convert the ship's 115vac (or 230vac) to +115vdc for operation of the receiver. In fact, if the receiver was ordered with the matching table cabinet, then the MM-555140-B was included and installed inside the cabinet. If the rack mounted version of the receiver was ordered then the MM-555140-B was separate and was installed externally (somewhere in the rack.) Since the AR-8516 is AC-DC in concept, all tube filaments are wired in series. Most of the tubes are 3-volt filament types, e.g., 3BZ6, 3BE6, 3CD6, 3AL5, etc. but (2) 5U8, (3) 7AU7 and (1) 12CU5 tubes are also used.

The SECTOR Control - The unfamiliar control on the AR-8516 is the SECTOR control which is part of the RF and Mixer 1 tuning in combination with the Crystal Oscillator. When the receiver is tuning from 80kc up to 4mc (first five bands) the RF and Mixer 1 circuits are bypassed and the Variable IF (acting as a RF amplifier, VFO and Mixer 2) tunes the receiver as a single conversion circuit. When tuning above 4mc, double conversion is used and the SECTOR control provides the correct tuning for the RF and Mixer 1 (and Crystal Oscillator) for each of the 2mc wide bands (4mc to 30mc.) Four sets of coils are used for ranges 4-12mc, 12-20mc, 20-28mc and 28-30mc. When combined with the selected Crystal Oscillator frequency the result is 2mc wide tuning ranges from 4 to 30mc. The "sector" refers to each 45 degree quadrant (A, B, C or D) of the RF/Mixer 1 variable tuning condenser's 180 degree rotation range. Selecting A, B, C or D will actuate a physical rotation of the RF/Mixer 1 tuning condenser rotor to the correct 45 degree quadrant required for tuning (plus the Crystal Oscillator) of the particular 2mc band range selected. The SECTOR switching is roller-chain coupled to the band switch. The RF/Mixer 1 tuning is gear-coupled to the main tuning control and the SECTOR selector knob will rotate as the TUNING knob is rotated. Although the SECTOR knob and RF/Mixer 1 condenser rotate with the main tuning, the correct quadrant selection is necessary and will require that the SECTOR knob be set to the "detent" in the correct position. The "PLACE SECTOR AT" indicator is illuminated and located in the viewing hole above the band switch. The main tuning dial, logging scale and meter illumination is also controlled by the SECTOR control in Bands 6 to 18. These lamps will only illuminate when the correct SECTOR position is selected.

Serial Number and Cabinet - The serial number of the AR-8516 shown is SN:5830. The serial number incorporates the year of manufacture in the first two digits, e.g., "58" is 1958 and the receiver number is "30." There is also another tag mounted to the rear chassis that gives the month and year of manufacture, in the case of SN:5830, the tag reads "JAN 1958." The odd size panel height of 9.5" (rather than the standard 10.5") does limit the receiver's installation into generic-type cabinets without an obvious gap. Mounting the receiver in a rack will solve that issue. Of course the proper RCA cabinet does fit the receiver panel correctly. As stated in the manual, the AR-8516 was available as a table model receiver with the cabinet (with the MM-555140-B installed) or it could be ordered as a rack mount without cabinet (with the MM-555140-B as a separate accessory.)

photo above: Top of the AR-8516 chassis

>>>  Antenna Connections per the Manual - There are four antenna terminals provided identified as A1, A2, GND and SHLD. The manual warns not to "ground" the SHLD terminal. This is because in Bands 1-5, SHLD is connected as part of the antenna input by essentially "shorting" the coax shield to the center conductor of a coax feed line. This allows the receiver to have a fairly efficient "T" antenna configuration in the LF and MW ranges. At Band 6, the band switch then connects SHLD to the receiver chassis allowing the use of a resonant, coaxial fed antenna. When using the typical unbalanced ham antenna, the center conductor of the coax is connected to A1, then A2 is jumped to GND and then just the shield of the coax is connected to SHLD. For the hook-up to work correctly, the antenna shield can't be grounded anywhere and must only be connected to the SHLD terminal. For hams the problem with this hook-up is for 80M (Band 5) and 160M (Band 4) operation.

A More Practical Antenna Hook-up - If you want to use a resonant unbalanced coaxial fed antenna for much better reception on 80M or on 160M, then connect the coax shield to GND, A2 also connects to GND and the coax center conductor to A1. Don't connect anything to SHLD. If an antenna tuner and tuned dipole antenna combo is used with this type of connection, then the tuner adjustments will "peak" normally. The ANTENNA (trimmer) won't function on Bands 1 thru 5 because the antenna signal is going to the Variable IF (the receiver is single conversion in Bands 1-5.)

photo above: Underneath the AR-8516 chassis

Building a Replica MM-555140-B

The AR-8516 manual states that if a very low hum on the audio output is desired or necessary then the receiver should be operated on +115vdc. I decided to build a MM-555140-B since finding an original would probably take a considerable amount of time. Luckily, the AR-8516 manual has the schematic for the MM-555140-B and there really aren't any unusual parts required for its construction. In fact, everything to build my homebrew version came from the various junk boxes I have around here. I used a 150VA 1:1 Isolation Transformer as the main component. The receiver requires about 80 watts for operation, so there's lots of reserve capability in the transformer. Other components were a full-wave bridge rectifier, two 40uf 300vdc electrolytic capacitors, a 22K 2W CC resistor, a 25 ohm 50W WW resistor and a 6.5 ohm 60W WW resistor. The two large dissipation wire wound resistors were made up of a parallel combo for the 6.5 ohm 60W (15+15+75 ohms) and a parallel combo for the 25 ohm 50W (two 50 ohm 25W resistors.)  >>>

>>>  These dissipation values are much higher than the original dissipation values called out on the schematic but I found that those values shown on the schematic did get very hot during operation. This was probably because of the 122vac line voltage I have. Actually, at the output of the isolation transformer the voltage is 124vac, versus the original MM-555140-B design for 115vac input. Also, the 6.5 ohm value differed from original value of 5 ohms but that again was due to the higher line voltage versus the original 115vac design.

The + DC output has to be connected to the receiver so +DC is routed through CR101 (the half-wave rectifier in the receiver.) This would correspond to the LINE (HOT) if the connection was directly to the AC line. The -DC corresponds to the AC NEUTRAL connection which is internally connected to the power supply chassis (transformer shield.) The B- inside the receiver is isolated from chassis to allow -DC to be connected to the power supply chassis  The chassis Ground of the receiver is connected just to chassis of the DC supply. I used a three-wire power cable so the power supply chassis and the receiver chassis go to ground.  >>> 

>>>  My version of the MM-555140-B was installed into a 3.5" x 6" x 8" aluminum project box. There is a standard single socket grounded AC outlet on the front panel. Also, two pin jacks are on the front panel that can be used for monitoring the DC voltage to the receiver. Other power supply features are an AC toggle switch and a 120vac neon pilot lamp. The fuse holder is on the rear panel and has a 2A SB fuse installed in the Line to primary. When connected directly to my 122 volt AC line, the DC voltage at the powered-up receiver is +115vdc +/- 1vac and the +115vdc remains very stable regardless of the various functional adjustments made to the receiver. DC voltage measured at the receiver at initial power-up will start at about +112vdc, climb to about +117vdc and then settle at +115vdc as the receiver "warms-up" which takes about 30 seconds. Hum is non-existent and the audio sounds very clean. With no load, this power supply output voltage will measure about +175vdc which isn't over the voltage rating of the components used but the design output voltage relies on an 80 watt load being connected and that's where it operates correctly. The upper-right photo shows my replica MM-555140-B.

Shown to the left is the schematic of the MM-555140-B and also the artwork of how the RMCA unit looked.

AR-8516 SN:5830 Refurbishment

When acquired, SN:5830 did turn on and did receive some signals, but it wasn't fully functional and really wasn't performing at its design capabilities. Initial inspection found five of the 18 tubes were bad with two showing significant leakage (shorts.) The AF gain pot was internally damaged and no amount of cleaning would fix it since the carbon track was cracked in several places. Luckily, I had a NOS 500K A-taper (audio taper) A-B pot to replace the bad original. De-Oxit was used on the Bandwidth switch which helped it to function correctly. The receiver was the rack-mount version so it didn't have a cabinet and for some reason RMCA never offered a dust cover. I guess they figured that the rack console would protect the receiver chassis. However, decades of storage in a greasy environment certainly had the chassis looking terrible. There was a little minor corrosion but most of the contamination was dirt, waxy-grease and probably tar-nicotine. The front panel had its share of "battle-scars," "rack rash" and even some green paint splatter. The knobs were just plain "grungy" with crud that seemed waxy-like in all of the flutes and recesses. There was white residue in the front panel screw slots implying that the panel and knobs had been rubbed-down with polish or wax in the distant past.

Thorough cleaning required major disassembly. The knobs were soaked in Dawn dish soap for two hours to loosen the waxy grunge after which they were scrubbed with a short-bristle brush to thoroughly clean. The knobs were then rubbed-down with a dry cotton flannel cloth to slightly polish the surface finish. The RCA meatball badge needed to be dismounted in order to remove the pitting (600 grit AlOx paper,) cleaned with Glass Plus along with a wooden toothpick used to "dig-out" the wax in the recesses between the letters and finally the badge was polished with Wenol's (British equivalent of Semichrome.) The chrome handles had minor pitting that was removed with Glass Plus and 600 grit AlOx paper followed by polishing with Wenol's. The grunge on the chassis was removed with WD-40 and an acid brush (with the bristles cut short.) This was followed by Glass Plus to remove the WD-40 residue. In some stubborn places, isopropyl alcohol would cut the grime better. The plastic dial cover was removed, cleaned and polished. The meter scale, the sector scale and the kilocycle/logging scale were made of photosensitive phenolic that had darkened but careful cleaning with Glass Plus brightened up the yellow-amber color significantly without damaging the nomenclature. The dial scale was a metalized over-lay applied to the drum. It cleaned up easily because the material and the nomenclature were particularly durable and could be cleaned a bit more aggressively than expected. I installed "matching" tube shields. These were the standard flat aluminum type because the IERC-type of heat-reducing tube shields are really only necessary when a receiver is going to be operated 24/7. I matched the original paint on the front panel (using 5 colors of Testor's model enamel - gray, black, white, dark blue and brown) and touched up most of the "battle-scars." The silver dial bezel was touched-up using thinned Testor's silver paint. A flat-razor blade was used to remove the green paint drops. The panel mounting screws were cleaned and painted black before reinstallation. After cleaning the tops of the two shield covers (RF-Sector condenser and Variable IF condenser) they both look silver in the photos. Actually, they appear slightly gold tint, though much lighter than the chassis. The gold finish is called iridite and it was a chemical bath that colored the sheet metal during manufacture and before assembly.

Disassembly issues not mentioned in the manual: The BFO pointer is dual set-screw mounted to the outer coaxial shaft of the vernier BFO adjustment. To remove the front panel this pointer has to be dismounted but you can't get at both set screws from the top. The right side panel has to be removed to access the set screws to dismount the pointer to allow front panel removal. All of the set screws in the knobs and couplers are Bristol (spline) types. There are four shaft bushings, two of which have "C" clips associated with them (ANTENNA and TUNING.) The Antenna (trim) coupler set screws are easy to access from the top. But, the TUNING shaft will require removal of the "C" clip to be able to dismount the front panel. Also, three of the bushings are standard size but one is much longer than the others. The long bushing is used for the TUNING shaft for better support of the shaft which does have a large counter-weight installed on it. The dial string assembly rail for the dial pointer is mounted to the front panel. When the four screws are removed this piece can then "flop around" and rub against and mar the dial scale. Place several paper towels over the dial scale to protect it during disassembly. Note that the PHONES jack is isolated from the chassis and panel by two shouldered fiber insulating washers. This jack should not be contacting the panel or chassis (problems could result if using direct AC line operation.) Removal of the dial bezel requires removing the front panel first, then plastic dial cover piece is dismounted from the rear, then the bezel is dismounted from the two retaining strips and then the two strips are dismounted from the back of the panel. This assembly can't be installed or removed if it is all put together, it has to be installed (or removed) by dismounting each individual piece.

Sea Trials - Dec 12-15, 2020 - After reassembly and completion of the DC power supply, I operated the AR-8516 for a few days to get the "feel" of the receiver. As acquired, this AR-8516 had already been meticulously and carefully re-capped so I was pretty confident that the receiver could operate for long time periods without any serious problems. The receiver seemed to function pretty well but it was obvious that some improvement should be possible. The next day I found a very minor problem that had a significant effect on reception. The jumper between A2 and GND had been a piece of solder (the lazy tech's jumper) and it had broken at the A2 terminal. I replaced the solder with a TC jumper. The signal reception level was greatly improved. I tuned around 12.965mc and copied one of the Seoul, So. Korea coastal beacon stations, HGW2. Also, a little lower in frequency and one of the Chinese coastal beacon stations, XSQ was heard. The next morning, I tried 40M and some of the SSB signals were intensely strong. I tried HGW2 on 12.965mc (should be at 12.915mc,...more on that problem in the alignment section) and it was very strong. Fire Drake, the Chi-Comm jamming signal on 9.85mc (it changes frequency all the time since it's jamming Radio Taiwan which also changes frequency all the time to avoid Fire Drake,) was very strong and showed about 25db over 1uV on the carrier level meter. Trenton Military Aviation Weather (Trenton, Ontario, Canada) on 15.031mc was heard with a very strong signal. One other thing noticed before, with the broken jumper, I couldn't seem to find a good match with the antenna tuner. With a jumper installed, now the tuner showed a very positive increase in reception when the antenna match was "tuned." Also, the ANTENNA (trimmer) on the receiver would "peak" now. It was also noted that the best results on SSB or CW was to reduce the RF Gain and advance the AF Gain which is typical of a receiver without a product detector. If the RF gain is advanced too far it seems to "mask" weaker CW signals with excessive background noise, again typical of a standard diode detector. In AM Voice, the RF gain can be fully advanced and the AVC allowed to control sensitivity. The RF tracking is fairly close with the error at 15.000mc being -6.0kc. The 500kc oscillator is off by about 400hz and this affects 45kc IF and the 1.5kc-800-100hz bandwidth performance (which barely worked) and the 500kc calibration marker accuracy. Although performance seems pretty good, a full IF/RF alignment will assure that the receiver is at its design specs. 

photo above: AR-8516 underneath with the shield covers removed from the Variable-IF bandswitch-coils, the RF bandswitch-coils, the Crystal Oscillator bandswitch along with the Xtal Osc trimmers and the Variable IF Mixer covers.

Return to the Workbench - Dec 18, 2020 - It's pretty much standard procedure to perform an initial refurbishment to get a set going, then use it for a while to assess how it's going to work out and then plan what else needs to be done to complete the rebuild. Although performance was generally good and it seemed likely that the AR-8516 could do a very good job as a station receiver there were other mechanical and electronic things that had to be done to enhance the receiver's functionality. Here's the list:

1. Mechanically aligned the slide rule dial to more accurately agree with the kilocycle dial. Pre-alignment necessity.
2. Cleaned with DeOxit and a small paint brush all band switches. RF sw, Vari-IF sw and Xtal Osc sw.
3. Cleaned rotor grounding contacts on both tuning condensers with DeOxit and a brush. RF-Sector and Vari-IF.
4. Cleaned (removed) dried Lubriplate grease from gear box gears and made sure backlash split gears were working. Lubriplate is designed for sliding surfaces not for gears. It dries like concrete if it isn't constantly in use and routinely cleaned off and reapplied. This Lubriplate must have been decades old. Light oil and an acid brush (bristles cut short) were used to remove the Lubriplate. Re-lubed split-gears and bushings with 10w machine oil.
5. Cleaned and readjusted tuning shaft over-riding clutch. The clutch is a spring-loaded mechanism that had been filled with Lubriplate - just what you need for a clutch - it was locked in place! Cleaned the Lubriplate off with light oil and a brush then washed the mechanism with Isopropyl Alcohol to remove oil residue. Then adjusted the clutch to "slip" at the dial end-stops.
6. All tube pins and sockets cleaned with DeOxit and a small paint brush.
7. Adjustments for 2mc C to 3.9mc L Variable IF tracking had GLPT lacquer "locking" them. Used lacquer thinner to remove the GLPT so these two very important adjustments would be moveable. Overall frequency readout accuracy depends on these two adjustments. Pre-alignment necessity.
8. Replaced the 30" long power cord with a 72" long power cord.

AR-8516 Manual Alignment Procedure Critique and Pre-Alignment Preparation - Normally, pre-alignment preparation isn't necessary but the AR-8516 manual alignment procedure is not written for "first timers" and assumes that the alignment tech has been doing a lot of AR-8516 receivers before and knows the locations of all of the components. I read through the procedure before starting the alignment and found that I was constantly having to go to chassis drawings and the schematic to physically locate key connection points. For example, when aligning the 45kc IF, the instructions indicate the VTVM should be connected to the junction of R191 and R192 but no resistor values are mentioned. There's no component location drawing showing smaller parts (like resistors.) Small components have to be located first on the schematic to find the values of these resistors so you know what to look for under the chassis. You'll know generally where to look from the schematic. First time is a hassle, that's why the pre-alignment checkout.

To say the alignment procedure is lacking details is an understatement. I found it necessary to make lots and lots of pencil notations about important component values, test point locations and even drawings of the IF transformer connections. The 455kc IF transformers pin-outs are important because the primaries or the secondaries or both have to be "loaded down" with 10K resistors during alignment. Luckily, the IF transformer pin-outs are vaguely mentioned in one of the schematic notes, where else,...and even that information is incomplete requiring a visual and continuity check to verify. NOTE: IF transformer pins viewed from the bottom, green dot is pin 1, go clockwise for pins 2, 3 and 4.

Next, the RF signal generator input for IF alignment is to the grid of V105 Mixer 1, problem,...except that the bottom of the V105 socket is located within a completely shielded compartment (with more than thirty 4-40 screws mounting the cover.) I used a seven-pin tube test extender socket to input the signal to the V105 grid.    >>>

photo above: Showing the RF/Mixer 1 tuning condenser (left) and the Variable IF tuning condenser (center) with the shield covers removed. Also, the gear box cover is removed showing the gear train.

>>>  Another complication in the IF and in the RF tracking alignment process is that the receiver has to be on its side since there are adjustments both on top of and underneath the chassis along with having to observe the front of receiver for the dial readout. I had to use a 4"x6" hand mirror to verify tuning dial readouts and sometimes the locations of various trimmers to keep from bending over or leaning around the side of the receiver for every adjustment. I guess I could have turned the receiver around so the panel was in front but then all of the difficult to see small chassis-located adjustments would have to either require a mirror and bending over and around the chassis or some other uncomfortable contortions.

I think that at least a couple of days of reading, making notes, visually locating the important component junctions and getting all of the resistor loads, coupling caps and alignment tools together will help to ease the alignment process.

IF Alignment - This starts with the 455kc fixed IF. The only unusual requirement is the loading down of some of the IF transformers primaries or secondaries with 10K resistors. This pretty easy to accomplish if the 10K resistors are on short flexible wires with small alligator clips on the ends. The bottoms of the IF transformers are easy to access for attaching the 10K loads. Two 10K loads are required because T124 and T125 have both primary and secondary loaded simultaneously when doing the adjustment. There is a trimmer capacitor for the mechanical filter that is adjusted for a relatively equal response between the 6kc bandwidth and the 3kc MF bandwidth. The 455kc BFO is also adjusted at this point in the procedure.

The 45kc IF does require a signal generator that will produce RF at that low of a frequency. I normally use the HP 606B for alignments but its lower limit is 50kc. Luckily, I also have an excellent General Radio Type 1001A RF generator that will produce a RF signal as low as 5kc. For the 45kc IF, I had to use the GR 1001A for the signal source. Four torriod type LCs are adjusted with trimmer caps and one transformer L is adjusted. The 45kc BFO is adjusted at this time.

The Variable IF is adjusted to track perfectly (or as close as possible) from 2.0mc to 3.9mc. This is accomplished with adjustable L and C and referenced to the kilocycle dial for maximum accuracy.

This completes the IF alignment. 

RF Tracking - The single conversion low frequency tuning is aligned first. This requires adjusting six L and C adjustments per band for a total of 30 adjustments. Only the 2-4mc single conversion band (Band 5) can be aligned using the kilocycle dial. Bands 1-4 are adjusted using the slide rule dial. After the tracking is adjusted on Bands 1-5, then the 45kc wavetrap is adjusted at 80kc before proceeding on to the double conversion RF tracking alignment.

Bands 5-18 are double conversion and are aligned in four major divisions, 4-12mc, 12-20mc, 20-28mc and 28-30mc. Each Sector A, B, C and D requires Crystal Oscillator peaking and RF-Mixer adjustments. Seven crystals are utilized but six are doubled and used on harmonics to allow the 13 bands to operate. There are 23 adjustments plus peaking the ANTENNA. 

I couldn't get "to the kilocycle" accuracy using the kilocycle dial. It's pretty close and does meet the specification of dial accuracy of "within 10kc" which seems to be pretty good (12-14mc is the exception - covered in the next section below.) For frequencies below 4mc, dial accuracy spec is 0.5% which is very easy to maintain.

The 500kc oscillator is adjusted using WWV and the beat note. Since the BFO is on when in the 500kc CAL position, the BFO should be set to zero so that only the 500kc oscillator and WWV are heard when listening for the heterodyne zero beat.

There's also two adjustments for the AF feedback that are adjusted for minimal "ringing" when in the narrow 100hz bandwidth.

Over 75 different adjustments are necessary for a complete AR-8516 IF/RF alignment.

Errors and Suggestions - There are two obvious errors or conflicting data in the manual's alignment procedure. T121 is identified as a 12mc adjustment in the manual's adjustment drawing. The receiver silk screening and the manual written procedure are correct indicating that T121 is for a 2.0mc adjustment. The second error is T115 is correctly identified in the adjustment drawing as 220kc and the receiver silk screening also shows the correct 220kc. The alignment procedure "frequency table" incorrectly calls out 210kc for the adjustment frequency. Some of the instructions are confusing in the wording used and require further checking to confirm what actually is required. The use of the IF 10K loads is a good example of confusing language. As a matter of poor layout, the manual has two chassis illustrations dividing the written alignment instructions. My manual was in a binder and the pages could be taken out as needed. If only a bound manual is available then print copies of alignment instructions so the book pages aren't worn out from flipping back and forth as the alignment proceeds. 

Post-Alignment Performance and Issues - No doubt, the alignment significantly improved sensitivity and certainly got the 45kc IF working correctly along with the Bandwidth 1.5kc, 800hz and 100hz working very well. Listening on 20M quickly turned up a ZS6 amateur from South Africa. Trenton Military out of Ontario, Canada was easily found at 15.035mc since their signal was very strong now and the dial accuracy noticeably improved. WWV 15mc was about 1.5kc off frequency readout and WWV 10mc was about 1kc off. However, WWV 5mc was off about 10kc (still within the factory specs.) 40M hams required reducing the RF Gain down to 4 for the very strong signals. 80M performance was quite good and the tracking on that band (Band 5) was easily within the 0.5% spec. Just a note,...there's no noticeable difference in using either a 3.2Z or a 4.0Z speaker (as expected.) So, that was the good news,...but two problems were noted.

12-14mc Band Tracking - For this problem I don't think there's any easy solution. The 12-14mc band tracking is off by about 45kc. Since all other bands are very close in their tracking, I would think that the Crystal Oscillator is the problem with the crystal being the most likely suspect. The crystal is Y103, an 8.00mc crystal and it is used twice, on 12-14mc and also on 28-30mc. A test on 29.0mc revealed an error of about 90kc which seems to confirm that the crystal Y103 is the likely problem. From what I've heard, there are virtually no companies that supply newly made crystals anymore. I checked to see if the R-390A happened to use that frequency crystal but it doesn't. In addition to finding an 8.00mc crystal there's also the physical access to the crystals in the receiver which is beyond difficult. Looking at the photo to the right, the crystals are soldered directly to the band switch (they have a green coating on the metal housing.) I looks like replacing a crystal requires extracting the entire Crystal Oscillator as a unit. It's not a module but note in the photo that the band switch shaft uses a slotted nylon Oldham-type coupler which implies the Crystal Oscillator is removable. Since performance on the 12-14mc band is excellent in all other areas, I'll probably just have to accept that frequency readout on that band (and on 28-30mc) is off. Even with a 45kc dial readout error, 12mc the accuracy is still better than 0.5% (0.5% is 60kc at 12mc.)   Note: Used 8.000mc crystals are available from Alltechs in California - seen on eBay - about $5 plus shipping.

200kc to 500kc Band Sensitivity - The second issue was poor performance on the 200kc to 500kc band (Band 2) and this was easily repaired. Low sensitivity on this band was due to the misalignment of VFO at 220kc. I must have initially set the RF gen to 210kc as indicated in the manual alignment instructions but actually had the receiver tuned at 220kc as indicated on the receiver chassis silk screening (after all, I was looking at the receiver dial in a mirror.) To correct the problem, obviously I realigned with the receiver tuned to 220kc and the RF generator also tuned to 220kc. The other alignment adjustments (six adjustments, three for 220kc and three for 500kc) were also peaked for Band 2 and afterwards the sensitivity was quite good and the tracking was excellent. In the evening, I set up the AR-8516 with the Pixel Loop and tuned in several NDBs in the 300kc to 420kc part of the spectrum. Best DX was LLD 352kc in Hawaii, several Canadian NDBs in Manitoba and Saskatchewan along with several Mid-west 25W marker beacons. With a wire antenna, signals were stronger but so was the noise which is normal. Dial accuracy was good but the dial's scaling is fairly vague.     Dec 23, 2020

Performance as a Station Receiver - I set up the AR-8516 with the Collins 32V-3 as a station to operate on 75M for the MRCG net and the Vintage Military Radio Net. A Collins 270G-1 speaker was used as the audio reproducer. The receiver doesn't have a remote standby circuit so the front panel standby position of the operation switch has to be used. Also, with the 32V-3 set up I use a vacuum relay for the T-R operation for isolating the receiver antenna input. Since 75M is tuned in on Band 5, the AR-8516 is operating as a single conversion receiver and because the signal routing is to the Variable IF, the ANTENNA (trimmer) isn't functional. However, using a tuned antenna (~50Z for the transmitter) will be a close match to the receiver input Z. Frequency readout is primarily on the slide rule dial where the accuracy seems reasonable. Sensitivity on 75M really isn't ever too much of a problem for just about any receiver. Selectivity is much more important. 6kc bandwidth can be used most of the time but it's nice to also have a 3kc mechanical filter for tough QRM. I run the AR-8516 on +115vdc and there's no hum at all on the audio output. It's very clean sounding and voice articulation is reproduced quite well. The result is easy-to-understand AM QSOs (most of the time.) The only complaint would be the lack of remote standby that is actuated with the PTT.

For the most part, any shortwave transmissions are easy to receive. CW also is reproduced nicely. SSB uses the adjustable BFO position to select either USB or LSB and selecting the "SSB" position on the operation switch does provide a slight increase in BFO injection. Stability in the CW or SSB mode is excellent. Using the narrow Bandwidth will depend on how well the receiver is aligned. When first tested, this AR-8516 hardly functioned in the narrow Bandwidth positions since alignment of the 45kc is crucial for these functions. After alignment, the narrow Bandwidth selections function great and are a benefit for CW in crowded band conditions. Also, the narrow bandwidth selections utilize the 3.1kc mechanical filter at 455kc and then use the 45kc IF to provide 1.5kc, 800hz and 100hz bandwidths. When properly aligned, the AR-8516 provides excellent selectivity options.     


Radiomarine Corporation of America - Model AR-8711 Direction Finder

Post-WWII found that there was a growing interest in the operation of private small boats and in piloting those craft on lakes, bays and maybe even some distance off the coasts. Larger private craft that provided sleeping berths and storage for supplies might be used for over-night to several day excursions that could involve operating the craft in the dark hours or perhaps in fog-bound waterways. Many small businesses were started for charter trips, excursions and fishing trips that may have required a moderate size boat to travel long distances in unfamiliar waters. At any rate, Radiomarine Corporation decided that there was enough of a market to build and sell small, easy to operate, radio direction finders that could be used to find correct bearings for navigation. Most of the larger bodies of water had marine beacons at various locations (many associated with lighthouses) and some buoys were equipped with radio beacons. At the time, Lighthouse Ships were also in use as navigation aids. Navigation charts usually had the locations and radio identifications shown. It was also possible to use a known AM-BC station to provide a "beacon" to a city. So, the intention of the AR-8711 was to offer owner-operators of cabin cruiser type boats a method of navigation if the visual options were unavailable. The original selling price of the AR-8711 was $149.00 in 1947.

The AR-8711 is an eight tube superheterodyne receiver that is designed to be used as a direction finder. Since the intended operation was to be onboard a boat, four different types of power supplies were available as options. There were three DC types of supplies for 6vdc, 12vdc or 32/115vdc operation and one AC supply for 115vac input. The DC supplies were vibrator based and the 32vdc unit could also use an adapter (RM-93) for 115vdc input. The AC supply used a 6X4 rectifier tube and the 32vdc DC supply used a 0Z4 rectifier tube. The 6vdc and 12vdc just used filters for the switched voltage out of the transformer. Power input to the AR-8711 was via a 21 pin Cinch-Jones male plug on the rear chassis. The same type of connector was used on the power supplies and an interconnecting cable was supplied.

The AR-8711 is a radio receiver that operates as a direction finder. Two antennae have to be connected for an indication of "true" direction. The simple wire or vertical antenna would be used as an omni-directional antenna when used by itself (BALANCE in REC position.) The loop antenna provides a "figure-8" pattern that is bi-directional with signal level peaks off the ends (in line with the loop axis) and deep nulls in signal level off of both sides. This is indicated at the BALANCE position on the receiver panel or with the BALANCE turned OFF. The loop alone provides ambiguous directional information which can be used in some types of navigation. When the two antennae are combined within the receiver front-end, then a "cardioid" pattern results, indicated at SENSE on the front panel. The advantage of the "cardioid" pattern is that only one null is produced which can be used for finding "true" direction. NULL was used to increase the sensitivity of the receiver for a better null indication. DF gain was used for controlling IF gain when DFing since the AVC was disabled for DFing.

If the boat pilot knew the approximate direction of the desired port, then bi-directional, that is, just the loop antenna could be used. By using a known AM-BC station that's located in the same city as the port as a "beacon" the boat pilot could tune in that AM-BC station on the AR-8711, then setting the loop athwartship, he would steer the boat for the minimum signal response from the AR-8711. This would have the boat heading directly for the AM-BC station's antenna. Listening to the AR-8711, if the boat pilot heard the signal increase in strength, he knew that he was slightly "off course" and could correct by steering the boat for minimum signal level. This method worked when there was an AM-BC station in the city that the desired port was located in and that the boat pilot knew the approximate direction of the city from the boat.

If a marine beacon or lighthouse ship (they were usually equipped with a radio beacon also) was in the direction of the desired course but the actual "true" direction of these beacons weren't known (you're lost, in other words) then their "true" direction could be determined by using unilateral DFing. This required the sense antenna in combination with the loop to provide a cardioid pattern. The procedure was to tune in the desired beacon using just the sense antenna since it was omni-directional. Once the beacon is tuned, then the loop only is switched in and the signal tuned for absolute minimum, the null. The compass bearing is noted and then the sense antenna is switched in combination with the loop. Then the loop is rotated 90?and the signal level noted. The loop is then rotated 180?and the signal level again noted. Which ever loop position had to lowest signal level (the null) that was the "true" direction with the loop axis pointing towards the signal. If you were really lost, using two maritime beacons (their positions and frequency shown on the navigation chart,) a bearing could be taken of each beacon and a line drawn from each beacon location on the chart and the point of intersection would be the your location (a form of triangulation.)

All of this assumes that the DF radio was setup in the boat with the loop axis oriented for 0?being the bow of the boat and that the boat was also equipped with a magnetic compass that gave the direction of North plus or minus whatever magnetic deviation for the area for "true" North. From the magnetic compass reading with compensation for deviation and the DF bearing, a correct course could be plotted for the boat to be steered towards the beacon. When the AR-8711 was operated by an experienced navigator it could provide a lot of information. For example, distance to a coastal beacon (lighthouse for instance) could be determined by taking two radio DF bearings as the boat passed. The angles measured and the distance traveled between the first and second bearing could be used to calculate the distance to shore where the beacon was located, essentially letting the boat pilot know how far out from land he was. Most of the larger boats that would have had a need for the AR-8711 would also have been equipped with other marine radio gear for ship to shore communications.


General Purpose Radio Communication Equipment

RCA - CR-88A

RCA began the AR-88 design in 1940 and because of WWII requirements it was rapidly completed so production could start. The intent of production was to provide our WWII Allies with a durable, high-performance receiver for intercept. Most of the WWII production was sent to England but Russia also received a significant share of the AR-88 receivers through Lend-Lease. Nearly all of the early WWII versions of the AR-88 are found in England with some turning up in Europe also. Russia also kept their AR-88s for quite a long time after the war ended and eventually sold them surplus to Russian hams. In the USA, RCA redesigned the AR-88 with various upgrades, especially to the Crystal Filter, for post-war commercial users. This version was designated CR-88.

The CR-88 is the standard version of the receiver with a carrier level meter installed. The CR-88A is the diversity version of the receiver with no carrier level meter (it's on the meter panel of the diversity rack) and with an IF GAIN control on the front panel to allow balancing the output of three CR-88A receivers operating in diversity. Very late in production (1953) the CR-88B was introduced but only a handful were ever built. It featured a significantly redesigned chassis, push-pull audio output and moderately redesigned front panel.

In 1950, the Signal Corps designed a triple-diversity receiver, the OA-58 that used a new version of the old AR-88, the SC-88. This receiver had a band-in-use dial mask, the chassis front end alignment adjustments were relocated and the panel was painted black wrinkle finish but otherwise, the SC-88 is very similar to the CR-88A. Only about 300 SC-88s were produced. Most that turn up today were sold by Fair Radio Sales as surplus in the late-1960s.

For more details on the entire AR-88 Series of receivers including history, circuit analysis, restoration, sweep alignment information and much more, go to "RCA's Amazing AR-88 Receiver - in Four Parts" use Home Index below for navigation.


Wickes Engineering & Construction Co. - Hammarlund Mfg.Co., Inc.  - R-270/FRR

Post-WWII modification of the BC-794 Super-Pro for the Signal Corps

A constant level signal, free from fading, was necessary for accurate copy of RTTY (Radio Teletype) signals. During WWII, RTTY was being used more and more by the Signal Corps. After WWII ended, the SC continued to develop better RTTY systems. Diversity reception would greatly reduce fading radio signals and provide the nearly constant signal level to the RTTY converter that would allow accurate copy. The diversity system would use widely separated antennas to respond to the different phases of the radio signal at different locations (called Space Diversity) and then the receivers would interact to provide a level of signal reproduction that was constant and based on which antenna-receiver combination was providing the strongest signal. The Signal Corps had different types of WWII receivers modified to allow their use in diversity RTTY systems. Early systems used modified BC-342 receivers. By 1947, the Signal Corps was using modified Hammarlund Super-Pro receivers. The initial modification was to improve frequency stability by installing MC-531, a kit that incorporated a three channel, crystal controlled oscillator to the Super-Pro. Other modifications required access to the IF output in order to drive the CV-31 Diversity RTTY Converter. When the modifications were finalized, the Signal Corps had Wickes Engineering & Construction Company professionally modify several BC-794 Super Pro receivers (designated as R-270/FRR) which would then be installed into the dual diversity receiver, AN/FRR-12. The AN/FRR-12 would interface with a CV-31A Diversity RTTY converter to drive the TTY machine.

The Wickes R-270/FRR changed some of the tubes in the BC-794 as part of the upgrade. The 6H6 AVC rectifier was changed to a 6SN7 to allow the use of one section (diode connected triode) as the AVC rectifier and the remaining triode to be used as an IF output buffer stage. The MC-531 kit added a 6SC7 crystal oscillator to the circuit. Additionally, the BFO could be crystal controlled also, if necessary, for precise reception of RTTY signals. A new aluminum front panel was installed with raised lettering and the steel bottom plate was replaced with an aluminum plate with the receiver schematic printed on the inside. The RA-74 power supply also got a new aluminum front panel and aluminum bottom cover with the schematic printed on the inside. The Crystal Oscillator front panel was installed over the Main Tuning dial escutcheon providing an ON/OFF switch with three channel selection and a separate frequency vernier control. All chassis component nomenclature was redone and the entire chassis given a heavy coating of MFP.

In 1951, Hammarlund released the SP-600 JX receiver that incorporated all of the Signal Corps upgrades along with totally redesigning the entire Super-Pro receiver. However, the Signal Corps was also beginning to use the Collins 51J receivers in their RTTY communications. Ultimately, the SC used Collins receivers for RTTY (in most instances) and the SP-600 for general surveillance. 


1953 Hammarlund SP-600-25C

Hammarlund Manufacturing Co., Inc.  -  SP-600 Series

Officially introduced in 1950 and with production starting in late-1951, the SP-600 was intended for the military and commercial user market. It was a very popular receiver even though the selling price was nearly $1000, but many thousands were built, especially for military applications. It's known that much of the SP-600 design input came from the U.S. Army Signal Corps, especially the selectable crystal oscillator and the turret band switching sections. The Signal Corps did have some WWII-version Super-Pro receivers modified with three-channel crystal oscillators in 1947-49 (R-270/FRR receivers, also other Super-Pro receivers with Improvement Kit MC-531 installed - see R-270/FRR section above.) The Signal Corps must have been working with other companies to find acceptable receiver manufacturers for their requirements because, in 1949, the Signal Corps contracted with Hallicrafters to build a Super-Pro "type of receiver" that met their design requirements. The Hallicrafters receiver was designated R-274/FRR with its first contract being issued in 1949. The SP-600 was first ordered in a 1950 contract that was for a R-483 receiver (JX-5.) The first R-274A Hammarlund version was on a 1951 contract (JX-1) and this was the first SP-600 officially produced in November 1951. Hammarlund used R-274A and C designations for Signal Corps receivers and R-274B for USN receivers. Hallicrafters versions was assigned the suffix D on the next contract (1952.) Eventually, the Hammarlund SP-600 was the choice of the military who ordered tens of thousands of them over the next decade while Hallicrafters' version was not ordered after only two contracts (1949 and 1952.) It's estimated that ultimately a total of around 150,000 SP-600 receivers were produced from 1951 up until 1972.

Though most Hammarlund SP-600 versions were built throughout the 1950s, the SP-600 continued to be produced in smaller numbers up into the early 1970s. The standard SP-600 tunes from .54 to 54MC in six bands. A "J" suffix indicates JAN parts were used in the construction and an "X" suffix indicates a selectable crystal oscillator for maximum stability as the LO. Hammarlund also offered a "JL" version with 100-400KC substituted for the .54-1.35MC band and a "VLF" version that covered 10-540KC (details on the SP-600VLF version in photo caption below.) Hammarlund made over 40 variations that were assigned a numerical suffix which identified the particular circuit, mechanical changes or sometimes the end-user. The last in the "time-line" was the model variation SP-600 JX-21A from 1969-1972 which utilized a product detector circuit, two additional tubes, different knobs and some other changes to make it "compatible" with SSB operations.

Most versions use a 20 tube double conversion superheterodyne circuit with a rotating turret bandswitch. The receivers also feature enormous proportions, robust construction and oversize controls - along with a super-smooth tuning system that only adds to the enjoyment of operating these fine receivers. Double conversion is switched in above 7.4MC and uses a crystal controlled conversion oscillator. Though the SP-600 has two dials, it has no bandspread - the right side dial is a logging scale allowing precise retuning of desired stations. On the left is the main tuning dial and the mechanically articulated dial pointer that indicates which tuning scale is in use (along with the small window between the dials that shows which tuning range is selected.) The tuning arrangement was an up-dated version of the "Continuous Bandspread" system introduced in RCA's AR-88 series receivers in the 1940s. The frequency readout accuracy is vague which is why a precise logging scale system is incorporated into the SP-600 design. The meter is not illuminated and a switch is provided to indicate either carrier level or audio output. Most (but not all) SP-600 model numbers usually will have a suffix with "J" or "JX" followed by a numeral. As mentioned above, suffix "J" indicated that, as much as possible, military level components and construction were used. Suffix "X" indicated that a selectable six-position, fixed-frequency crystal-controlled oscillator was installed that allowed the user to install HC-6/U type crystals for specific desired LO frequencies. The VFO position allowed the receiver to operate with the standard LO while the positions 1 to 6 turned off the LO and turned on the Crystal Oscillator while allowing selection of any of the six crystal-controlled frequencies. Although the user could switch to any of the crystal LO frequencies for increased stability for that particular frequency, the receiver still has to be "tuned" to the desired frequency for the RF and Mixer stages to be in tune.

Many SP-600 receivers were set-up for diversity operation and the standard diversity model was the JX-17 version. This version was produced in large numbers and can be easily spotted by observing that it has two extra controls and uses three red colored knobs. The SP-600 Audio output is about 2 watts from a single 6V6 using a balanced split-winding audio output transformer for 600 ohms Z. The audio quality from a rebuilt SP-600 is communications-grade audio with the lower end rolled off at 125Hz 3db down. This audio shaping, while noticeably lacking bass response, was designed into the SP-600 to allow excellent copy in all modes whether it be CW, RTTY (or other data modes) along with greater intelligibility of weak signals in voice modes (either AM or SSB.)

The number following the letter suffix generally indicates specific features for that version, e.g., contract or end user, circuit upgrades, etc., with the number ranges being more or less chronological until the last of production. Though the number suffixes were more or less chronologically assigned, many of the versions were built over a fairly long time period. This meant that engineering and component changes were being added as receiver production continued. The end result today is that there are early and later versions of many of the numbered suffix models and documentation is not always specifically accurate based just on the number suffix. It is more accurate to use the build date of the receiver and use documentation that is dated close to the receiver manufacture date.

SP-600 VLF-31  -  Shown in the photo to the left is the SP-600VLF-31 receiver from 1955. This version covers 540kc down to 10kc in six tuning ranges and uses 21 tubes. The SP-600VLF is NOT a standard SP-600 with LF coils installed in the turret. The VLF version  uses very different circuitry but with the mechanics and general construction being standard Hammarlund SP-600. The circuit of the SP-600VLF uses a 705kc IF and is a single conversion receiver. Double pre-selection is employed on all bands (two TRF amplifiers.) The "X" option crystal oscillator provides four channels (instead of six) and uses FT-243 type crystals. Five selectivity positions are provided (instead of six.) The 455kc IF output is accomplished by mixing the 705kc IF with a 1160kc crystal oscillator. The 455kc IF was for driving data devices such as RTTY converters. The chassis and the side panels are iridite finish and the side panels are similar to those used on the R-390A receiver. Since the VLF receivers are from the late-fifties, ceramic disk capacitors are used throughout the circuit. 

The SP-600VLF provides tremendous sensitivity and requires a "tuned" antenna for it to operate at its full potential. Generally, a "tuned loop" antenna will greatly reduce the RFI noise that is very high below 500kc and it can also provide directivity that may also help in noise reduction. Nearly all signals in the LF spectrum are now digital data signals or various types of beacons with virtually no voice signals at all.

Best results using the SP-600VLF will require an "RF-quite" location, a tuned loop antenna or a shielded magnetic loop of reasonable size (minimum 3' diameter) and using a headset for the audio output since many signals are "in the noise" and barely detectable.

The Tubular Capacitor Problem - All early versions of the SP-600 receivers were built using molded tubular paper dielectric capacitors of various manufacture - Cornell-Dubilier (most common) and Sprague (sometimes) are the types encountered. Nearly all molded capacitors are defective nowadays, requiring extensive replacement work when rebuilding an SP-600. In fact, it's quite common to find a few burned resistors in an un-rebuilt SP-600 due to leaky or shorted molded tubular capacitors. Later versions had much more reliable ceramic-disk type capacitors installed rather than the problem-prone molded capacitors. All early SP-600s will require a rebuild for the receiver to operate at the high level of performance that it is capable of. Molded capacitor replacement requires some major disassembly of the various units in the receiver. The turret bandswitching assembly has 6 capacitors inside, the RF platform has 20 capacitors inside, the IF transformers have 1 or 2 capacitors inside, T1 has 1 capacitor inside and the conversion crystal oscillator has 3 capacitors inside - all these units have to be partially disassembled to access these molded capacitors that need to be replaced. The JX versions will have the switchable crystal oscillator that also needs rebuilding (2 molded caps inside.) Additionally, there are many other molded capacitors under the chassis. Most SP-600s will have over 50 capacitors that will need replacement - a challenging task but absolutely necessary and well worth the effort required. After a rebuild, the SP-600 will need a full IF-RF alignment for a performance level that meets or exceeds original specifications. The decision of whether or not to rebuild an early SP-600 is not really an option - all early SP-600 receivers need to be rebuilt for safe and proper operation.

For more details on rebuilding the Hammarlund SP-600 receiver, read our article - "Rebuilding the Hammarlund SP-600" - use Home/Index for navigation

photo right: 1953 Hammarlund SP-600 JX-21


Signal Corps/Hallicrafters R-274/FRR sn: 762

 Signal Corps U.S. Army/The Hallicrafters Co.  -  R-274/FRR, R-274D/FRR (aka: SX-73)

In the post-WWII era, the U.S. Army Signal Corps needed a source of high-quality, frequency-stable receiver that had specific Signal Corps design requirements. The main features were a selectable crystal oscillator to virtually eliminate frequency drift in the LO and BFO for reliable RTTY reception. A rotating turret band switch was required for ease of maintenance and eliminating the conventional, problem-prone band switch. Both Hammarlund and Hallicrafters built successful versions of these types of receivers. Although WWII Hammarlund Super-Pro receivers had been modified by the Signal Corps to have the selectable crystal oscillator in 1947 through 1949, the quantity of available receivers was limited. The Signal Corps needed a manufacturer that could deliver a new receiver with the specified options in fairly large quantities. It's fairly obvious that most of the R-274 is a Signal Corps design and that Hallicrafters was the contractor chosen to build the receivers. The first R-274 receivers were built on a 1949 contract. It's very probable that Hammarlund had also been working closely with the Signal Corps and the SP-600 was their combined effort. Hammarlund had been advertising their SP-600 since 1948 (as the SPC-600-X) but apparently the receiver wasn't available until 1950. The Signal Corps didn't want to assign a new designation to a similar receiver (since they were so closely involved with the design of both receivers) so the Hammarlund SP-600 was assigned R-274 with suffixes A, B or C used for specific identification. Future R-274 receivers contracted to Hallicrafters would be designated as R-274D.

The typical post-WWII military contract production quantity was probably relatively small, perhaps 1000 receivers per contract. It doesn't seem likely that Hallicrafters would have gone through the effort for one contract. Hallicrafters probably thought there would be many more contracts in the future but that doesn't seem to be the case. The Signal Corps obviously favored the Hammarlund version and literally tens of thousands of SP-600s were supplied to the Signal Corps over the next decade. Since Hallicrafters had invested in some production line tooling and had obviously set up component suppliers for production, they decided there might also be a commercial or even a ham market for their receiver. The civilian designation assigned was SX-73 and these receivers are virtually identical to the R-274D except for the ID tag, which shows "SX-73" as the receiver type. Some advertising mentions that a cabinet was supplied with the SX-73 though advertising artwork generally shows the receiver in the rack mount configuration. Selling price was quite high at $975 which certainly limited purchases of the SX-73 by the civilian market. The SX-73 version is seldom seen and production must have be very limited. The R-274D and SX-73 were available from 1952 up to around early-1954.

The R-274/SX73 is generally referred to as "Hallicrafters' version of the SP-600" or the "Hallicrafters' Super-Pro" since there are so many similarities between the two receivers. The similarities are to be expected since both receivers were "designed" with major influence of the Signal Corps and both receivers had to meet Signal Corps' design specifications. The most obvious similarity is the turret band switch which, while functionally the same at the SP-600's, is not nearly so robust in construction using heavy-duty plastic module bases with stub pins pressing against flex contacts. The SP-600 coils are on a ceramic base with longer pins passing thru dual pinch contacts. The tuning dial provides a main dial and a logging dial as the SP-600 does but behind a single escutcheon rather than separate dials behind two escutcheons as the SP-600 does. There is a selectable six-channel Crystal Oscillator that functions like the SP-600 "X" option and provides improved stability for RTTY and other data modes. Like the SP-600, the bandwidth is selectable in six selectivity steps with three of those steps using a Crystal Filter for narrow bandwidth (a front panel Phasing control is also provided.) A 600 ohm balanced audio output is also similar to the SP-600 audio output.

One major difference between the R-274/SX73 and the SP-600 circuit is the conversion frequency of the SP-600 is 3.955mc while the R-274/SX73 uses 6.455mc. Also, the placement of the conversion frequency with reference to tuning range four has the double conversion starting at 7.0mc on the R-274/SX73 while it is 7.4mc on the SP-600. This results in double conversion being used for the 40 meter ham band on the R-274/SX73 but not on the SP-600.

The R-274/SX73 frequency coverage of each tuning range is beneficial to the ham user in that 160, 80, 40 and 20 meters are on separate tuning ranges while the SP-600 combines 80 meters at the low end and 40 meters at the high end on tuning range three. In the audio section of the R-274/SX73, the coupling capacitors are .01uf in the R-274/SX73 while the SP-600 uses .0015uf capacitors. This results in the "communications-grade audio" found in the SP-600 while the R-274/SX73 has a more conventional audio response. Additionally, the R-274/SX73 provides an Antenna Trim control while the SP-600 does not. Possibly the most important difference between the R-274/SX73 and the SP-600 is that the former receiver utilizes almost entirely ceramic disk capacitors in the circuit rather than the "leakage-prone" molded capacitors that have negatively influenced the reliability and reputation of all early SP-600 receivers. In considering the restoration of the R-274/SX73, the ceramic capacitors will certainly and positively reduce the amount of rework that is going to be necessary.

Some of the components used in the R-274/SX73 are of a better quality than those found in the SP-600 - IF transformers and the bulk of the capacitors used, for example. But some other parts and components are not as high of quality as those found in the Hammarlund - band switch turret, the dial gear train and the dial lock, for instance. The R-274/SX73 tuning condenser bearings are of a very poor quality and can rust excessively in a humid environment which can cause "sticking" and "jamming" of the tuning condenser's rotation. The R-274/SX73 tuning dial itself along with the logging dial are difficult to read (some users find the same fault with the SP-600) and the R-274/SX73 dial illumination is subtle (dim really) while the SP-600 dial illumination is dazzling. The Carrier Level meter has only a Decibel scale that references 0db as mid-scale on the meter which is equal to 50uv input signal level. Performance is the final judgment though and the R-274/SX73 will easily provide the user the same high sensitivity and quality reception as the SP-600 along with much better sounding audio reproduction.

It's obvious to R-274 owners who have taken the time to carefully examine their receivers that Hallicrafters' engineers did not design the R-274. It's a product of Signal Corps engineering with Hallicrafters manufacturing the R-274 to Signal Corps specifications.

photo above: The R-274 mounted in its military table-top cabinet, the CY-699/FRR


MIL-COMM RADIO GEAR - Part Two                                           Return to Home Index



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