Vintage Longwave Receivers
by: Henry Rogers WA7YBS
Radio Towers and Station House at Arlington, VA ca: 1920
Information in Part 1
Restoration and Performance
Testing the Following Vintage Longwave Receivers
1. IP-501-A - RCA/Wireless Specialty Apparatus Co. - 1923
2. Type 105-A - Mackay Radio & Telegraph Co. - 1932
3. RAA-3 - USN-RCA Manufacturing Co.,Inc. - 1931 (1935 ver.)
4. RAG-1 - USN-Hygrade Sylvania Corp. - 1933
5. RIO - DOC-National Co.,Inc. - 1933
6. SP-100LX Super-Pro - Hammarlund Mfg. Co.,Inc - 1938
7. RAZ-1, CRM-46092 - USN-Radiomarine Corp. - 1941
8. AR-8510 - USN-Radiomarine Corp. - 1944
9. RAK-7, CND-46155 - USN-RCA-Andrea Radio Co. - 1944
Information in Part 4
What to Listen to on LW
NDBs, LW Stations, VLF Stations
SAQ 17.2kc Alexanderson Alternator
Loop Antennas, Long Wire Antennas
Operating Vintage Gear on the 630M
(472-9 khz) Band with Log incl'd
NBD Stations in Nevada & NDB Station Log
Vintage Longwave Receivers - Part 1
Testing Classic Vintage Longwave
IP-501-A - MW & LF Receiver-Amplifier
Commercial Shipboard Receiver from 1923
40kc to 1000kc
"Listening on longwave with a 1923, battery operated, regenerative receiver? You gotta be kidding!"
One has to remember, the IP-501-A was the commercial shipboard receiver that was built to the highest standards of the day. It was well-known for its superior performance and reliability. It is the "R-390" of the 1920s.
The initial versions of this receiver were built at Wireless Specialty Apparatus, a company that was owned by United Fruit Company. UFC was a member of the cross-licensing "Radio Group" headed by General Electric and included Westinghouse, AT&T, RCA and the United Fruit Company. WSA, through its original founder, Greenleaf Pickard, owned all of the crystal detector patents at the time (John Firth was also an original founder of WSA.) WSA built a few broadcast radios for RCA in 1921 and 1922 but by 1923 they had become part of RCA. For the next few years, RCA continued to have the shipboard radio business of WSA operate using the WSA plant and using WSA components to build shipboard equipment. In 1927, RCA combined WSA with Independent Wireless Company and that organization became Radiomarine Corporation of America, a division of RCA. Radiomarine continued to build the IP-501-A up into the early thirties. Manuals for the IP-501-A were available from RMCA into the late-thirties. Removal of the IP-501-A from shipboard use probably started just prior to WWII since the receiver's regenerative detector easily coupled to the antenna and radiated the oscillating detector quite well. Ships often could pick-up an oscillating IP-501-A as far as five miles away (at sea.)
This three tube receiver uses a three-circuit tuner with a regenerative detector and two transformer coupled audio frequency amplifier stages - not exactly unheard of for a lot of radio receivers in 1923. What really sets the IP-501-A apart from the other three-circuit tuner regen sets is its incredible Antenna Tuner section that is entirely shielded from the main part of the receiver (which is also entirely shielded.) The Antenna Tuner allows exact tuning of the antenna's impedance so the load remains the same on the Secondary circuit. It's like having a built-in pre-selector. The only transference of signal happens by way of the small variable coupling coil located inside the Antenna coil. The fact that the receiver cabinet and front panel are entirely shielded results in no hand-capacity effects when the receiver is operated as an autodyne detector. This makes tuning CW super-easy. The Secondary Tuner has six frequency ranges from 1000kc down to 40kc and the dial is calibrated in meters. The Tickler coil is actually a variometer built into the Secondary coil form and includes load windings from the Secondary inductance to improve regeneration on the lower frequencies. The audio amplifier section is standard and uses two RCA interstage transformers. The audio gain is more-or-less controlled by the filament voltage and the operator can also select how much gain is required by using one of the phone jack outputs. The phone jacks also control the filament voltage to the tubes and only the tubes needed are in operation when that jack is selected. Maximum audio is from the AF2 jack which provides Det + 2 AF stages. In high noise level conditions or for very loud signals, AF1 saves the operator's ears by using just one audio amplifier. If the DET jack is used, only the detector tube is in operation - this would be for receiving local transmissions. Intended audio output is to Hi-Z earphones but the IP-501-A will drive a horn speaker loudly from the AF2 jack. To power the receiver up requires 6vdc at .75A for the filaments, 45vdc and 90vdc for the B+ requirements and -4.5 for C bias. The filament adjustment pot controls the A battery into the receiver and is used to turn off the receiver. Pulling the phone plug from one of the jacks will turn off the tubes but the meter will still show A battery voltage unless the filament pot is turned off. The tubes normally used in the IP-501-A were UX/UV-201A triodes. Operating any radio receiver that uses batteries for its power source can be a hassle and expensive unless you are all ready set-up to run battery receivers. Usually highly-filtered power supplies provide "close to pure" DC voltages to operate these types of receivers. I use a Lambda 6vdc 4A power supply for the A supply, a 1920s RCA Rectron B Eliminator for the B supply and a 4.5vdc battery for the C bias. Hi-Z earphones are necessary for the audio output and I generally us a set of 2200 ohms dc, Western Electric 518W 'phones. The IP-501-A also requires a fairly large antenna worked against a true earth ground for best performance.
||In operation, the filaments are set to about 4.5 to 5.0vdc using the panel meter as reference. Tuning is accomplished with the Secondary Condenser and then "peaking" the signal with the Antenna Condenser. Sensitivity is controlled by use of the Tickler. Since an adjustable resonance and load can be controlled by the Antenna Condenser control, the Tickler control can be set to one position and doesn't require too much adjustment per each tuning range. Selectivity is controlled by the Coupling control. Changing the settings of any of the controls will always cause an interaction in any regen set when it is used as an Autodyne Detector (oscillating regenerative detector.) When the IP-501-A is used as a three-circuit tuner with Autodyne Detector, the Coupling control must be set to "Critical Coupling" for best performance. This requires the operator to tune through the Antenna Condenser's resonance while listening for a "double-click" (and for the oscillating to stop.) If the clicks are heard, this indicates too much coupling. Continue to loosen the coupling and retune the Antenna Condenser until no clicks are heard at resonance. Now the Coupling is set properly. Large changes in tuned frequency will require minor adjustments to the Coupling setting. All tuning can usually be accomplished using just the Secondary Condenser control for tuning stations and then using the Antenna Condenser for adjusting the signal to maximum. Now and again you will have to slightly re-adjust the Tickler. For tuning in NDBs, the IP-501-A should be operated as an Autodyne Detector receiver. This provides a heterodyne so the NDB carrier can be easily heard. Regenerative detectors can become unstable at the oscillation point and good construction helps to stabilize the regeneration. The IP-501-A is very stable and easy to operate in the Autodyne set-up since that was one of its intended uses - to copy the CW from arc transmitters.|
I have had this IP-501-A since 1979. A ham friend sold it to me after he had traded a telephone pole for it. I have performed three restorations on the set over the years. The last one in 1984 brought the IP-501A back to full original configuration and appearance internally and very good restored condition externally. I used the receiver back in the 1980s with a 125' EFW antenna and tuned in all the normal AM BC stations one would expect. As far as Airport Non-Directional Beacons (NDB,) the only one I remember tuning in was SPK 251kc, located at the old Reno-Cannon AP. I remember SPK because they used to transmit voice weather with the MCW ID "SPK" in the background. I really didn't know how to get a lot of performance out of the IP-501-A back then. The AM BC performance was fine but listening to AM BC over a horn speaker gets boring after awhile. When I opened the museum in 1994, the IP-501-A was installed in a display case and it stayed in the case for almost 15 years. Lately, I had been thinking about trying something different, as a challenge to the performance capabilities of early regenerative receivers. Since the IP-501-A was the commercial receiver of choice in difficult environments and it had every indication of being the "best" of its day, I decided to give it a try. I used my ham antenna, a 135' tuned dipole, but with the feedline shorted. This would provide a vertical with large capacity hat configuration similar to the large "T" antennas of the twenties. Our initial tests turned up a small problem with the IP-501-A's circuit selector switch. We had no detector plate voltage but it was just a bad contact that needed a bit of cleaning and we were up and operating,...sort of. Lack of audio output was another easy fix. The bias SS power supply had failed and was at -25vdc, definitely in the cut-off region for UX-201As! I sub'd a battery for the bias and then the IP-501-A sprang to life. Before power-up, I had tuned the receiver to around 800 meters as a pre-set and, to my complete surprise, SX 367kc in Cranbrook, BC, Canada was coming in (this was at about 5PM local time in December.) I tuned in a few more NDBs and then decided to wait until about 10PM and try again. At 10PM, I received around 25 more NDBs tuning from 326kc up to 414kc. Best DX was the 2KW transatlantic beacon DDP 391kc in San Juan, Puerto Rico.
|IP-501-A NDB Log - 2009 - The following is the log of the NDBs copied using just the IP-501-A receiver and the 135' tuned dipole antenna with the feedline shorted. NDB location, frequency and power (if know) are listed. Total was 103 NDBs copied in a three-week period in January 2009.|
|AA - 365kc - Fargo, ND - 100W
AEC - 209kc - Base Camp, NV
AOP - 290kc - Rock Springs, WY
AP - 260kc - Denver, CO - 100W
AZC - 403kc - Colorado City, AZ
BKU - 344kc - Baker, MT - 80W
BO- 359kc - Bosie, ID - 400W
CII - 269kc - Choteau, MT - 50W
CNP - 383kc - Chappell, NE - 25W
CSB - 389kc - Cambridge, NE - 25W*
CVP - 335kc - St. Helena, MT - 150W
DC - 326kc - Princeton, BC, CAN
DDP - 391kc - San Juan, Puerto Rico - 2KW
DPG - 284kc - Dugway Proving Gnds, UT
DQ - 394kc - Dawson Creek, BC, CAN
EUR - 392kc - Eureka, MT - 100W
EX - 374kc - Kelowna, BC, CAN
FCH - 344kc - Fresno, CA - 400W
FN - 400kc - Ft. Collins, CO
FO - 250kc - Flin Flon, MB, CAN
GLS - 206kc - Galveston, TX - 2KW
GUY - 275kc - Guymon, OK - 25W
GW - 371kc - Kuujjuarapik, QC, CAN
HQG - 365kc - Hugoton, KS - 25W
IOM - 363kc - McCall, ID - 25W
ITU - 371kc - Great Falls, MT - 100W
IY - 417kc - Charles City, IA - 25W
JW - 388kc - Pigeon Lake, AB, CAN
LBH - 332kc - Portland, OR - 150W
LFA - 347kc - Klamath Falls, OR
LV - 374kc - Livermore, CA - 25W
LW - 257kc - Kelowna, BC, CAN
LYI - 414kc - Libby, MT - 25W
MA - 326kc - Midland, TX - 400W
MEF - 373kc - Medford, OR
MF - 373kc - Rogue Valley, OR
MKR - 339kc - Glascow, MT - 50W
MLK - 272kc - Malta, MT - 25W
|MO - 367kc - Modesto, CA - 25W
MOG - 404kc - Montegue, CA - 150W
MR - 385kc - Monterey, CA
NO - 351kc - Reno, NV - 25W
NY - 350kc - Enderby, BC, CAN
ON - 356kc - Okanagan, Penticton, BC, CAN*
OT - 378kc - Bend, OR
PBT - 338kc - Red Bluff, CA - 400W
PI - 383kc - Tyhee, ID
PN - 360kc - Port Menier, Anticosti Is., QC, CAN*
PTT - 356kc - Pratt, KS - 25W*
QD - 284kc - The Pas, MB, CAN
QQ - 400kc - Comox, BC, CAN
QT - 332kc - Thunder Bay, ON, CAN
RD - 411kc - Redmond, OR - 400W
RPB - 414kc - Belleville, KS
RPX - 362kc - Roundup, MT - 25W
RYN - 338kc - Tucson, AZ - 400W
SAA - 266kc - Saratoga, WY - 25W
SB - 397kc - San Bernardino, CA - 25W
SBX - 347kc - Shelby, MT - 25W
SIR - 368kc - Sinclair, WY
SX - 367kc - Cranbrook, BC, CAN
SYF - 386kc - St. Francis, KS - 25W
TAD - 329kc - Trinidad, CO
TV - 299kc - Turner Valley, AB, CAN
TVY - 371kc - Tooele, UT - 25W
ULS - 395kc - Ulysses, KS - 25W
VQ - 400kc - Alamosa, CO
VR - 266kc - Vancouver, BC, CAN
WG - 248kc - Winnepeg, MN, CAN
WL - 385kc - Williams Lake, BC, CAN
XD - 266kc - Edmonton, AB, CAN
XH - 332kc - Medicine Hat, AB, CAN
XS - 272kc - Prince George, BC, CAN
XX - 344kc - Abbotsford, BC, CAN
YAZ - 359kc - Tofino, Vancouver Is., BC, CAN
YBE - 379kc - Uranium City, SK, CAN
|YCD - 251kc - Nanaimo, BC, CAN
YHD - 413kc - Dryden, ON, CAN
YJQ - 325kc - Bella Bella, BC, CAN
YK - 269kc - Castlegar, BC, CAN
YKQ - 351kc - Waskaganish, QC, CAN*
YL - 395kc - Lynn Lake, MN, CAN
YLB - 272kc - Lac La Biche, AB, CAN
YLD - 335kc - Chapleau, ON, CAN
YLJ - 405kc - Meadow Lake, SK, CAN
YMW - 366kc - Maniwaki, QC, CAN*
YPH - 396kc - Inukjauk, QC, CAN
YPL - 382kc - Pickle Lake, ON, CAN
YPO - 401kc - Peawanuck, ON, CAN
YPW - 382kc - Powell River, BC, CAN
YQZ - 359kc - Quesnel, BC, CAN
YTL - 328kc - Big Trout Lake, ON, CAN
YWB - 389kc - West Bank, BC, CAN
YWP - 355kc - Webequie, ON, CAN
YY - 340kc - Mont Joli, QC, CAN
YYF - 290kc - Penticton, BC, CAN
YZH - 343kc - Slave Lake, AB, CAN
ZP - 368kc - Sandspit, QC IS., BC, CAN
ZSJ - 258kc - Sandy Lake, ON, CAN
ZSS - 397kc Yellowhead/Saskatoon, SK, CAN
ZU - 338kc - Whitecourt, BC, CAN
Z7 - 408kc - Claresholm, AB, CAN
3Z - 388kc - Taber, AL, CAN*
* = Newly NDB heard
Mackay Radio & Telegraph Company
Radio Receiver Type 105-A
Serial No. 32081
Commercial Shipboard Receiver from 1932
16kc to 1500kc
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 J. 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. Mackay Communications is still in business, located in North Carolina.
|The Type 105-A is a pre-WWII commercial shipboard receiver that
dates from after the Federal Telegraph move to New Jersey since
the ID tag lists Newark, N.J. as FTC's location. Later Mackay radios
incorporate the year of manufacture into the first two digits of the
serial number. It looks like this is also the case with the Type 105-A
and, with the serial number 32081, this receiver was built in 1932. The
circuit uses four tubes that were originally four-pin triode tubes with
Either 201-A tubes (5vdc to 6vdc filament voltage) or type 30 tubes
(2vdc to 2.5vdc filament voltage) are specified in the Mackay 105-A brochure.
However, this receiver was modified in the past to use five-pin cathode-type tubes. It's 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
16kc in seven tuning ranges. Power was supplied by batteries. Like
earlier designs for shipboard receivers, 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
front panel, the cabinet (if I had one) and most of the shield panels
are made from cadmium-plated brass. The cabinet and panel was painted
Mackay Gray. Originally, sn: 32081 could have
been in a metal cabinet (as shown in the sepia photo above) but the
Mackay brochure also indicates that the receiver could be supplied
without the cabinet for panel mounting, possibly in one of the Mackay Marine Radio Units that would have housed the majority of the radio gear for the ship.
The installation shown below (S.S. Manhattan) shows the various Mackay
equipment mounted in their individual cabinets. It's also possible that
when the receiver was rebuilt to use five-pin tubes the cabinet was
discarded then to allow for panel mounting.
photo right: Mackay Type 105-A sn: 32081
photo above: This is the radio room onboard the S.S. Manhattan, ca. 1938, entirely equipped with Mackay Radio and Telegraph Company gear. The Type 105-A receivers are flanking the central transmitter in the photo. The receiver to the right of the "right-side" Type-105-A is a shortwave receiver, the Type 104-B. This photo is from the frontispiece of Sterling's THE RADIO MANUAL, 3rd Edition, 1938.
photo right: The underside of the Mackay 105-A. The lower coils are the Antenna Tuning coils and the upper coils are the Detector Tuning coils.
This Type 105-A was an eBay find that was purchased in October 2009. The receiver has vintage modifications that were probably installed during its life as a "shipboard receiver." The original concept appears to have been designed for exclusive DC operation. The Filament control has been bypassed since cathode tubes were now being used and since cathode type tubes are used, AC could be supplied to the tube heaters. However, AC voltage won't read on the panel meter since it doesn't have an internal rectifier - also the internal series resistor is burnt out for the B+ section of the meter. Additionally, there was a DC voltage input filter on the filament line that has been bypassed. I have examined this Type 105-A carefully and the five pin tube sockets are not original but the rework looks vintage. Several years ago, there were many small marine radio equipment companies that were usually located near shipyards that specialized in repair, rebuilding or upgrading and selling used shipboard radio equipment. Most likely, this Type 105-A was modified by such a business. The good news is that this Type 105-A is a working receiver. It operates very much like the IP-501-A in that the position of the regeneration control is dependent on how you set-up the Antenna Tuning. Though there is no coupling control, the interaction between the Antenna Tuning and Regeneration does about the same thing as setting the "Critical Coupling" on the earlier IP-501-A. The Antenna Series Condenser switch compensates for use of a single antenna length and adds to the range of the Antenna Tuning. The Tone Control knocks down the static noise on the LF and VLF ranges. At first, I used an old Signal Corps power supply that provides 6.3vac and regulated 135vdc to power up the Type 105-A. Using the 135' Tuned Dipole antenna with the feed line shorted at the receiver antenna terminal, I was able to easily receive all of the usual longwave signals using WE 509W 'phones for the audio output. Some of the NDBs tuned in were MM 388kc from Fort McMurray, Alberta, ZP 368kc Sandspit, BC for best DX but also consider SYF 386kc, a 25W marker beacon in St. Francis, KS. The VLF reception included the Navy NSRTTY stations in Jim Creek, WA (24.8kc) and Cutler Maine (24.0kc.)
Update on Mackay Type 105-A Performance: The high noise level of the Type 105-A seemed to be limiting the reception of very weak signals. I finally decided to run the heaters on DC voltage which was a subtle change and hardly noticeable but very weak signal detection was improved. I was able to receive WG 248kc in Winnepeg, MB and RL 218kc in Red Lake, ON. Note that both of these NDBs are in the 200kc - 250kc part of the spectrum - a particularly noisy area. DC voltage on heaters does help on weak signal detection.
Additional Note on Set-up and Performance: I decided to try an entirely different DC power supply set up using a 6vdc 4A Lambda power supply for the tube heaters and a vintage B eliminator, the RCA Duo- Rectron, to test if the noise would be further reduced. The change was amazing! Apparently the old Signal Corps power supply wasn't filtered as well as the Duo-Rectron because now the MCW signals from NDBs have no distortion and the tone sounds like a good sine wave. Luckily, there happened to be a true CW station operating on 425kc during my test. This was probably an "events" type of operation of one of the old Point Reyes stations since the signal was very strong and was only "on the air" for about one hour. This CW also was very pure in tone. The operation and performance of the Mackay Type 105-A only seems to improve that closer one gets to operating it on pure DC (as original.) November 21, 2009
RCA Manufacturing Co., Inc.
Tuner - CRV-46034-A
10kc to 1000kc
|This is a teaser,...like a preview,...a way to let longwave enthusiasts know what's coming up pretty soon. I've been working on the 1935 RAA-3 since June 2017. I'm hoping to have the receiver operational someday but the electronic restoration work still necessary is formidable. However, the effort put forth on the mechanical and cosmetic restoration has changed the RAA-3 from a "corroding hulk" to an impressively enormous and beautiful Navy LW receiver. This 465 pound, over three feet wide, behemoth of a receiver has three sections,...the RF Tuner, the IF/AF Amplifier and the "not shown in the photo" Power Supply. The RAA was the first long wave superheterodyne built for the US Navy in 1931. This receiver is the RAA-3 from 1935. It has 14 tubes, 4 individual 2-stage IF amplifiers that are selected by the band switching and a lot more. This ultra-rare receiver was in absolutely horrible condition after having been stored outside for decades, wrapped-up in a tarp. However, it's electronic restoration is progressing and will be coming together in the near future. Go to "Navy Department - RCA RAA-3 Superheterodyne Long Wave Receiver" article that details the restoration process so far - use navigation Home Index at the bottom of this page.|
Hygrade Sylvania Corp.
CHS-46042 - Receiver
15kc to 600kc
TRF Receiver with Tracking BFO
|The Navy needed a less expensive alternative to the
gargantuan RAA receiver. A receiver that would be easier to fit into a
smaller ship's radio room. A receiver that was easier to maintain and
more reliable. The result was the RAG-1 and RAH-1 combination of
receivers that allowed frequencies from 15kc up to 23mc to be received.
The RAG-1 was the low frequency receiver covering 15kc up to 600kc.
There was only one contract for RAG-RAH receivers and it was from July
13, 1933. These receivers were used by the Navy from about 1934 up to
around 1940. The RAG-RAH receivers were removed from ships before WWII
because their lack of heavy-duty construction limited their reliability
for wartime use at sea.
Circuit Description - The RAG-1 Type CHS-46042 is an eight tube, TRF receiver that also employs a tracking BFO that is adjusted to always be 1kc higher than the receiver's tuned frequency. The tuning condenser is a "stacked" assembly with the lower five-gang air variable being the main tuning of the RF amplifier grid LC. The smaller upper five-gang tuning condenser adjusts the coupling between the RF plate on the stator and the next stage RF grid on the rotor. Since the RF amplifier grids can't be all connected together, the rotors are mounted on a Garolite shaft (a dense type of fiberglass.) The gang on both condensers that's closest to the front panel tunes the BFO as the main tuning is adjusted. The BFO is always operational since the RAG-1 is "primarily a CW receiver." The RAG-1 Sensitivity is controlled by varying the negative "grid bias" on the three RF amplifier tubes and that RF amplifier output is routed to a triode detector stage. The detector output is interstage transformer coupled to a tunable Bandpass audio filter, called "AUDIO TUNING" which allows the operator to adjust the resonant frequency of the filter thus "peaking" the desired CW frequency. There are two ranges selectable for 450hz to 750hz or from 750hz to 1300hz and a "wide band" position selected with the "OFF" position that removes the tunable audio filter from the circuit. The AUDIO TUNING adjustment mechanically varies the position a hinged section of the inductor's laminated iron core. The audio signal is then routed to a large Audio Bandpass Filter assembly that provides a narrowed bandwidth of 450hz to 1300hz with the center frequency optimized for CW at 800hz. The BP Filter output is interstage transformer coupled to the first audio amplifier and then the audio is RC coupled to the audio output. The audio line also has an adjustable bias-controlled AVC LEVEL control that acts like an output limiter to keep the receiver's audio output from over-driving the operator's ear with unexpected strong signals or static bursts. The AVC limiter tube is a full-wave rectifier that is on the audio line to chassis, acting something like a clipper limiter on the output of the first audio amplifier. The AVC can be switched OUT in quite reception conditions. The 600Z ohm audio output is available at the TEL jack on the front panel and is also routed out the power cable to connect to the Control Unit. >>>
>>> The tuning ranges are as follows:
Tuning Range 1 - 15kc to 38kc
Tuning Range 2 - 38kc to 95kc
Tuning Range 3 - 95kc to 240kc
Tuning Range 4 - 240kc to 600kc (actual top end is 650kc)
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 along with +180vdc for B+ and -55vdc bias voltage. The 80 rectifier required 5.0 vac which was supplied by the PS power transformer and the -55vdc bias voltage was probably obtained by connecting the center tap of the power transformer's HV winding to chassis through a wire wound resistor. This will result in a negative voltage present at the center tap that can be used for bias purposes. The 600Z audio output power is only 250mW implying that earphones were the intended reproducers to be used. As mentioned, the audio is also routed out the power cable at the back of the receiver for connection to the Control Unit CHS-23067 that allowed switched 'phone connections between the RAG-1 and its companion receiver, the RAH-1. Sensitivity is rated at an impressive 1uv to 4uv. Power Supply was identified as CHS-20032.
|RAG-1 Accessory Components - In addition to the RAG-1 receiver, the complete setup included a Control Unit CHS-23067, AC Power Supply CHS-20032 and four interconnecting cables (one cable is permanently connected to the receiver as its power cable.) The Control Unit was a device that allowed the radioman to control the basic operation of both the RAG-1 and the companion receiver, the RAH-1. The Catalog of Navy Radio Equipment isn't detailed on how the Control Unit works just stating that with the two receivers a total frequency coverage from 15kc up to 23mc. Generally, this allowed one radioman to "guard" two frequencies simultaneously. This implies that one Control Unit was provided for operation of the pair, RAG-1 and RAH-1. The RAG-1 was connected to the Control Unit via its 8 foot long power cable. The RAH-1 also was connected to the same Control Unit via its 8 foot cable. Also, a 16 foot long cable connected the Control Unit to the AC Power Supply which was capable of powering both receivers. It's possible that there were two 16 foot interconnecting cables from the Control Unit to the AC Power Supply each with its own input connections (the Catalog of Navy Radio Equipment isn't specific about this but shows two 16 foot cables in the parts list.) The Control Unit would be able to "switch on" the AC Power Supply which in turn powered up either RAG-1 or the RAH-1 or both using switches on the Control Unit. The RAG-1 front panel ON-OFF switch was only functional if the receiver was to be operated on batteries in an emergency situation. This probably also applied to the RAH-1. Besides the basic "power on" function, it's likely that the Control Unit also provided audio outputs from each receiver that could probably be switched between the RAG-1 and the RAH-1 to a single set of 'phones using a switch. This would allow one radio operator to easily "guard" two pre-set frequencies by just switching back and forth between the two receiver's audio outputs. Normally, these types of Control Units would be on located on the radioman's table thus the shorter receiver power cable of 8 feet length. The AC power cord from the PS to the AC line was 18 feet 8 inches long.|
Restoration - Sometime in RAG-1 Serial Number 1's past it was dropped from probably a high storage shelf (at least six feet off of a concrete floor.) This severely damaged the front panel and chassis. The receiver required extensive mechanical repairs. Additionally, the physical damage also extended to several of the circuit components that then also needed rebuilding. Beside three broken ceramic coil forms, the garolite shaft of the grid to plate tuned-coupling capacitor was broken. Electronically, the detector plate interstage coupling transformer had an open primary. Also, the circuitry had been slightly compromised by a minor modification and needed to be put back to original in order for the RAG-1 to function correctly. Compounding the restoration difficulty was the fact that absolutely no documentation exists for the RAG-1. No schematic, no manual, no photos,...nothing. The ONLY source of information is the somewhat brief description in the Navy Catalog of Radio Equipment (no photo in the Navy Catalog either.) The RAG-1 SN: 1 restoration is covered in great detail with lots of "in process" photos in our web-article "USN RAG-1 LW Receiver" use the Home/Index to navigate.
Powering the RAG-1 - I use a HP 712B power supply since this one piece of equipment can provide all of the voltages required by the RAG-1. I had to build an extension for the RAG-1 power cable since it had been cut to just 12" in length. The extension cable used seven pin Amphenol mating plug-socket and lengthened the power cable to five feet. The connections to the HP 712B are via the 5-way binding posts on its front panel. The voltages provided are 6.3vac at 10A capability (the RAG-1 only requires 2.5A,) +180vdc B+ is provided by the adjustable +HV supply (0 to +500vdc range) and the negative bias (-50vdc) is supplied by the adjustable 0 to -150vdc bias supply. Full voltage monitoring is easy with B+ and negative bias metered by the HP 712B and the 6.3vac monitored by the RAG-1 filament meter. B+ current has full-time monitoring on the HP 712B with the RAG-1 typically drawing 47mA while operating.
- There aren't very many RF Signal Generators that will provide a sine
wave output down to 15kc. While a Function Generator can be used, I've
found these typically don't have a very fine control of the frequency adjustment and many aren't very
stable when in the higher kilocycle range (high for a function generator.) One piece of vintage equipment that's really ideal for LW
alignments is the General Radio Type 1001-A RF Signal Generator. It's
very stable, frequency accuracy is very good, the attenuator is great
and, best of all, it can provide a RF signal down to 5kc. The
disadvantage is the GR-874 output connectors require General Radio
874 adaptors but they are easily available and make connections to the
Type 1001-A easy.
photo left: Under the chassis of the RAG-1 after restoration. The bottom cover is removed showing the extensive shielding. Of note is the huge Audio Bandpass Filter which is built on its own chassis and is comprised of six large solenoid coils and associated capacitors.
Alignment - I aligned the RAG-1 like I would any TRF receiver. I set the dial for 9.50 which is very near the high end of each tuning range. The RF signal generator was coupled through a 400pf capacitor to the antenna input. The generator was tuned until the the signal was heard in the receiver. There are five adjustments per band. The ANT/1RF stage, the 2RF stage, 3RF stage, DET stage and the BFO stage. Once the RF signal generator is heard, then starting at the ANT/1RF stage that coil's trimmer is adjusted for peak response. This is repeated moving forward in the stages except for the BFO. The BFO requires the signal to be tuned exactly with the BFO disabled, press the TEST OSC switch to disable the BFO. Once the signal is exactly tuned, the TEST OSC button is released and the BFO trimmer is adjusted to approximately 1kc higher than the tuned frequency. Check that the BFO is higher by adjusting signal generator 1kc lower in frequency and the BFO note should "zero beat" if adjusted correctly.
As part of the alignment, I also created a "Tuned Frequency to Dial Readout Correlation Chart." This allows finding where in the spectrum the RAG-1 is tuned since its dial readout is 0.00 to 10.00 reading two dials, one that's 0 to 100 and the other being 0 to 10. One rotation of the 100 dial equals 1 increment on the tens dial. Creating the tuning chart showed two minor tuning discrepancies. On band 1, the high end of the tuning range is 35kc at 9.90 on the dial and 38kc can't be tuned (on Band 1, it can be tuned on Band 2.) This error is just about 10% which was probably within spec. On band 4, the high end of the tuning range extends to 650kc at 9.85, not 600kc, which may have been intentional in the design to give a slight extension to the high frequency coverage (it's still less than 10% off.) Otherwise, frequency coverage is just slightly extended above and below the ranges indicated on the front panel nomenclature.
Performance - There's no doubt that the Navy wanted a shipboard receiver that was easy to operate and while providing solid reception on all of the tuning ranges. All that's necessary to receive signals is to apply power, advance the SENSITIVITY until background noise is heard, peak the ANTENNA TRIMMER and then begin tuning in signals. If the noise level is high, the AUDIO TUNING can be used to enhance a specific heterodyne beat note, reducing noise and increasing selectivity. If there are static crashes, the AVC can be switched on and the AVC LEVEL adjusted to reduce the static noise. The RAG-1 is a fine receiver that is easy to operate. Performance is especially good on VLF when using the AUDIO TUNING. Noise is greatly reduced and the increased selectivity allows separating the USN VLF MSK stations, even NML 25.2kc from NLK 24.8kc (only 0.4kc separation.) AUDIO TUNING also helps with WWVB and JJY, both transmitting pulse encoded signals. NDBs are easier to tune without the AUDIO TUNING on. With the BFO operating 1kc above the tuned signal, the NDB morse ID will be heard slightly "off zero" which actually makes the MCW signal seem stronger and easier to copy. Below is a log of some of the signals that the RAG-1 received during testing in late-May 2020.
|The RAG-1 was powered by a HP 712B power supply
providing 6.4vac tube heaters, +180vdc B+ and -55vdc bias. The
antenna was a 275' "T" wire antenna. Reproducers were 600Z ohm
Daytime Test Reception:
NML - 25.2kc - LaMoure, ND - Very strong signal -
6.39 on RAG dial
All of these USN VLF MSK stations are very strong signals that are easy
to receive. HOLT is a bit more difficult but is still receivable. All
USN MSK stations transmit almost 24/7. Good reception of these
stations in this particularly noisy region of the spectrum required the use
of the AUDIO TUNING which greatly reduces noise and enhances a specific
audio frequency that increases the MSK tones significantly above the
noise. A really great feature of the RAG-1 for VLF reception. AVC was ON
but set to about 5 which only limits very strong pulse-type noise.
Tuning range 1, SENSITIVITY 2
WWVB - 60kc - Ft. Collins, CO - Very strong signal - 5.36
on RAG dial
WWVB is easy to receive anywhere in the USA at anytime. Again, AUDIO TUNING is able to really enhance the pulse encoded signal of WWVB while greatly reducing noise. Tuning range 2, SENSITIVITY 4. JJY 40kc (Japan's PE Time Station) requires listening mornings just before sunrise. Rcv'd 0545 hrs May 28, 2020. JJY ID in Morse CW 15min and 45min after each hour.
MOG 404kc - Montegue, CA - 5.64 on RAG dial
Night Test Reception:
May 26, 2020 2150hrs to 2215hrs PDT
STATION-FREQ-QTH RAG DIAL
1. MOG 404kc - Montegue, CA -
occasional crashes, no wind, no weather fronts. Best reception for NDBs was with the AUDIO TUNING OFF, AVC ON and set
to 3 for the occasional crashes heard, Tuning Range 4, SENSITIVITY on 9.
AM-BC - Although several AM-BC stations can be received, the RAG-1 audio is very restricted by the Audio Bandpass Filter making voice reception difficult and music programming unlistenable. AM-BC is good for testing purposes only.
U.S. Department of
Commerce, Aeronautics Branch,
National Company, Inc.
Intermediate Frequency Receiver (LF and MW)
160kc to 630kc
TRF Receiver with Tracking BFO
- National Co., Inc. built their first contracted "airport" receivers in
1932. The first superheterodyne was designated as "RHM" and was part of a contract with the Department of Commerce, Aeronautics
Branch, Airways Division & Lighthouse Service. The DOC wanted to upgrade
all radio communications and navigation equipment at the many airports
that were already servicing Air Mail routes and were beginning to provide air travel and
other types of air services throughout
the USA. General Electric was contracted to build the ground
transmitters and Aircraft Radio Corp built the airborne equipment. National
was contracted to build the ground receivers. The RHM was a nine tube,
superhet with plug-in coils that covered 2.3mc to 14.8mc, a micrometer
Type-N tuning dial and all aluminum construction. The circuit used single preselection
and two IF amplifiers operating at 500kc. Three plug-in coils were
necessary for each of the five bands thus totaling 15 coils. Each
installation also included a Model 58C Monitor receiver, a GRDPU dual
power supply, coil rack and rack speaker all mounted in an open frame
About 100 RHM receivers were built to fulfill the initial contract. National wanted to benefit from the prestige of the government contract by selling these types of receivers to the ham and shortwave listener market. After installing a few upgrades the new receiver was released as the "AGS" in 1933. Frequency coverage of the AGS was from 1.5mc to 20.0mc. The IF remained at 500kc. In late-1933, the "Single Signal" AGS-X was introduced. This version had the Lamb Crystal Filter installed and also moved the BFO frequency adjustment to the front panel. The "Single Signal" AGS-X had other accessories such as band spread coil sets for 160, 80, 40 and 20 meters. By late-1934, 10 meter coil sets were also available. The high price of the AGS-X limited its market. National never produced any of the RHM-AGS line in any significantly large quantities.
As airport communications and airways navigation requirements evolved so did the National receivers that were being supplied for these services. The RHM was upgraded to the RHQ, a receiver that ganged the three plug-in coils into one assembly that allowed plugging in all three coils at once. In other services the RHQ was designated as the AGU. The RHQ-AGU frequency coverage was greatly reduced to specifically what the airports needed, 2.5mc to 6.5mc (only two coil sets were supplied.)
Not all airport operations were on HF and much of the Airways Navigation radio needs were on lower frequencies. National also provided an Intermediate Frequency receiver that covered 160kc up to 630kc and was designated as the RIO. The RIO wasn't a superhet, however. This receiver was three stages of TRF amplification followed by a detector and single stage audio amplifier. The circuit also provided a "tracking BFO" that utilized a section of the five-gang tuning condenser to allow the BFO to always track at the tuned frequency. An AVC tube controlled the TRF amplifiers grid bias dependent on signal strength from the detector tube. Only two tuning ranges are provided with the higher frequency range A covering 275kc up to 630kc and the lower frequency range B covering 160kc up to 330kc. The tuning ranges were selected by a panel switch. The RIO was powered by the same type GRDPU rack power supply that the RHM used or it could also be powered by the 5886 "dog house" power pack (as could all of the other receivers in the RHM/AGS family.). Like the RHQ/AGU receivers, the RIO used a Type BX tuning dial (an illuminated SW-3-style dial.) In some National advertising the RIO was also identified as the AGL receiver.
photo right: Airways Radio Range Station showing the National RHM receiver and, below it, the RIO receiver. Photo from Aeronautic Radio Bulletin No. 27 - DOC/BAC 1937
|Serial Number 3
- Details - This RIO receiver is virtually "all
original." Only one resistor, a cathode resistor on the second TRF
amplifier, had been replaced and one paper capacitor was changed. Both changes
were performed decades ago
based on the resistor style and the orange paper capacitor used for the repairs. As standard for the RHM/AGS
receivers, the original capacitors were built by Sprague in the black "postage stamp" style along with a few metal tub capacitors. Resistors are the standard
National-made types with the white ceramic body and hand-written values
(in blue ink ).
Tubes used are as follows: 1RF = 78, 2RF = 78, 3RF = 78, Det = 37, Audio Output = 89, AVC = 36, BFO = 36. The power pack will also account for a type 80 rectifier tube.
Unlike later USN longwave TRF receivers with a tracking BFO, rather than have the BFO set slightly higher than the tuned frequency (usually 1kc,) the RIO alignment instructions have the user set the BFO frequency to "zero beat" with the tuned frequency. This allowed the user to tune a ICW (Interrupted CW used a rotating chopper wheel to "break" the continuous wave at an audio frequency rate resulting in a modulated CW signal) or "true" MCW signal. The user would then zero the carrier frequency and the modulated CW would be heard somewhat normally. With this method of tuning it was much easier to find weak signals since the carrier was usually easily heard. The RIO uses an output transformer that is a fairly high impedance indicating that hi-z 'phones were probably the intended audio reproducers. If a loudspeaker was needed then either a hi-z armature-pin loudspeaker or a voice coil-type speaker with a matching transformer could be used.
When testing the tubes it was noted that every tube was a "U.S.N." tube.
The tubes had not been out of their sockets in many decades. This could
imply that this RIO was used by the Navy. Not surprisingly, all
of the tubes tested good. I also tested a few important components for
shorts or continuity and didn't find any problems. I used a National
5886 Dog House power pack (6.3vac and +180vdc) as a voltage source. I
connected a set of Navy Baldwin Type C 'phones for audio reproducers.
With power applied, the dial lamp came on and within about 20 seconds
audio was being received. I had the RIO on Band B at the top end so KPLY
on 630kc in Reno was coming in strong. All switches were noisy or
expected) but AM BC signals were being received. I later tried Band A with the BFO on and tuned in the DGPS
node on 314kc and it was coming in fairly
strong. Any switching to either Band A or B resulted in extreme "static"
in the 'phones. If the BFO is on then the AVC
must be turned off so any noisy contacts are routed thru the audio
system unattenuated - ouch!
Even through the RIO seemed to function, it really needed a thorough servicing, especially contact cleaning and alignment. To clean the tuning condenser and the bandswitch required removal of the tuning condenser shield and the bandswitch shield. DeOxit was brushed onto the contact surfaces and the component operated to work out the corrosion. I also lubricated the bearings on the tuning condenser to reduce the drag on the BX dial as much as possible. The improvement was dramatic. The bandswitch could be now operated without severe noise being generated and the BFO operation was stable. Tuning was light and didn't slip. The SPKR-TEL switch needed contact adjustment since in the SPKR position there was no connection to the output terminals. The switch was a cam-operated finger-contact type of switch that needed cleaning and a slight adjustment for a positive contact.
|photo left: The tuning condenser shield removed to show the
five-gang air variable tuning capacitor. The rear-most section is the
tracking BFO capacitor.
photo right: The bandswitch shield removed to show the five section, two position bandswitch.
- Alignment is very easy since the RIO is a TRF receiver. Peaking the RF
coil trimmers and aligning the BFO are all that's required. The
procedure starts with Band A and finishes with Band B. There is a pencil
notation (see photo right) on the back of the front panel indicating "Realigned 9/12/35
FHS Tubes OK" and that probably was the last time this RIO
was aligned. Alignment provided a major improvement on both tuning
ranges. The trimmers on most of the RF coils needed considerable
adjustment to "peak" the test signal. After alignment the MVC gain could
be reduced to only 20% advanced for headset listening. BFO has adjustments
on both bands since it has to track the tuned signal. Band A BFO was
slightly off but Band B BFO was so far off that several turns were required
on the trimmer to get the BFO in tune with the signal. After alignment
was complete the RIO was connected to a the 135' CF Inv-vee with 96' of
ladder line that was shorted together. This antenna, although not designed
as a LF antenna, does a good job as a sort of "T" antenna. During
test-listening I heard KPH on 400kc running true CW at 25WPM to honor Memorial
Day. This was at 1100 PDT 5/26/18. Strong signal with clear note. Easy
The fact that the RIO does function on almost all original parts after many decades of idle storage and static display is an indication that National used the very best components available in 1933 for the construction of the receiver. Of course, the circuit is fairly simple and that also might account for the reliability.
- The first thing to remember when listening on the RIO is that there
isn't any type of noise limiter or bandpass filters or output
limiters,...nothing to suppress noise. Any pulse-amplitude noise is
going to disrupt your hearing when using 'phones for audio reproduction.
The old radio ops always kept the 'phone cups slightly in front of their
ears to avoid painful discomfort when listening in a noisy environment.
The RIO is very sensitive with the manual specs indications as low as 1uv. But at MW and LF that level of sensitivity is usually lost in the ambient noise. Using a low-noise antenna is a real benefit. The remotely tuned loop antenna provides the low noise necessary to take advantage of the receiver's sensitivity. Of course, late-spring isn't the best time of the year for testing a longwave receiver but some regional NDBs can be heard. Listening at 10PM on 5/27/2018 using the wire antenna I heard, MOG 404kc, XX 344kc, ZP 367kc, NY 350kc and DC 326kc. Except for MOG, the remaining NDBs heard were all Canadian beacons that tend to run more power. Conditions were terrible with high static levels that sometimes bordered on painful. The next test will be with the loop but the best test results will be those conducted during November through January. This performance information will be updated when better listening conditions allow for a more thorough test.
Hammarlund Mfg. Co., Inc.
"Series 100" Super-Pro - SP-100-LX SN: 2730
SW Receiver - 1938
"Series 200" Super-Pro - SP-200-LX
LF, MW, SW Receiver - 1940
100kc to 400kc and 2.5mc to 20mc
- Hammarlund began work on the Super-Pro design as early
as 1933,...while they were producing their famous Comet-Pro, the first
successful, commercially built, shortwave superheterodyne. The new Super-Pro was ready by mid-1935 when Hammarlund supplied them to the Signal
Corps as the SPA receiver. In March 1936, the official announcement for
the civilian Super-Pro (later called the SP-10) appeared in QST magazine
with a two page spread that included a letter from Lloyd Hammarlund (son
of Oscar, the founder of the company) about the design of the new
receiver. The SP-10 was intended to be a commercial receiver that could
also be used by affluent hams. The SP-10 version was produced for about
nine months. It was a receiver that was very easy for inexperienced
operators to misadjust and overload the AVC control of the front end with too much
RF gain resulting in some signal distortion. The manual explains how to
set up the SP-10 for any conditions and not experience any overloading
(but who read the manual?)
Customer complaints forced Hammarlund to redesign the SP-10 with the new version designated as "Series 100 Super-Pro" receiver. The SP-10's separate RF and IF gain controls were combined into a Sensitivity control on the SP-100 and the former's fully adjustable coupling in the IF-Det-AVC section replaced with fixed coupling on the Det-AVC transformers leaving only the variable-coupled IF transformers. The "all glass" tubes were partially replaced with the SP-100 utilizing eight metal octal tubes and six glass tubes along with two glass tubes in the separate power supply. The new Super-Pro would be difficult to misadjust to the point of overloading the front end and the new design provided an excellent receiver, either for the professional or amateur.
The military and commercial users needed different frequency coverage than the standard SP-100X receiver provided (.54 to 20mc.) Hammarlund introduced the SP-100SX with 1.25 to 40mc coverage and the SP-100LX with 100 to 400kc and 2.5 to 20mc coverage. The SX was generally considered the "ham receiver" since it did tune all the ham bands from 160M to 10M and had bandspread on all five tuning ranges.
The LX was considered the military or commercial receiver since it covered a large section of LF and MW frequencies with two tuning ranges, 100 to 200kc and 200 to 400kc. For airport navigation/communications or military surveillance/communications the LX's LF tuning was ideal. The exception was for maritime users. For the Navy and for many other shipboard users, the lack of any 400 to 500kc tuning was a distinct disadvantage with the result being few Super-Pros were ever used at sea.
Much of the airport communication at the time was using frequencies in the 6 to 7mc range, so the combination of LF/MW navigation frequency coverage plus SW coverage made the SP-100LX a good choice for airport use. The price wasn't cheap however. The list price for the SP-100 receivers was around $450 which included the power supply and loudspeaker.
Other Super-Pro features were infinitely-adjustable Band Width (between 3kc up to 16kc) accomplished with variable coupled IF transformers, band-in-use dial mask, logging scale band spread that operated only in the high frequency tuning ranges (therefore the LX only has band spread in the 2.5 to 20mc ranges, not in the LF-MW ranges) and 14 watts of push-pull audio from triode-connected 6F6 tubes. The early versions of the Super-Pro used 8.0Z ohm audio output but very late LX versions might have 600Z ohm speaker and 8000Z phone outputs. Remote standby was provided as was a "phono" input that allowed access to the first audio amplifier grid for various uses. The Antenna input was about 110Z ohms balanced with no antenna trimmer provided. The user had to make sure his antenna provided a good match for best performance. The Carrier Level meter measured the total IF amplifier plate current and therefore stronger signals increased the AVC and that cut the gain in the IF and reduced the IF plate current, resulting in a meter that read lower for stronger signals - you had to tune for the lowest meter reading when tuning in an AM station. On CW, a large delay capacitor was switched into the AVC line that allowed the user to operate the receiver with the BFO on, the AVC on and still tune in CW signals (this also works great for SSB nowadays.)
The Super-Pro SP-100LX isn't seen very often*. Probably a few hundred LX receivers were produced and most of those were used by the Signal Corps although there were a few commercial users too. Total (X, SX and LX) "Series 100" production was around 1200 receivers. The Super-Pro was just too expensive even though its performance was superior to almost any other contemporary receiver. Also, it does use somewhat delicate fiberboard parts that did break easily if "roughly treated" and the reliability of the Cornell-Dubilier TIGER paper-wax capacitors wasn't the greatest. Maintenance issues may have ultimately limited the number of SP-100 receivers used commercially or by the military.
Performance - The SP-100LX shown is fully restored and it's an incredible performer. Powerhouse audio, fully adjustable bandwidth, ample sensitivity and even a good ability to cope with the noisy conditions below 500kc make the Super-Pro LX versions great receivers. For quite a while I've had the SP-200LX listed in the "Other LW Receivers" section of this article. I had used that Super-Pro with the six foot remotely tuned loop and the performance was pretty good but that was in Virginia City. I hadn't tested an "LX" receiver here in "low noise" Dayton. I recently finished the restoration of the SP-100LX (2019) and it gave me the opportunity to test the "LX" here in Dayton. Since the noise level is very low, I was able to use a wire antenna that has about 25db signal increase over the loop. The performance was impressive. So much so, I decided to add the "LX" versions of the Super-Pro to the detailed write-ups on Vintage Long Wave Receivers. Below is a log of three nights and one early morning listening to NDBs during mid-spring 2019 (not the best conditions.) 135' x 96' "T" Antenna and loudspeaker.
UPDATE: Aug 31, 2019 - I tried listening with the SP-100LX SN: 2730 this morning and tuned in 17 NBDs in about 20 minutes. Quiet conditions allowed easy reception of two NDBs from Hawaii, LLD 353kc and POA 332kc and two NDBs from Alaska, HBT 390kc and RWO 394kc. RWO is a TWEB NDB so it broadcasts voice weather reports in AM with their MCW ID in the background. Log is shown below.
May 13, 2019 22:15 to 22:35 PDT
May 15, 2019 22:15 to 22:30 PDT
May 24, 2019 21:55 to 22:25 PDT
Aug 31, 2019 05:35 to 05:55 PDT
|YYF - 290kc - Penticton, BC, CAN
YCD - 251kc - Nanamio, BC, CAN
UAB - 200kc - Anahim Lake, BC, CAN
SBX - 347kc - Shelby, MT
NY - 350kc - Enderby, BC, CAN
YQZ - 359kc - Quesnel, BC, CAN
RPX - 362kc - Roundup, MT
ZP - 368kc - Sandspit, Queen Charlotte Is., BC, CAN
HQG - 365kc - Hugoton, KS
YK - 371kc - Yakima, WA
QV - 385kc - Yorkton, SK, CAN
YWB - 389kc - West Bank, BC, CAN
PNA - 392kc - Pinedale, WY
ULS - 395kc - Ulysses, KS
QQ - 400kc - Comox, Van. Is., BC, CAN
MOG - 404kc - Montegue, CA
|MOG - 404kc - Montegue, CA
ULS - 395kc - Ulysses, KS
QV - 385kc - Yorkton, SK, CAN
ZP - 368kc - Sandspit, Queen Charlott Is, BC, CAN
RPX - 362kc - Roundup, MT
YAZ - 359kc - Tofino, Vanc. Is, BC, CAN*
YQZ - 359kc - Quesnel, BC, CAN
YCD - 251kc - Nanamio, BC, CAN
XC - 242kc - Cranbrook, BC, CAN*
* = new to this listening
|QL - 248kc - Lethbridge, AB, CAN*
YCD - 251kc - Nanamio, BC, CAN
YYF - 290kc - Penticton, BC, CAN
DC - 326kc - Princeton, BC, CAN*
SBX - 347kc - Shelby, MT
NY - 350kc - Enderby, BC, CAN
YAZ - 359kc - Torfino, BC, CAN
RPX - 362kc - Roundup, MT
6T - 362kc - Foremost, AB, CAN*
ZP - 368kc - Sandspit, Queen Charlott Is, BC, CAN
GC - 380kc - Gillette, WY*
QV - 385kc - Yorkton, SK, CAN
YWB- 389kc - West Bank, BC, CAN
PNA - 392kc - Pinedale, WY*
ULS - 395kc - Ulysses, KS
QQ - 400kc - Comox, Vancouver Is, BC, CAN*
MOG - 404kc - Montegue, CA
|ONO - 305kc - Ontario, OR
UNT - 312kc - Penticton. BC,CAN
DC - 326kc - Princeton, BC, CAN
RYN - 338kc - Tuscon, AZ
XX - 344kc - Abbottsford, BC, CAN
NY - 350kc - Enderby, BC, CAN
LLD - 353kc - Lanai City, HI
YQZ - 359kc - Quesnel, BC, CAN
ZP - 368kc - Sandspit, Queen Charlott Is., BC, CAN
HBT - 390kc - Sand Point, AK
YWB - 389kc - West Bank, BC, CAN
RWO - 394kc - Kodiak Is., AK - TWEB
SB - 397kc - San Bernadino, CA
QQ - 400kc - Comox, Van.Is., BC, CAN
ZT - 242kc - Port Hardy, BC, CAN
YCD - 251kc - Nanamio, BC, CAN
POA - 332kc - Pohoa-Hilo, HI
|*NOTE: I'm not totally convinced of that SN: 2730 is an original "LX." When doing the total rebuild on this receiver I found several indications that the entire RF box was actually from a BC-779 (SP-200LX.) The bypass capacitors were military style molded caps, there were plate load and AVC resistors inside fabric sleeving, MFP is found only on the RF box and there was a serial number stamped on the back panel of the RF box in the 9000 range. These are all found in the later BC-779 RF box. During the rebuild, I changed all of the conflicting components and "made" the RF box the SP-100LX type. That way, performance is like a SP-100LX would have been. But, why was the BC-779 RF box installed in the first place? The most probable scenario is that serious problems with the original SP-100X RF box may have been readily solved by installing a RF box from a surplus BC-779. The most likely culprit? A ham.|
- In 1939, Hammarlund updated the Series 100 Super-Pro receiver. The
upgrades were mainly to bring the Super-Pro circuit current with modern
receiver design and with what the competition was offering. Gone was the
odd-ball Carrier Level meter that measured IF plate current and read
backwards. It was replaced with an illuminated meter that operated off
of the AVC line and indicated increased signal strength with higher
meter readings. The four IF amplifiers with separate input and output
detector transformers were reduced to three standard IF amplifier stages
with two variable-coupled IF transformers. A Noise Limiter was added and increased the
tube count to 16 tubes in the receiver and two in the
separate power supply. The 6D6, 6C6 and 6B7 glass tubes were replaced
with metal octal equivalents. The Crystal Filter was modernized with
switched steps of selectivity and a phasing control. A dual secondary
audio output transformer provided 600Z ohms and 8000Z ohms outputs with
the 8000Z going to a standard phone jack on the front panel. The 600Z
was routed to rear chassis terminals. The frequency coverage was
optional with "X" covering .54mc to 20mc, "SX" covering 1.25mc to 40mc
and the "LX" covering 100kc to 400kc and 2.5mc to 20mc. Price was
reduced to $375 list.
With WWII starting in Europe and the USA preparing for war, the Signal Corps began to buy more Super-Pro receivers. By 1943, the circuit components were upgraded along with the power supply to military components. Steel panels replaced the aluminum panels and the black paint was changed to varying shades of grayish-green. Designations were also changed and the "X" became the BC-1004, the "SX" became the BC-794 and the "LX" became the BC-779. Demand for receivers required that Howard Radio Company become an alternate contractor for the Super-Pro. At the end of the war, no more LW receivers were produced by Hammarlund until the introduction of the SP-600 receiver in the early-fifties (SP-600-JLX and the SP-600VLF-31.)
Radiomarine Corporation of America
Department - Bureau of Ships
MW, LF & VLF Radio Receiver - 1941
Serial Number: 65
CRM-46092, CRM-50092, CRM-20096 aka: AR-8503, AR-8503-P, RM-6
15kc to 600kc
The Radiomarine Corporation of America was a division of RCA that specialized in the operation of RCA's Communications Stations and sold RCA-built equipment for both major communications stations and for shipboard installations. The AR-8503 was introduced around 1938 and was designed mainly for shipboard installations. A matching pre-selector was also included, designated as the AR-8503-P. Additionally, an AC power supply was offered, the RM-6. Although in an emergency, the AR-8503 could be operated from a battery pack the preferred method of operation used the RM-6 to supply the required 6 volts for tube heaters, +22 vdc for the detector B+ and +90 vdc for the amplifier plates. Sometime around 1941, the US Navy wanted to install the AR-8503 on some of their smaller ships and a contract was issued for a small number of receivers. "RAZ-1" designated a complete set of equipment that included the CRM-46092 Receiver (AR-8503) with the matching CRM-50092 Pre-selector (AR-8503-P) and the CRM-20096 Power Supply (RM-6.) The contract date was just five days before the attack on Pearl Harbor, Dec 2, 1941.
The CRM-46092 receiver uses four metal octal tubes in its regenerative circuit. The RF amplifier, detector and first audio are all 6K7 metal octal tubes while the audio output tube is a 6F6. The CRM-50092 preselector uses a single 6SG7 metal octal tube as a tuned RF amplifier. The CRM-20096 uses a 5Z4 metal octal tube for the rectifier. The CRM-50092 pre-selector receives power from the CRM-20096 power supply via a three foot long, three conductor cable that is connected to the power supply ground terminal along with the 6vac terminal and the +90vdc terminal. The CRM-46092 receiver has four tuning ranges covering 15 KC up to 600 KC. Three bandswitches - two on the receiver and one on the preselector - have to be utilized for changing tuning ranges. The National Type-N dials are scaled 0 to 100 and have a 180 degree layout. A tuning chart is provided in the manual to correlate the dial reading to tuned frequency. Coupling, Regeneration and Volume controls are on the front panel and the preselector also has an RF Gain control. Audio output is provided for a single audio stage or for full audio output via two telephone jacks on the front panel. Output is designed for the Western Electric 509W earphones and, although any Hi-Z 'phones will work, the 509W phones seem to give the best immunity to noise. The receiver case is shock mounted and is made of copper plated steel painted a grayish-brown color. The preselector case is made of aluminum and painted to match the receiver although it is not shock mounted. The power supply is a standard steel box painted gray. The front panels of the receiver and the preselector are machine textured aluminum that has been matte chromium plated.
|Left photo: The CRM-46092 chassis showing the large bee's
wax dipped coils and the sparse layout of components. The tuning
condenser is inside the shielded box in the center of the chassis.
Right photo: The CRM-50092 preselector chassis showing the tuning condenser and the 6SG7 RF amplifier tube. The RF coils are under the chassis.
I first saw this RAZ-1 in 1997 at the home of W3ON, John Ridgway. It was setting next to the SX-28 he was going to sell me (if I could lift it off of the table.) I asked John if he wanted to sell the RAZ-1, to which he replied, "You wouldn't take a longwave receiver away from an old Navy radioman, would you?" John was living in Galena, Nevada at the time but since he was 85 and now alone, he was moving back to Maryland. John lived to the age of 93, becoming an SK in January 2006. To my surprise, in the summer of 2006, I got a 'phone call from an estate agent who said that they had found a letter among John's papers that stated that he wanted his radios and parts to be sent to the "Radio Museum in Virginia City, Nevada." The agent was calling me to see if I really wanted any of "this junk." I told them I did. The estate paid to ship the parts and equipment back out west. The shipping of the 22 boxes was spaced out over about a six week period. In the 21st box was the RAZ-1. Shipping had caused one small problem, one of the largest coils had broken from its mount. The large buss wiring had kept it in place and all that was required was to glue the mount back together and screw the coil form back in place. I acquired the correct shock mounts from N7ID. I did have to replace the filter capacitors in the power supply for quiet reception.
The RAZ-1 is very sensitive and almost any station on LW can be tuned in however the lack of a calibrated dial makes this somewhat difficult if looking for a specific frequency just using the RAZ-1 dial alone for reference. Though I could use a heterodyne frequency meter if it is important to determine the exact frequency being received, I find it is easier to know approximately where I am tuning by listening to known adjacent signals. In other words, if the NDB MOG is zero beat (or being heard in the background) and I'm trying to copy another weaker signal partially obscured by MOG, I know that weak NDB is on 404kc or very close to it, since that is MOG's frequency. I can usually determine an unknown NDB's frequency within 1 or 2 kc by this method. The lack of any kind of limiter is sometimes a problem if local noise is present, however switching to the loop antenna has greatly reduced local noise. To reduce noise to a minimum, the Coupling is set very close to zero (0 to 25% maximum,) the Volume about 25% to 60% advanced, Regeneration right on the oscillation point (autodyne detection) and then signals are peaked with the the Preselector and then slightly manipulated with the Trimmer control. The Preselector gain is usually set to about 85%. These settings usually result in the best response of signal to noise along with the greatest selectivity. Although very strong signals are encountered from local or powerful stations, very weak MCW signals are the norm when searching for DX NDB stations. Usually, with several NDBs on the same frequency it is possible to slightly de-tune the loop antenna to one side or the other of the frequency and enhance one or more of the NDB signals for successful copy. I have probably logged more NDBs with the RAZ-1 than any other LW receiver. However, that might be because it was one of the first LW receivers that I used when I started logging NDB stations. But, it can always be relied upon to pickup whatever is out there as long as reasonable conditions are present.
Radiomarine Corporation of America
MW, LF & VLF Shipboard Receiver - 1944
15kc to 600kc
|The AR-8510 was the replacement receiver for the AR-8503 (aka RAZ-1 for
the USN - profiled in a section above) and 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 input
and tuned plate output using a three-section ganged tuning capacitor.
The antenna switch allows the user to select which receiver will be
connected to the antenna - either the AR-8510 or an emergency receiver. The audio output can drive the panel mounted loud speaker or headsets
either simultaneously or, using the Loudspeaker switch, the panel
speaker can be turned off. 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 supply) was used. There was
also an RM-37A unit that provided 90vdc B+ output with a 115vdc input
from the ship's power. This
was to be used if it was necessary to conserve the B batteries that
normally provided the +90vdc. The AR-8510 requires 6.3 volts at 1.8A (AC
or DC) and 90vdc at 15mA. The vacuum tubes used are four 6SK7 tubes
and one 6V6G or GT.
photo right: Top of the chassis showing the antenna connections (far left front of chassis) and the power input connections (far right back of chassis.) The tuning condenser is under the central cover.
The AR-8510 could be 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, battery charger switching, clocks and more. The 4U console used the RMCA AR-8506 MW-SW receiver with a larger transmitter. The 5U console had both receivers installed along with transmitters and auxiliary equipment. Mackay Radio and Telegraph Company also supplied Marine Radio Consoles MRU-19 or 20 that had their equipment installed.
The AR-8510 was approved by the FCC for shipboard use in 1942 (concerning minimum radiation from the antenna.) The schematic drawings are dated 1943. It's likely that it was at least 1944 before any AR-8510s were in use and this particular AR-8510 is dated NOV. 1944 (with a serial number of 2774) making it an early example. It seems that most of the installations during WWII were onboard Liberty ships. Post-WWII installations were generally on commercial ships. The AR-8510 found a lot of use and longevity with some receivers still in use onboard some old oil tankers as late as the 1980s.
Unfortunately, most AR-8510 receivers led a pretty hard life
and the sea environment didn't help preservation. Most examples have
been worked on or have missing parts (or non-original parts.) The
AR-8510 shown in the header photo is cosmetically restored with nearly all original parts. The
exception is one capacitor under the chassis, the speaker grille and the RCA pointer knobs. The
paint job on the front panel is VHT Gray wrinkle finish which is
slightly darker than the original RMCA gray.
|Later manuals and some Internet sites will show a slightly
different AR-8510 that has silk-screened nomenclature on the panel
including the information on the data plate silk-screened onto the
panel in the upper right part of the panel. The B&W photo (shown
right) in the 1950 manual shows this later version with a date on
the panel of 1947. It's probable that the WWII version (early
version) used the easy-to-replace nomenclature tags as an
ease-of-maintenance function. Later post-war receivers were probably
not going to be subjected to the rigors that the wartime versions
experienced so the silk-screened panels could be used and provided
an excellent appearance.
I was given the AR-8510 shown in the photos as payment for some radio repair work. It probably was taken off of one of the Liberty ships that were part of the "moth-balled" fleet that was moored outside of Benicia, California since the receiver originally was obtained from the SF Bay Area. The "as received" condition was fairly good considering how the ships were taken care of - they weren't. Of course, the front panel has been repainted in the past - probably with a brush. The perf-metal grille had more than its share of paint applied (and it wasn't original either.) The receiver came without any type of power supply (the RAZ-1 power unit RM-6 can be used as a power source.)
|This AR-8510 required a little bit of work to get it
operational. Bands 1 and 2 functioned okay but needed alignment.
Band 3 and 4 were non-functional due to broken leads from the coils
that are in the plate circuit. The open coils resulted in an
absence of plate voltage to the first RF amplifier when bands 3 or 4
were selected. I had to remove the coils and rebuild them then finishing them
off with a re-waxing job. After reinstalling, bands 3 and 4 had to
Performance using a "T" antenna of 98 vertical feet running to a 135 foot horizontal section was very good. Since the AR-8510 was the replacement for the AR-8503, it's fair to compare the two receivers. First, with a direct readout dial there's no need for the charts and graphs that are necessary for finding where you're tuned on the AR-8503. The preselector is built-in with the AR-8510. Also, only a single band switch is necessary on the AR-8510 while two band switches are used on the AR-8503 plus a band switch on the preselector. Sensitivity on the AR-8510 is about the same as the AR-8503 with preselector. Regeneration action is very similar in that it's a very sharp adjustment between maximum sensitivity (either non-oscillating or oscillating) and any adjustment below either point greatly reduces sensitivity (this is typical of regenerative detectors though.) The AR-8510 seems to hold its adjustments better across the band especially the Trimmers that only require a slight adjustment from one band end to the other. This is expected since it's part of the alignment process. I find that the loudspeaker is actually pretty good for some reception. If you want to use Hi-Z phones, it's better if you leave the loudspeaker on. Without the speaker load the 'phones seem to respond to more noise than signal.
Radio Corp. of America
Navy Department -
Bureau of Ships
contractors: Andrea Radio Co. or Magnavox
photo right: Andrea
Radio Co. CND-46155, RAK-7 - Accepted by USN Jun 1945
The RAK and the RAL used glass tubes that were large six-pin type, 6D6 tubes for the two RF amplifiers, a 6D6 for the regenerative detector, a 6D6 for the first audio amplifier, a 41 for the audio avc amplifier and another 41 for the audio output. The power supply, CRV/CND-20131, was a separate unit that used a 5Z3 rectifier, an 874 regulator tube and an optional 876 ballast tube. The 876 can be left out of the power supply if the AC power is stable and noise free. An internally mounted switch routes the 120vac to a different tap on the power transformer if the ballast is not required (it should still be removed from its socket if switched "out.") If the ballast tube remains installed it will be "connected" regardless if it is switched in or not (although less current is flowing through it when it is switched out of the circuit.) When the 876 is switched in, the 120vac actually is IR dropped through the ballast and a different tap on the power transformer is used (~70vac) thus providing the regulation of the AC to the transformer if the line voltage is not stable. Since the ship had to generate its own power and most of the equipment onboard (including motors to rotate gun turrets) ran on this power, the varying switching loads were what caused the line voltage fluctuations that required using the ballast regulators. In shore set-ups, on standard AC line power, ballast regulators were not required. However, the Navy manual (NavShips 900,480) recommends that the RAK can be operated without the 876 ballast if the AC line maintains 10% regulation, BUT the RAL receiver should always be operated with the 876 ballast tube installed and switched "in" regardless of the AC line stability. The RAL had to operate at much higher frequencies than the RAK and slight AC line variances could cause receiver instability. The instability only occurs when tuning above about 10mc and when receiving CW (or SSB nowadays.) With AM reception above 10mc it's difficult to detect the instability since the detector isn't oscillating. With modern AC line stability the only time slight variances occur is when heavy-load appliances turn on or off within the users home. While the instability can be noticed, it is not to the extent that it would affect copy. Operating the 20131 power supply with the 876 ballast installed will generate quite a bit of heat as it dissipates about 140 watts. It's up to the user to decide whether the slight instability only on CW above 10mc is that much of a problem versus the power dissipated as heat resulting from having the 876 in the circuit.
The RAK was designed primarily for CW, ICW or MCW reception. The receiver has a low pass filter that is permanently connected in the audio circuit to roll off the upper audio frequency starting at about 1200 hz. An elaborate audio avc circuit allows the user to limit the audio or noise peaks at an adjustable level. This was to provide the radio operator relief from "static crashes" common near storm fronts. Also a selectable-frequency audio bandpass filter was provided to enhance the CW tone reception for noisy conditions. Voice transmissions can be received (nowadays in the 540kc to 600kc portion of the AM-BC band) but the RAK audio response is restricted to about 400hz to 1300hz, so voice or and music transmissions sound "hollow" with little depth. The manual states that another receiver should be used if voice reception is required, implying that the RAL should be used since its audio filters can be switched out of its circuit. The tuning of the RAK is heavy duty, gear driven and the tuning dial readout is shown on two circular dial scales of 0 to 10 and 0 to 100. The actual tuned frequency has to be correlated with a graph that is in the manual. The receiver does provide a logging chart on the front panel for a "most used frequencies" reference. A RF trimmer, an antenna trimmer, sensitivity and regeneration controls are on the lower panel of the receiver. The meters monitor audio output level in db and tube heater voltage (doubles as the "power on" indicator.) The RAL receiver is almost identical construction but has nine bands covering 300kc to 23mc. Additionally, (as mentioned) the low pass filter and the tuned audio filter can be switched out of the RAL circuit for voice reception. Also, a vernier tuned frequency control was provided. Normally, the two receivers operated together through a control box (CND-23073) that allowed the radio op to monitor two frequencies simultaneously. The control box also could be used to switch the AC to the receivers on or off.
photo left: The RAK and RAL in use aboard a US Navy ship. Also, National RBL and RAO receivers far left, the LM-type frequency meter by the telephone handset and a Scott SLR-type receiver below the order binders.
|Nowadays, a complete RAK and RAL set up will require a heavy-duty
table for the set-up since the total weight of the two power supplies
and the RAK and RAL receivers is well over 200 pounds.
In my installation I had the power supplies for the RAK/RAL receivers
bolted to the underside of the table. I provided for a space of about
3.5" above the supplies to allow good ventilation for the ballast tubes.
I ran the power supplies with their ballasts even though wasn't
necessary. The actual difference in power consumption is significant -
the ballast dissipates about 140 watts. I had run the receivers
both with and without ballasts and I noticed that the received noise seemed to be
slightly less with the ballast in use. Of course, this was operating the RAK and RAL in Virginia City, a city notorious for noisy, low voltage AC
In actual operation, the RAK is a very sensitive receiver that spreads the LF tuning range over several bands. This bandspread action is nice for tuning in weak stations or trying to separate several stations that are on the same frequency - as many NDBs are. The major problem is that calibration is relying on the readout versus a graph and that graph is in the manual. The first thing to do is make a copies of the frequency graphs to keep with the receivers (RAK and RAL have separate frequency to tuning dial charts.) Then it's easy to keep track of where you are in the LF spectrum. If it is important to know the exact frequency, use a heterodyne freq-meter set up (or a RF Signal generator can also be used as a calibrated heterodyne freq-meter.) The Audio AVC will help with static crashes and to a certain extent, noisy conditions but, like most output limiters, if it is advanced too far it severely clips the audio with high distortion. With the AVC control, 10 on the scale is minimal AVC action and reducing the setting (CCW rotation) will increase the AVC action. A setting lower than 3 will usually begin to affect the CW audio tone. The adjustable frequency audio bandpass filters seem at first to be almost useless due to the seemingly high frequencies chosen - 450hz to 1300hz. All NDBs use MCW with a 400hz tone. When tuning a NDB, with the detector oscillating to provide a heterodyne, the tendency is to tune for carrier zero beat but that will only allow the 400hz note to be heard. By selecting a higher tuned audio frequency, for example 800hz, and tuning to enhance just the MCW note (that won't be 400hz anymore,) the selectivity of the filter will allow hearing the Morse in the clear but the carrier will be attenuated. It takes some practice and a moderately strong NDB signal. For DX NDBs, the Tuned Audio Filter should be switched off.
A tuned loop antenna, with its high Q, really helps reduce the noise and increase the signal to noise ratio. I used a ten foot diameter remotely tuned loop with the RAK-7 when I was in Virginia City and the signal reception was excellent. Unfortunately, that large of a loop didn't survive the wind very well. I have yet to test the RAK-7 using the Pixel Technologies shielded magnetic loop that I now use but, as soon as I do, I'll add the results here. The easiest access to the audio output is from the front phone jack. It's 600Z ohms and, while the RAK will easily drive a 600 ohm loudspeaker, many more weak signals can be copied using 600Z earphones rather than using a loudspeaker. The 600Z audio out the back of the receiver was normally routed to the Control Box. It has some filtering inline and can be used but the front panel phone jack is using the same source for its connections and is much easier to access. I have tuned in all of the normal LF signals with my RAK-7. The best NDB DX were several in North-Eastern Canada and Puerto Rico's powerhouse transatlantic beacon, DDP. At lower frequencies, the RAK seems to get better and better with JJY at 40kc being a regular copy. The Navy MSK-RTTY signals from 19.8kc up to 25.2kc are always present. WWVB, JJY and all of the USN MSK stations will require using the Tuned Audio Filter set to about 800hz for best results.
Note the operator's desk. It has the "well" for the mill (the lowered center section of the desk) and the riser for the receivers along with drawers and cubby areas. The interesting thing is that it's made out of wood. The shipboard radio operator desks were all-metal construction.
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