Radio Boulevard
Western Historic Radio Museum


Vintage Longwave Receivers

Restoration and Performance Testing
Various Models of Vintage LW Receivers


Four Parts





by: Henry Rogers WA7YBS


Radio Towers and Station House at Arlington, VA   ca: 1920

Information in Part 1

Restoration and Performance Testing the Following Longwave Receivers

1.  IP-501-A - Radiomarine Corp. of America - 1923 

2.  Type 105-A - Mackay Radio & Telegraph Co. - 1932

3.  RIO  -  National Co.,Inc.  -  1933

4.  SP-100LX Super-Pro - Hammarlund Mfg. Co.,Inc - 1938

5.  RAA-3  -  USN-RCA Manufacturing Co.,Inc.  -  1935

6.  RAZ-1, CRM-46092 - USN-Radiomarine Corp. of America - 1941

7.  AR-8510 - USN-Radiomarine Corp. of America - 1944

8.  RAK-7, CND-46155 - USN-RCA-Andrea Radio Co. - 1944

9.  BC-344-D - Signal Corps-Farnsworth Radio&Telev Corp. - 1944

10.  RBL-5, CNA-46161-B - USN-National Co., Inc. - 1945

Information in Part 2

Restoration and Performance Testing the Following Longwave Receivers (cont.)

RBA-1 (CFT-46154) & RBA-6 (CFT-46300)
 USN-RCA-Federal Tele.&Radio Corp - 1941-45

12.  Type 3001-A - Mackay Radio & Telegraph Co. - 1948


Information in Part 3

Restoration and Performance Testing the Following Longwave Receivers (cont.)

R-389/URR - Signal Corps-Collins Radio Co. - 1951

14.  SP-600VLF-31 - Hammarlund Mfg Co., Inc.- 1955

15.  RA-17C-12 & RA-237-B - RACAL Engineering Ltd. - 1966

Information in Part 4


What to Listen to on LW

NDBs, LW Stations, VLF Stations

SAQ 17.2kc Alexanderson Alternator

USN VLF MSK Stations

Loop Antennas, Long Wire Antennas

Operating Vintage Gear on the 630M (472-9 khz) Band with Log incl'd

USCG  Loran C Master Station"M"- Fallon, Nevada
2007 Photo Tour

NBD Stations in Nevada  &  NDB Station Log


Vintage Longwave Receivers - Part 1

Performance Testing Classic Vintage Longwave Receivers


Radiomarine Corporation of America
(Wireless Specialty Apparatus)


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 part of the cross-licensing "Radio Group" headed by General Electric and included Westinghouse, AT&T, RCA and the United Fruit Company (who owned Wireless Specialty Apparatus (WSA.) WSA built a few broadcast radios for RCA in 1921 and 1922 but by 1923 they had become part of RCA. Soon WSA became Radiomarine Corporation of America and was building shipboard radio gear for RCA.

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.

photo above: Inside the IP-501-A receiver showing the high quality construction

 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*




* = New NDB heard




Mackay Radio & Telegraph Company


  MW, LF & VLF Radio Receiver  Type 105-A

Serial No. 32081

Commercial Shipboard Receiver from 1932

16kc  to  1500kc


built by: 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 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.

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 chief engineer for most of FTC's history. FTC bought Brandes and created a new division of FTC called Kolster Radio Company specifically 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, Federal Telegraph Company continued to do most of the Mackay Radio work under contract to ITT. Federal Telegraph moved to New Jersey in 1931 after it was purchased by ITT. 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 in the 1940s.


photo left:  Federal Telegraph Company building in California about 1927

photo above: The chassis of the Mackay 105-A. The rectangular box on the right side of the chassis contains the power input filters. The right side cylinder contains the RF choke while the remaining cylinders contain the AF interstage coupling transformers.



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.

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 are five-pin cathode-type tubes. 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 16kc in seven tuning ranges. Power is 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. Originally the receiver was probably in a metal cabinet but later it could also have been panel mounted in one of the Mackay Marine Radio Units that  would have housed the majority of the radio gear for the ship.

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.
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 it appears that the five pin tube sockets are not original but the rework looks vintage. It could be that the receiver was rebuilt at the factory sometime in its past resulting in professional looking rework and the patina of age appearing on the solder joints. 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 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 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



National Company, Inc.


Intermediate Frequency Receiver (LF and MW)


Serial Number:  3


160kc to 630kc


History - 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 relay rack.

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.

Power-up - 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 intermittent (as 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 - 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 copy.  

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.

Performance - 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

LF, MW, SW Receiver - 1938

"Series 200" Super-Pro  - SP-200-LX

LF, MW, SW Receiver - 1940


100kc to 400kc  and  2.5mc to 20mc


Super-Pro History - 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 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 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 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 session

NOTE: Conditions tonight (15th) were very poor due to wind and storm front. Lots of static crashes preventing weak signal reception. Conditions on the 13th and the 24th were surprisingly good though static crashes were numerous on all nights

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
SP-200LX - 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.)



RCA Manufacturing Co., Inc.

Navy Department
Bureau of Engineering


RF Tuner - CRV-46034-A
IF/AF Amplifier - CRV-50022-A
Power Unit - CRV-20016-A


Superheterodyne Receiver - 1935

10kc  to  1000kc

This is a teaser, 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 by the end of the summer of 2019. 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 restoration is now finally coming together. 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.


Radiomarine Corporation of America

  U.S. NAVY - RAZ-1

 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


Model AR-8510

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.

photo above: Under the chassis showing the bee's wax impregnated coils.

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 be aligned.

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 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.



U.S. Navy


contractors: Andrea Radio Co. or Magnavox

RAK - 15kc to 600kc, RAL - 300kc to 23mc

Andrea Radio Co. CND-46155,RAK-7 & Andrea Radio Co. CND-46156,RAL -7  - 1944

The Navy wanted more modern LF, MW and SW receivers in the mid-thirties so RCA provided the Navy with the RAK/RAL series. The RAK/RAL were to replace the aging RAG and RAH receivers built in 1933. The RAG/RAH were TRF receivers with non-regenerative detectors and tracking BFOs built by Sylvania. There was an early version of the RAL that was supplied to users other than the USN that was designated as the TBR.

Any receiver built for shipboard use had to be "bullet and bomb" proof, in other words, the ship had to take a couple of torpedoes, be sinking fast and the radio gear would still be working. Additionally, steel and iron was kept to a minimum in shipboard radio construction to reduce corrosion problems that were common on marine equipment. The RAK/RAL series were built like the battleships they served on. The construction is something to marvel at - so over-built, so heavy-duty with no expense spared - it's no wonder that most RAK or RAL receivers still function with all original parts even though they are pushing seventy years old. The design concept was to provide maximum reliability in severe service by simplicity of design - and it paid off since the receivers were in use up until the end of WWII with their last service on board submarines.

RCA was the primary designer and builder of the first contracts of RAK and RAL receivers. During WWII, Andrea Radio Corp. became another contractor building RAK and RAL receivers. The RAK, (aka CND-46155 by its Andrea/Navy designation, substitute "R" for the "N" for the RCA /Navy designation) covers 15kc up to 600kc in six tuning ranges. There was a RAK-8 and RAL-8 produced with Magnavox as the contractor.

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, CNV-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. If the ballast tube is installed it will be on regardless if it is used or not although less current is flowing through it when it is switched out of the circuit. When switched in, the 120vac actually is 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 are 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.

The RAK is designed for CW or MCW only. The receiver has a low pass filter that is permanently connected in the audio circuit to roll off the upper audio frequency 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 allow the receiver to be used in heavy static conditions. Also a selectable audio bandpass filter was provided to enhance CW reception in noisy conditions. Voice can be received but it is severely limited on the higher audio frequencies making copy difficult. The manual states that another receiver should be used if voice reception is required - like the RAL. 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 frequency trimmer, an antenna trimmer, sensitivity and regeneration controls on on the lower panel of the receiver. The meters monitor audio output level in db and tube heater voltage. The RAL receiver is almost identical construction but has nine bands covering 300kc to 23mc. Additionally, the low pass filter can be switched out of the circuit for voice reception and a vernier frequency control is provided. Most of the concern about a stable AC line voltage was directed at the RAL receiver which itself can become unstable at high frequencies if the line input varies. 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 RAK and RAL set up will require an ample and heavy-duty table for the complete set-up. 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.

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 copy of the frequency graph to keep with the receiver. Next is to calibrate the RAK so the readout is fairly close to the graph. Then it is easy to keep track of where you are in the LF spectrum. If it is important to know the exact frequency, I use a heterodyne freq-meter set up. 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. The adjustable bandpass filters are almost useless. This is due to the high frequency chosen for the first audio frequency cut-off - 450hz. This may have been fine for true CW but that is seldom encountered anymore in the LF bands. All NDBs use MCW with a 400hz tone. The lowest setting of the filter works okay on NDBs but the other bandpass frequencies are even higher in frequency and so are not very useful.

The tuned loop antenna, with its high Q, really helps reduce the noise and increase the signal to noise ratio. The audio output is taken from the front phone jack. It's 600Z ohms and, while the RAK will easily drive a 600 ohm speaker, many more weak signals can be copied using earphones rather than a speaker. 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 a fairly regular copy. The Navy MSK-RTTY signals from 19.8kc up to 25.2kc are always present.

photo above:
  "Women Marines - USMC PHOTO - 24-8" -  showing a Women Marines Reserve radio op using the RAK and RAL receivers. She's tuning the RAK and is ready to copy on the "mill" (the typewriter.)  RAK and RAL power supplies are on top of the receivers in this set up. Note how she has the 'phones slightly in front of her ears. This helps reduce "ringing ears" from static crashes or unexpected loud "pops" common on the lower frequencies. The RAK and RAL have audio AVC circuits for an output limiter function.



U.S. Army Signal Corps

Farnsworth Television & Radio Corp.



BC-344-D - LF/MW Radio Receiver - 1944

150kc  to  1500kc


SN: 5     Contractor: CFN (Farnsworth) 


Order No. 10651-PHILA-44


The BC-344 belongs to a family of radio receivers that were designed by the U.S. Army Signal Corps at Fort Monmouth, New Jersey in the late-thirties. The early versions of the receiver used more aluminum in the chassis and some early versions were painted with a "leatherette" finish paint (properly called "Crackle Finish.") By the time WWII contractor manufacturing had started, the receiver used an all-steel chassis and the paint used was the standard black wrinkle finish. Farnsworth Television & Radio Corporation built the AC operated versions, those being the BC-342 and the BC-344. RCA Manufacturing Co., Inc. built the DC operated versions, those being the BC-312 and BC-314. The BC-312 and BC-342 are shortwave receivers with six frequency ranges that cover 1.5mc up to 18.0mc. The BC-314 and BC-344 are LF (low frequency) and MW (medium wave) receivers with four frequency ranges the cover 150kc up to 1500kc. The BC-344 has two RF amplifiers and two IF amplifiers (four 6K7 tubes.) A separate Mixer (6L7) and LO (6C5) are used. The BFO uses a 6C5 triode, the detector-AVC-first AF is a 6R7 and the AF Output tube is a 6F6. There are ten tubes used in the BC-344 including the 5W4 rectifier tube used in the RA-20 power pack that mounts internally. No selectivity filter is used in the BC-344. Audio output is generally 4000Z ohms but may be set to 250Z ohms by moving a jumper inside later versions of the receiver. The LS-3 loudspeaker was commonly used with the BC-344 receiver although a headset would allow copy of weaker signals.

photo above: Army communications set-up for local ground transmissions. The telephone box allowed interfacing a telephone contact through the BC-191 transmitter. Note that the vernier dial locks are removed from all three receivers.

The Signal Corps utilized the BC-312/314/342/344 receivers in many applications. Fixed stations generally used two or more receivers paired with at least one BC-191 transmitter. A typical Army station is shown in the photo to the left. Note that three BC-342 receivers are utilized with an AC operated BC-191 transmitter (the RA-34 power supply is barely visible under the transmitter table.) Mobile stations ranged from small vehicle set-ups to the SCR-299 that used a BC-342 and BC-344 set up with a BC-610 transmitter, all powered by an AC generator towed in a trailer. Most DC set-ups operated on 12vdc from vehicle battery-charger systems. There was a 28vdc BC-312 version, the BC-312-NX.

BC-344-N on Long Wave? - I've been interested in how well the BC-344 would perform on MW and LF reception for some time. I have a "rough-condition" receiver that had been painted "metallic" green. I even had it on the work bench once but I lost interest when I saw some "hamster" rework inside. Recently (2016,) I obtained another BC-344 receiver that happened to have the scarce shock mount installed. This receiver seemed to be pretty original and virtually complete. Replacements for the few missing or poor condition parts were easily located in my BC-312/314/342/344 junk box.

Reworking the BC-344-D - Since the BC-312/314/342/344 were essentially "workhorses" for the Army, many are in rough condition today. All will require some work and many require a full restoration in order to function correctly. The receiver is difficult to disassemble and this has saved many BC-344s (and 342s) from being modified too much by hams. Most hams preferred to modify the easy-to-work-on BC-348-Q. Most 344/342 receivers will be very close in alignment because all of the adjustments were "locked" if they were easy to access, like the IF transformers, or hidden by covers or plugs in the case of the RF adjustments. Most receivers will be missing the dial lock since this piece often times interfered with the rotation of the vernier knob. It's not unusual to do mostly a cosmetic restoration and have the receiver work with all original parts with the exception of the dual electrolytic filter capacitor located in the RA-20 power pack.

Front Panel Dismounting - One of the common problems with any of this series of receivers was present on this BC-344. That is, the plastic dial index that always seems to have warped, cracked and discolored to the point where it's no longer even transparent. Luckily, I had an excellent condition replacement. Unfortunately, to install the index requires completely dismounting the front panel.

I say "unfortunately" because this task is unbelievably complicated by the Army's mechanical design that never seemed to even consider the possibility that the receiver might need to be disassembled someday. Besides the abnormal amount of front accessed screws there are three screws that mount from the backside of the panel that must be removed. The two fuse holders must be unsoldered and removed. All of the phone jacks and controls must have their mounting nuts removed. The wires to the dial lamps have to be unsoldered. All of the wires going to the front panel "trunk" connector have to be unsoldered. The power input terminal strip has to be dismounted. All screw connections to Antenna and Ground connections have to be dismounted. Knobs and control nuts must be removed. At this point, you'd think the front panel would easily come off, but not yet! The Fast Tuning gear has a pinned shaft that is flanged. You can't remove the front panel unless the gear and shaft are taken apart and that requires driving out the pin. However, once the gear and flanged shaft are apart, the front panel finally can be dismounted.

photo right: A rear view of the chassis showing the stout construction of these receivers

At this point, installing the replacement dial index is easy (it's mounted with eight screws!) Since the front panel is off, now is the time for a thorough cleaning and touch-up. I use jet black nitrocellulose lacquer that is thinned about 3:1 for touch-ups. After the lacquer has set for a while, I rub down the panel with light weight machine oil. Usually, this will blend the color of the touch-ups with the original black wrinkle paint making the touch-ups invisible.

When remounting the front panel, it will be noted that all of the 6-32 screws are the same length. However, there are three different length 4-40 screws. The four long 4-40s are for the wire mounts. There are two short 4-40s and the rest are all the same length. The two short 4-40s are very important. There are two places where, if long 4-40 screws are used, they will protrude far enough thru the rear panel mounted nut to contact the dial mask with possible scratching of the mask when the band switch is operated. The "short" 4-40 screws must be mounted as follows: One "short" screws is by the band change switch and is the screw nearest the "G" in "CHANGE" in the nomenclature of that switch. The other "short" 4-40 is used near the lower left side of the data plate (the screw head almost is contacting the data plate edge.)

The other observation is the small component board that is mounted to the back of the front panel uses two different length stand-offs. The reason is that the lower screw must past thru a part of the chassis mounting flange and the top screw doesn't. The longer stand-off is the upper one.

Patience is required in any BC-321,314,342,344 rework. The work is tedious and none of the component parts are easy to access without removing another part or assembly first.

  Electrolytic Filter Capacitor - Most military gear used oil-filled paper dielectric capacitors as the filters. The disadvantage of this type of capacitor was its large size but the advantage was long-term reliability. Since space was at a premium in the RA-20 power supply, a dual electrolytic filter capacitor was used. The advantage was a small size for two 8uf 450vdc caps but the disadvantage now is, after 75 years, the dielectric paste has dried up and drastically reduced the value of the capacitor.

There's ample room to mount new electrolytic capacitors inside the old capacitor can. There are many methods to accomplish this "re-stuffing." I cut the cap in two and remove the old cap and black wax using a heat gun to soften the wax. This allows the old capacitor to be easily pulled out of the can. I install new capacitors connecting them to the correct terminals and then epoxy the can back together. I remount the rebuilt cap and connect it into the circuit.

The photo to the left shows the underside of the BC-344 and in particular the RA-20 power pack. Note that the RA-20 is "swung out" on the right side hinge mount. Also note the wiring harness that is connected to the power terminal strip. 

Miscellaneous Rework - Sometimes the OFF-MVC-AVC switch contacts don't allow the operator the ability to turn the receiver on. These are special build, stacked switches that have the MVC and AVC switches enclosed. Luckily, the AC on/off switch is only covered with a fiber disk that can be bent to allow access to the arm and contacts. It's usually only dirt or maybe minor corrosion that is causing the problem. This can be cleaned off using a small piece of Alu-Ox paper or a very small wire brush. It might be necessary to bend the arm slightly to have better contact. If the AC contacts are damaged beyond reconditioning then the entire switch has to be replaced with a used-good original. These are sometimes difficult to find except from parts sets. The same switch is used on all of the versions of the BC-312, 314, 342 and 344. Alignment Notes - IF is 92.5kc. Note in the photo above left that the large LO box (far left) has four plugs that are covering access to the trimmers for each band. Also note the shield cover over the two RF amp boxes and the Mixer box that is preventing easy access to these trimmers. Each IF transformer adjustment has a lock nut on the threaded shaft to prevent tampering (see photo above right showing top of chassis.) Other than the lower frequencies involved, the BC-344 is straight forward in its alignment procedure.
Performance on a Wire Antenna -  The antenna used was my 135' CF Inv'd Vee with 96' of ladder line with the two feed line wires tied together. This is something like a "T" antenna and it performs fairly well on LF. The test listening was on January 29, 2018 from 1915 to 1945 PST. I only tuned the BC-344-D from 405kc down to 300kc. Within the frequency span and time period I tuned in 25 NDBs. Greatest USA DX was IN 353kc in International Falls, MN. Greatest DX was YMW 366kc in Manawaki, Quebec, CANADA. Another Canadian NDB tuned was YXL 346kc in Sioux Lookout, Ontario. Listening QTH was Dayton in Western Nevada.

All signals were heard over a headset, not by loudspeaker. BFO was on and stations generally tuned for zero beat of carrier to then hear the MCW tone correctly. Sometimes it's advantageous to tune the BFO about 400hz to 1000hz from the IF frequency. This will enhance the 400hz on some weak NDB stations. The AVC was off. Noise level wasn't too bad and I'd rate the conditions as "very good."

The BC-344-D is a capable LW receiver that has ample sensitivity in the medium wave portion of the spectrum using a wire antenna. There is no crystal filter or any other method to reduce IF bandwidth so many signals are "heard" over what seems to be a fairly wide IF passband. It's easy to select one particular signal and then concentrate on that tone or sound of the signal to then successfully copy the call. Most of the time this process will be with very, very weak signals that are within the passband with stronger signals. If the received noise seems to be covering up some weaker signals it's possible to slightly "detune" the antenna trim (ALIGN INPUT control) to reduce the noise while not affecting signal copy.

While the BC-344-D is kind of a "basic" superhet with no fancy filters or output limiters, it does a good job with the few controls provided. It's certainly sensitive enough and the reduction of the VERNIER tuning mechanism allows for easy tuning of all stations, including NDBs. Without an output limiter though there isn't any way to reduce "pops and clicks" or heavy static. Wearing the 'phones just ahead of the ears is recommended. I'm sure if the BC-344-D was used with a wire antenna in a modern RFI-noisy, urban-type location the weak signal DX reception would be terrible. But, in the rural RFI-quite area here in Dayton, NV, the BC-344-D does a very good job of pulling in NDB DX using just a wire antenna.

Performance on  a Tuned Loop Antenna - The remotely tuned loop is six feet "point to point" of its diamond-shape and tuned using varactor diodes with a variable bias voltage source. The receiver is connected to a pick-up loop that is mounted within the main loop. Test listening was on February 1, 2018 from 1930 to 2000 PST. I only tuned the BC-344 from 325kc up to about 390kc. 23 NDB stations were tuned in during that time. Conditions were very good. Greatest USA DX was FIS 332kc in Key West, FL. Greatest Canadian DX was GW 371kc in Jarpik, Kuujjuarapik, Quebec, CAN.

All signals were heard over a headset, not by loudspeaker. BFO was on, AVC was off. Loop Antenna was pointing NE/SW for the entire test listening period.

The BC-344-D running with the tuned loop antenna is a surprisingly good performer. Without any bandwidth controls, one would expect noise to be a problem but the loop does keep the noise level somewhat lower than the wire antenna. Additionally, the "loop tuning" provided the ability to slightly detune the peak adjustment which lowered the noise without causing loss of the signal. This was a nice advantage for signals that were "in the noise." Also, the ALIGN INPUT (antenna trimmer) could be used to slightly detune the antenna and reduce the noise peaks allowing some weaker signals to be copied. These two features, loop tuning and antenna trimmer, do a lot to make up for the "wide open" IF bandwidth which does have a tendency to be somewhat noisy.

As mentioned in the "wire antenna test" section, the BC-344-D is a pretty basic receiver with no filters or no bandwidth control. Yet, I was still able to copy NDBs all the way to the east coast and Canadian NDBs out to Quebec. I don't think any vintage LW receiver enthusiasts will be going out to purchase a BC-344-D to use as their main LW receiver, but I was impressed with its performance and would rate the receiver as one of the better medium wave and low frequency receivers. It is fully capable of receiving DX LW signals and, with its incredible "WWII Military" appearance, makes a fine addition to any collection of vintage LW receivers. Additionally, if you get tired of listening to MCW signals on 'phones, you can always tune in almost the entire AM BC band and listen on an LS-3 loudspeaker - very cool.





National Co., Inc.


   RBL-5, CNA-46161-B  - MW, LF & VLF Radio Receiver - 1945

15kc to 600kc


National Company also provided a great LW receiver for the Navy in WWII - the RBL series of regenerative receivers. Following the long Navy tradition of  National providing NC-100A types of receivers - like the RAOs and similar HF receivers, the RBL series uses the same general appearance with a similar dial layout and a familiar band switching feel. Though the bandswitch looks like the RAO catacomb system, it isn't. The mechanism uses several large gears to simultaneously actuate two large ceramic switches to provide band changes. The RBL is the same approximate size as the RAO receivers so it was probably intended that they be paired up for coverage from 15kc to 600kc on the RBLs and 540kc to 30mc on the RAOs. Unlike the earlier LW receivers described above, the RBL has a built in power supply and has direct frequency readout on the illuminated dial. Like the RAO receivers, Wells-Gardner Company was a second contractor and built the RBL-3 and RBL-4 versions under contract using many National parts for assembly (see RBL-3 at the bottom of this write-up.) The RBL-6 receivers were supplied with a one-piece, larger cabinet that had the shock mounts installed on the bottom of the cabinet. The front panel of the RBL-6 receiver was 19" wide however the receiver wasn't intended for rack mounting. Side panels were installed on the chassis and the entire chassis could be easily removed from the cabinet for maintenance.
The circuit uses a cascade of three 6SK7 RF amplifier stages. The detector is a 6SG7 regenerative autodyne detector followed by a 6H6 audio limiter circuit followed by a 6K6G audio tube. The power supply rectifier is a 5U4 in early RBLs but later was changed to a 5Y3G. Like the RAO, some RBL receivers were built by Wells-Gardner Company. Heavy duty construction, ample shielding, copper-plated cabinet under the black wrinkle paint are standard construction used in the RBL receivers. They were normally bolted to a cushioned mount that attached to the holes in the lower front and rear corners of the cabinet. Nowadays these mounts are usually missing. Included in the circuit is an audio filter for wide or narrow bandwidths (switch on left side of escutcheon below ON-OFF switch) and an adjustable audio limiter (switch and control on right side of escutcheon.) The limiter is very well designed and works wonders in reducing the static crashes while not distorting the audio signal. The direct frequency readout on the dial is the major advantage of using the RBL receivers and the accuracy is impressive considering the receiver's age. The illuminated dial is quite a departure from the usual military LF receiver. The lower controls (l to r) are gain, regeneration, bandswitch, antenna trim, oscillation push button and frequency trim.

This RBL-5 was acquired from a ham neighbor here in Virginia City. It required a little work before it was functioning to its specifications. The tubular antenna connection input that attaches to the box that bolts to the back of the cabinet was shorted internally so essentially whatever antenna was connected was shorted to chassis. Removal of the tubular connector and just running the coax through the box directly to the antenna and ground terminals fixed the problem. Also, there was a soldering job at the audio output transformer that was poorly done. Exactly what the object of the solder job was is not known but it probably was in search of the lack of output that was really caused by the shorted antenna input. Fortunately, no original parts were removed and only the connections to the audio output transformer were moved to incorrect terminals. We just returned everything to the original connections and then the receiver output returned to normal.


photo left: The chassis on this RBL-5 is immaculate and all original. RF section is on the right side of the chassis and the power supply, limiter and audio sections are on the left side.


I have logged a lot of NDBs using this RBL-5 receiver, primarily because the RBL-5 is easy to use, very sensitive, has direct frequency readout and the limiter functions quite well. The limiter makes long sessions of receiving comfortable since the static crashes are reduced to the point where they aren't causing headaches anymore. I take the audio output right from the earphone jack on the front panel running 600 ohm 'phones for best copy on weak signals. The NBDs normally copied are multiple stations operating on the same frequency, with two and sometimes three different CW identifications being heard simultaneously. Using the RF trimmer and the Antenna Compensator controls, it is usually possible to enhance one or the other of the MCW signals and identify the particular NDB, (the RAK and RAZ LW receivers also have this ability to manipulate the signal a little to enhance copy.) The RBL works particularly well with the tuned loop antenna and this provides the ability to add some directional characteristics to the reception. Additionally, the loop can be slightly de-tuned to allow enhancing NDBs that are on one side or the other of antenna resonance which can sometimes help with copy.




photo right: The underside of the RBL-5 is also immaculate and all original. The photo shows the multiple gears that drive the two ceramic bandswitches. Construction is first rate as expected from National Company. Note that the alignment trimmers are all clustered together. The bottom cover has a sliding access panel that allows the receiver to be aligned with the bottom cover installed - probably why the RBL-5 has such an accurate dial readout.

Shown in the photos right and left is the Wells-Gardner Co. version, the RBL-3. This receiver is also in excellent condition. Note the difference in the transformers and chokes used in the W-G version compared to the National RBL-5. Typically, W-G used mostly National parts but did use their own transformers, chokes and smaller components. Note the rectifier tube in the upper left corner is a 5U4G rather than the 5Y3GT used in later versions of the RBL.  Performance of the W-G RBL-3 is the same as the National RBL-5.



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