Radio Boulevard
Western Historic Radio Museum

WHRM Radio Photo Gallery

1932 - 1942  &  1946 - 1969
(Airport, Shipboard, General Purpose & Military Gear)

(For WWII gear go to "WWII Communications Equipment"  -  Nav Link in Index Below)

 Photo: Army Troubleshooting  

Commercial & Military Communications Gear - 1932-1942 & 1946-1969

Pre-WWII Gear - 1932 - 1942


Airport Communication Receivers

National Company, Inc.  -  RHM Airport Receiver

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

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

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

To take advantage of the prestige the Department of Commerce contract had given them (and to profit through additional sales to the general public,) National adapted the RHM for ham use and dubbed it the AGS. The AGS was upgraded with newer tube types and other changes during its short production life (probably two or three production runs totaling no more than 300 receivers.) The major change was with the introduction of the AGS-X which added a front panel BFO control and  a James Lamb type crystal filter to the receiver. In 1934, optional 10 meter coils were added as the AGS frequency coverage was increased to reflect the needs of a "ham receiver" - although at $265, not many hams could afford it. If the ham really wanted the AGS-X he could wait for the introduction of the HRO (in early 1935) at which time Leeds was selling the AGS-X for $123. Or, if the ham couldn't wait, he could opt for the Hammarlund Comet Pro, the only other commercially built shortwave superhet available at the time. Actually, the Comet Pro was only $155 (and it had a built-in power supply) but it didn't have an RF stage and required an external pre-selector for image-free reception above 10mc. The Comet Pro came out in 1931 and, from 1932 up to about early 1934, only National and Hammarlund were offering commercially-built, shortwave superhets.

In addition to the RHM and AGS receivers, National also produced the RHP and RHQ receivers that were very similar to the RHM within the circuitry but ganged the three coils together behind a small panel that created a plug-in "coil set" for each tuning range covered. These receivers were specifically designed for airways communication and therefore were only supplied with two coil sets that allowed tuning from 2500kc up to 6500kc. The RHQ was also identified as the AGU receiver in some uses. Also, National produced a long wave receiver built along the same lines as these early airways receivers, the RIO. The RIO didn't use plug-in coils but had two switched tuning ranges that covered frequencies below 500kc.

I have owned the RHM  receiver shown above since 1990. It is all original and has its complete original coil set (15 coils) in the original rack mounted coil holder. In March 1933, Radio News published an article by James Millen titled "Testing a Modern Superhet" that described National's procedure for testing and aligning the AGS receiver. In the article there was a photo of the Radio News' AGS set-up that showed they were using a National Dog House power supply and a set of Hi-Z 'phones for the audio output. I've been running my RHM in this fashion also, using a National 5886 PS (6.3vac Filament and +180vdc B+) and then listening on a pair of Type-C Navy Baldwin 'phones. This has given me the best results, although if I don't want to use the "Baldies," I can connect up a Hi-Z magnetic cone speaker like a Radiola 100A which then eliminates the need for an audio output transformer and provides ample volume (the Hi-Z speaker solenoid coils connect between AF plate and B+ - just like an audio output transformer.) The RHM functions quite well with 75 year old components - every part was the best that was available at the time. Today, the RHM performance seems antiquated and crude but in 1932 it was "state-of -the-art" and the fact that the receiver is still operating and is still fairly accurate in its dial readout is testament to National's build quality and Herbert Hoover Jr. and James Millen's design capabilities. This same design team again worked together in 1934, producing the famous HRO receiver.

photo left: The RHM Airport setup (bottom to top) power supply, SW-58 monitor receiver, RHM receiver, spare coil holder and loud speaker.


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

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

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

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

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


National RCE, ca: 1937

National Co., Inc. - Airway Communication Receivers

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

Although the Airway Communication Receivers are based on the NC-100 chassis, there are some significant additions to the circuit. The most obvious is that the receiver is rack mounted and has a very different front panel when compared to the striking "art deco" panel of the standard NC-100 receiver. The Airway receivers use a 3/16" thick aluminum front panel that is black wrinkle finished.

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

Shipboard Radio Equipment

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

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

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

The Type 105-A is actually a pre-WWII commercial shipboard receiver that dates from sometime after the Federal Telegraph move to New Jersey since the ID tag lists Newark, N.J. as FTC's location. It is a four tube receiver using 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. It is possible that this Type 105-A was updated either at the factory or by a professional radio work shop for the cathode type tubes since there are some indications that the original design may have used direct-heated filament type triodes. The frequency coverage is 1500kc down to 15kc in seven tuning ranges. Power is supplied by batteries. Like earlier designs for shipboard receivers, e.g. the IP-501-A, the Mackay 105-A utilizes an LC Antenna tuner ahead of the regenerative detector to increase gain and selectivity. An Antenna Series Condenser switch selects various value capacitors to match the ship antenna to the receiver input and a stepped Tone control provides some relief from static. The panel meter is a dual meter that normally reads filament voltage but B+ voltage can also be monitored by activating a panel switch. The left large tuning knob tunes the Antenna Condenser, the middle large knob controls the Regeneration Condenser and the right large knob tunes the Detector Condenser. The Mackay 105-A is built for shipboard use being physically stout and very heavy. Originally the receiver was panel mounted in one of the Mackay Marine Radio Units that housed the majority of the radio gear for the ship. (See our "Vintage Longwave Receivers" webpage for an in depth article about this receiver.)

Mackay Radio & Telegraph Co. - Type 102-B Frequency Monitor


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



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

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

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

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

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

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

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

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

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


General Purpose Communication Receivers

RCA AR-88D ca.1944

Radio Corporation of America  -  AR-88 Series

     includes: AR-88D, AR-88LF, CR-88, CR-91, SC-88, R-320/FRC - also Triple Diversity Receivers DR-89, RDM and OA-58A/FRC 

RCA's greatest communications receiver creation was the AR-88. Designed in 1940-41 by Lester Fowler and George Blaker, the AR-88 was a 14 tube superheterodyne that covered .54 to 32MC in six tuning ranges and featured incredible sensitivity (even up to 10 meters), excellent stability and high fidelity audio (from a single 6K6.) Most of the production was sent to England, Russia or other Allies during WWII because of Lend-Lease which accounts for the relative scarcity of the early versions of the receiver in the USA. The AR-88 was used extensively in Great Britain during WWII for various purposes. RCA and Radio Marine Corp.of America also used the AR-88 and its variants in their own installations for various purposes. Even the US Military used some of the later AR-88 variations in their installations. Contrary to some published estimates of unbelievably high production levels in excess of 100,000 units, serial number analysis seems to indicate that around 30,000 AR-88 series receivers were built between 1941 and 1953.

The AR-88 series receivers use three stages of 455kc IF amplification with stagger-tuned IF transformers. Two under-coupled IF transformers and two over-coupled IF transformers are utilized when the receiver is operated in the "BROAD" selectivity position. To assure that the passband is symmetrical usually requires a sweep generator and oscilloscope for proper alignment. However, if fidelity is not an issue, there is a procedure to align the IF section using just a VTVM but the results are usually not as good as the sweep method. There are five steps of selectivity with position 1 and 2 being rather broad for good fidelity while positions 3,4 and 5 use the crystal filter for increasingly narrow bandwidth. A Noise Limiter and a Tone control were provided. The standard table top version was designated as AR-88D and it sometimes had a Carrier Level meter incorporated into the circuit however many AR-88D receivers didn't have CL meters installed due to a shortage of meters that occurred during WWII. The wiring for the meter was sometimes included in the harness for future installation of a CL meter, if they became available. Generally, the wires for the meter connection are bolted to the lamp bracket behind the receiver's illuminated ID window. Early in WWII, some of the Allies required receivers that covered MF frequencies and the AR-88LF was created, covering 70kc to 550kc and 1.5mc to 30mc. The first 3000 or so AR-88LFs used different power transformers and different audio output transformers from the AR-88D. The IF was at 735kc to allow complete coverage in the 400kc to 500kc range. All AR-88LFs were built at the RCA plant in Montreal.

photo right: Early version of the CR-91 variation of the AR-88, ca. 1945

Many of the AR-88 receivers were used in RCA Triple Diversity Receivers like the DR-89 - a seven foot tall rack loaded with three AR-88F receivers and all of the auxiliary equipment necessary for professional diversity reception. The Navy designation for the DR-89 was RDM. The Diversity AR-88F receivers did not have CL meters installed because the Diode Load current output from each receiver was routed to the Tone Keyer terminal board but the actual three Output Level meters were mounted in the Monitoring Unit Panel of the DR-89/RDM rack. All Diversity AR-88 receivers (and their variations) that were used in RCA Triple Diversity Receivers will have a "DIVERSITY IF GAIN" control on the front panel. This provided a method of adjustment for balancing each of the receiver's output for equal diversity effect (using the actual desired signal) even if the receivers/antennas were not exactly identical in their performance. The U.S. Army Signal Corps had their versions of the RCA Triple Diversity DR-89 with the Signal Corps ID of OA-58A/FRC. The Army SC diversity set-ups used a slightly different, upgraded receiver, the SC-88.

The AR-88D, CR-91 and some of the other variants that weren't specifically for RCA diversity racks didn't have the "DIVERSITY IF GAIN" control and were usually installed in a table cabinet although this wasn't always the case. Note that in the case of the AR-88D, the "D" does not indicate a "diversity" receiver, only the AR-88F, CR-88A and SC-88 are true diversity models of the '88 receiver. A matching speaker was available for all table models. The CR-91 was the version of the AR-88LF that was built in at Camden, NJ. The AR-88 series of receivers were produced from 1941 up into the early fifties. The CR-88B was the last of the AR-88 series, available from 1951 up to 1953. The CR-88B features push-pull audio, two position tone switch, dial masking, crystal calibrator, 15 tubes and a three position selectivity switch.

photo above: CR-88A version from 1947 in RCA "umber"

1950 Signal Corps R-320/FRC, aka RCA SC-88, part of OA-58A/FRC

The AR-88 series of receivers are well known for their amazing performance with fabulous audio reproduction and great reliability. Unfortunately, a great many AR-88s led a hard life and then were stored in poor environmental conditions that nowadays results in the receivers being found in "rough" condition with missing shields, missing parts and usually other more serious problems even though the receiver may still function. Many receivers are found as victims of careless repairs or needless modifications. A "well-cared for," original AR-88 that has not been modified will usually function quite well but if it has been carefully and correctly rebuilt and then aligned (using the sweep method for the IF) will have tremendous sensitivity and flawless audio reproduction. The front panel adjustable Crystal Phasing used on the later versions allows the user to not only adjust the selectivity of the receiver but also to use the Crystal Filter to reduce or eliminate heterodynes. The AR-88 was one of the first receivers that was designated as a "continuous bandspread" receiver due to its substantial gear reduction but its tuning accuracy relies on the logging scale for precise reset ability. The direct frequency readout resolution is vague. The AR-88 receivers are stoutly built using all steel construction and they are heavy, weighing in at nearly 100 lbs when installed in the cabinet. 

Shown in the photo to the left is the SC-88 (Signal Corps designation R-320/FRC, SN 214, used in OA-58A/FRC diversity receivers) one of the later of the AR-88 versions from 1950, featuring "band-in-use" masking and the crystal phasing control on the front panel (the AR-88's is internally adjusted.) Since the SC-88 was specifically built for the Signal Corps' diversity racks, these receivers are rack mount configuration only and have the "DIVERSITY IF GAIN" control on the front panel. The total production of SC-88 receivers was quite small with estimates usually being around 300 receivers built. Although the SC-88 appears similar to the earlier AR-88 series, many changes took place inside which moved the locations and designations of the front end alignment adjustments. Using an AR-88 manual for aligning an SC-88 will not provide accurate information. The proper manual for the SC-88 is TM11-899. 

Shown in the header photo is the AR-88D from WWII. The first inset photo shows the early version of the CR-91, a receiver that is very similar to the AR-88 but with a frequency coverage of 70kc up to 550kc in the first two bands and then 1.5mc to 30mc in the remaining four bands. The CR-91 uses a different IF frequency of 735kc to allow continuous coverage in the LF and MF spectrum. Additionally, the audio output tube was changed from the 6K6 to a 6V6. The slightly different (wider) IF bandwidths are a product of the higher IF frequency. The CR-91 version was introduced during the last part of WWII with these receivers built at Camden and having the non-adjustable crystal filter phasing and a black wrinkle finish panel. The later CR-91A essentially took the place of the AR-88LF with all manufacturing at the RCA plant in Montreal. The CR-91A was an updated version that has the front panel crystal filter phasing control and a smooth gray finish on the front panel. Most early CR-91 receivers were in cabinets and were probably used for surveillance or LF/MF communications onboard ships (some CR-91 manuals warn about excessive LO radiation on the antenna if the A2-G link is removed.) Shown in the second inset photo is the 1947 CR-88A. These receivers were generally for the later versions of the DR-89 and RDM Triple Diversity Receivers but sometimes they are found as individual receivers that were used for a multitude of purposes. This example of the CR-88A is installed in a matching RCA cabinet. 

For the ultimate information source on the AR-88, including more history, the triple-diversity receivers, serial number analysis, how to do sweep IF alignment, restoration hints and performance details go to our web article "RCA's Amazing AR-88 Receivers" - Link below in the Navigation Index.


Post-WWII Gear - 1946 - 1969


Airport Communication Receivers

National Company, Inc.  - RCR Airport Receiver

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

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

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


Shipboard Communication Receivers

Mackay Radio & Telegraph Company  -  Type  3001-A

The Mackay Radio & Telegraph Co. Type 3001-A is a Longwave regenerative receiver covering 15kc to 640kc in four bands and dates from as early as 1948 but with most manufacturing dating much later. The receiver shown was built in 1952. Mackay receiver serial numbers generally incorporate the last two digits of the year of manufacture as the first two digits of the serial number. The 3001-A was mainly for commercial shipboard (non-military) use where it could be set up as the main receiver or as the emergency receiver. The receivers were sometimes installed in the Mackay "Marine Radio Units," like the MRU-19/20, a shipboard radio console which contained two 3001-A receivers along with transmitters and other auxiliary equipment (the MRU receivers were panel mounted.) The 3001-A uses an AC-DC circuit and can operate on 115vac or on batteries. Various filament battery options were available with 6vdc, 12vdc and 24vdc being the most popular. B+ was supplied by standard dry cell B batteries when used. The receiver uses a four pin Amperite ballast tube along with six octal tubes. A small built-in speaker provides for radio room monitoring but earphones would normally have been used by the shipboard radio operator. Selectivity is controlled by a combination of the RF Gain setting and the setting of the Regeneration. The 3001-A is very sensitive and capable of receiving any of the NDBs and other LW stations found in the spectrum below 500kc. These type of Mackay receivers were in use for several decades and were commonly found still operating on commercial ships as late the 1990s. These types of Mackay receivers date from the late forties and were manufactured through the fifties. (See our "Vintage Longwave Receivers" webpage for an in depth article about this receiver.)


Radiomarine Corporation of America  -  AR-8506-B

The origins of Radiomarine Corporation of  America date back to the 1920s,...a time when RCA was controlled by General Electric and Westinghouse. When RCA was created in October, 1919, the US Navy wanted a radio company that could handle operation of the coastal wireless stations, service all the equipment, provide sales of new wireless gear. General Electric created RCA out of some of their own assets and, mainly, with the purchase of American Marconi. As radio developed, GE directed what was to become known as the "Radio Group" - a patent-sharing association of five companies that dominated radio in the early twenties. General Electric, Westinghouse, RCA, United Fruit Company and Wireless Specialty Apparatus owned all of the important radio patents. As radio broadcasting became more and more popular, GE and Westinghouse somewhat controlled that end of business while United Fruit Company and Wireless Specialty Apparatus was more involved with the maritime radio business. Eventually, RCA was able to purchase Wireless Specialty Apparatus (around 1925) and this company became Radiomarine Corporation of America. The virtual domination of radio by GE, Westinghouse and RCA ended in 1930 with a settlement of an anti-trust suit by the government (which actually gave the Navy what they had wanted back in 1919.) The settlement took patents from GE and Westinghouse (and others) and gave them to RCA. Westinghouse and GE couldn't compete with RCA for two years (until 1933) and RCA was freed of any debt to Westinghouse or GE. This meant that the millions of dollars that RCA owed Westinghouse and GE for the loans necessary to buy the Victor Talking Machine Company didn't have to be repaid. In 1930, RCA Victor became the division of RCA that handled consumer radio and Radiomarine became the division that handled all shipboard radio equipment and operated all of the RCA Coastal Ship to Shore stations.

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

After WWII, the AR-8506-B continued to be offered by RMCA for maritime use on various types of ships. The post-war versions are somewhat different in appearance in that the individual celluloid control identification plates are replaced with a "raised letter" type of panel nomenclature. Additionally, the data plate was removed and the manufacturing information became part of the front panel nomenclature. The AR-8506-B shown in the photo above is from 1953 and shows how the later versions looked when installed in the table top cabinet (with shock mounts.) Ship owner's reluctance to replace radio gear had the RMCA consoles and the associated equipment in-use well beyond their normal life-span with examples still in use as late as the 1980s.   

The AR-8506-B has an internal 1700kc wavetrap. The wiring and adjustment of the wavetrap should be checked if BC signal leakage is encountered. The wave trap should be adjusted on Band 3 for minimum response with a 1700kc RF signal input to A1 on the antenna input of the receiver. If it is correctly adjusted and still there is BC signal leakage then using an antenna that is "tuned" for the specific frequency desired should be tried. This could be a resonant antenna cut for the specific frequency desired or an antenna with an antenna tuner. The "tuned" antenna will be selective and should reduce the BC interference. Like a lot of RCA receivers, the AR-8506-B doesn't have a standby switch (either remote or panel.) To use as a station receiver requires either an antenna relay with good isolation for the receiver in "transmit" or you can also use an electronic TR switch.

The WWII version of the AR-8506-B is profiled in the "WWII Communications Equipment" page - navigation index at the bottom of this page.


Radiomarine Corporation of America  -  Model AR-8510

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

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

The AR-8510 was approved by the FCC for shipboard use in 1942. The schematic drawings are dated 1943. It's likely that it was at least 1944 before any AR-8510s were in use. The AR-8510 shown in the photo above is the post-WWII version and the photo is from the manual. Like the AR-8506-B, the AR-8510 replaced the round celluloid nomenclature plates and went to an embossed nomenclature panel. Production continued into the 1950s and actual use of the AR-8510 lasted quite a bit longer. It wasn't uncommon to find AR-8510 receivers still being used on old oil tankers as late as the 1990s.

The WWII version of the AR-8510 is profiled in the WWII Communications Equipment page - navigation index at the bottom of this page.


General Purpose Radio Communication Equipment

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

Post-WWII use of the BC-794 Hammarlund Super-Pro - Signal Corps

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

The Wickes R-270/FRR changed some of the tubes in the BC-794 as part of the upgrade. The 6H6 AVC rectifier was changed to a 6SN7 to allow the use of one section (diode connected triode) as the AVC rectifier and the remaining triode to be used as an IF output buffer stage. BFO was changed from 6L7 to 6SL7 and the MC-531 kit added a 6SC7 crystal oscillator to the circuit. Additionally, the BFO could be crystal controlled also, if necessary, for precise reception of RTTY signals. A new aluminum front panel was installed with raised lettering and the steel bottom plate was replaced with an aluminum plate with the receiver schematic printed on the inside. The RA-74 power supply also got a new aluminum front panel and aluminum bottom cover with the schematic printed on the inside. The Crystal Oscillator front panel was installed over the Main Tuning dial escutcheon providing an ON/OFF switch with three channel selection and a separate frequency vernier control. All component nomenclature was redone and the entire chassis given a heavy coating of MFP.

In 1950, Hammarlund released the SP-600 JX receiver that incorporated all of the Signal Corps upgrades along with totally redesigning the entire Super Pro receiver. However, the Signal Corps was also beginning to use the Collins 51J receivers in their RTTY communications. Ultimately, the SC used Collins receivers for RTTY (in most instances) and the SP-600 for general surveillance. 


1950 Collins 51J-2 in original style cabinet. Note the green highlighting of the 40 meter amateur band

Collins Radio Co. - 51J Series
(includes 51J-1, 51J-2, 51J-3, R-388/URR and 51J-4)

Introduced in 1949, the 51J series was developed as a general coverage receiver primarily for military but also for the commercial user or individual/enthusiast providing accurate frequency readout and great stability. Since the earliest versions of the 51J receiver had military designations it's likely that Collins was working with the military to design a thoroughly modern, general coverage receiver that had the requirements needed for dependable RTTY and other data modes of reception. The initial 51J receiver utilized a permeability tuned circuit using the 70E-7 PTO along with a dual tunable IF system and a multiple frequency Crystal Oscillator to cover .5mc to 30.5mc in thirty (1mc wide) bands. The dual tuned IF is switched between odd and even bands (referencing the band number not the frequency.) Three fixed frequency (500kc) IF amplifiers are used. Adjustable selectivity is provided by a Crystal Filter. A standard envelope detector and Noise Limiter are also in the circuit. Band 1 actually is triple conversion but only to allow coverage of the AM BC band. Bands 2 and 3 are single conversion (since they are the output of the dual tunable IF) while all of the remaining bands are double conversion (working with the 10 frequency Crystal Oscillator.) 16 tubes are used in the 51J-1 and J-2. On the early versions, the ham bands are high-lighted in green on the megacycle drum dial but, at nearly $900, not many hams could afford a 51J as their station receiver. Early versions also have a metal dial bezel, the Collins' "winged emblem," no grab handles and an illuminated S-meter. Audio response is restricted at 200 to 2500Hz and is definitely not high fidelity, usually sounding somewhat "muffled" when receiving AM voice signals. The most apparent difference between the 51J-1 and 51J-2 is that the latter added an Audio Output function to the S-meter which became a Carrier Level/Audio Output meter that was actuated by a toggle switch next to the meter and the 100kc Crystal Calibrator nomenclature was changed from "100 KC OSCILLATOR" to "CALIBRATE." Some later production 51J-2 receivers may be found with the 70E-15 PTO installed but whether this was a post-sale retrofit or a Collins engineering-production upgrade is unknown at this time. The 51J-1 was produced in very small quantity in 1949 while the 51J-2 was somewhat higher in production quantity being produced from late 1949 thru most of 1950. 51J-1 receivers were supplied to the military as the R-381/URR and 51J-2 receivers went to the military were identified as the R-381A/URR receiver.

In 1950, the updated and improved military version 51J receiver, the R-388/URR, was introduced, featuring an 18 tube circuit (adding a voltage regulator and vfo buffer,) a new version of the PTO (70E-15) and eliminating the fixed 300 ohm Z antenna input (by removing the primary winding on the antenna coils) and redesigning the antenna input to a more flexible design with an Antenna Trim control. This revision was probably at the request of the Signal Corps, who found the R-381 (51J-1) or the R-381A (51J-2) difficult to use for some of their teletype installations because of the fixed 300 ohm Z antenna input requirement  (most of the SC installations used either 75 Z ohm dipoles or Lo-Z vertical whip antennas.) Production of R-388/URR receiver was rather high with estimates that over 12,000 receivers were produced. The contracts started in 1950 but production levels were very small in 1950. The greatest quantity of receivers were produced in 1951, 1952 and 1953. There were additional contracts in 1954, 1955, 1956, 1957 and 1962 but the production levels in these later contracts total less than 1000 receivers. With the R-388, grab handles were added to the front panel along with a high quality Burlington Co. sealed meter although some receivers may be found with a Marion Electric meter (or other makes) installed but whether these were "factory" or a later "field replacement" is not known. Additionally, the 51J-3/R-388 added an internal relay to isolate the antenna and remove plate voltage from the IF section as a Remote Standby function. This addition required the user to provide approximately 12vdc at 135mA to operate the internal relay from the auxiliary contacts on a T-R relay.

Hallicrafters was contracted to build at least two different types of military cases for the R-388 receivers. These cases were made out of heavy gauge steel and featured glides that allowed rollers to be mounted to the R-388 chassis to ease installation and removal of the receiver from the cabinet. A dual receiver cabinet was also produced. The CY-1206/G is shown to the left with a 1952 contract R-388 receiver installed.

A standard phone jack provided 4 Z ohm output and a PL-68 type jack provided 600 Z ohm output both mounted on the front panel on the early versions and later versions had a "Break-in" switch that replaced the front panel speaker jack. Some early receivers were later reworked by the military to replace the front panel speaker jack with the "Break-in" switch. All R-388 receivers are MFP coated and all have an irradite treatment of the sheet metal used for the chassis and side panels that give the pieces a gold color appearance. However, the side panels are steel, not aluminum (as the R-381s were.) Nearly all R-388 aluminum top covers will have the receiver schematic on the underside of the cover. Very late R-388s will have the pin jacks on the rear chassis apron for AVC and Diode Load (same as the 51J-4.) The serial number that is stamped on the rear of the chassis apron is a Collins' assigned number that usually is sequential for the entire production however the ID tag's serial number is sequentially assigned for the specific contract only. These two serial numbers never match and considering how they were assigned, shouldn't. The civilian version of the R-388/URR was the 51J-3 which was produced in very limited numbers since the major demand for the receiver was from the military and that demand was satisfied with the R-388. Some 51J-3 receivers do turn up and their ID tags will identify the receiver as such, however be aware that many so-called 51J-3 receivers are actually R-388 receivers with their ID tags removed. The 51J-3 was introduced in 1951 and produced in limited numbers up to around the introduction of the 51J-4 in 1955.

photo above: 1951 contract R-388/URR

1957 Collins 51J-4 sn 2392 in original style cabinet (now owned by KB6SCO.)

In 1955, the 51J-4, with 19 tubes and three selectable mechanical filters became available. The 51J-4 added a fourth stage of IF amplification to compensate for the insertion loss of the mechanical filters. The mechanical filter assembly utilized two 6BA6 tubes as input and output amplifiers which, while providing a total of four IF amplifiers, actually only three IF stages are tuned. The bandwidth filters used on the 51J-4 were 1.4kc, 3.1kc and 6.0kc. The filters are quite different physically from the other Collins contemporary receivers in that the 51J-4's are rectangular units that are for 500kc IF rather than the round cylindrical types found in the 75A-4 or R-390A receivers that are for 455kc IF. The 51J-4 also added an adjustment to the overall gain of the IF amplifier section in the form of a chassis mounted potentiometer. The 51J-4 sold for $1099 and despite its expense was still a popular receiver that could be found in coastal stations such as KPH and KMI, in overseas embassies, in commercial laboratories like Beckman, universities like Stanford and even wealthy SWLs and enthusiast-ham set-ups. Some of the very late production 51J-4 receivers can be found with light gray panels and black nomenclature with some even sporting Collins S-line knobs. 51J-4 receivers are not MFP coated. Ultimately, the 51J-4 serial numbers exceeded 7000. Navy catalogs specify an R-388A and R-388B - the R-388 with the mechanical filter assembly installed. It likely that these receivers were probably actually identified on their tags as "51J-4" which would account for the lack of any examples of the R-388A or R-388B.

The 51J-4 was the ultimate evolution of the design but the earlier 51J-1, J-2s and R-388s have their own appeal and can provide top-notch reception. Though thousands of R-388 and 51J-4 receivers were produced, very few 51J-1 or 51J-2 receivers ever turn up indicating that their production was at a fairly low level. Fortunately for those who enjoy using these receivers, the most common version, the R-388, is the best performer providing good quality audio and competitive sensitivity with 1kc dial accuracy.

For more information on the 51J Series including rebuilding and restoration go to our web-article "Rebuilding the Collins 51J Series Receivers." Navigation link in Index below


1953 Hammarlund SP-600-25C

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

Officially introduced in 1950 and selling for nearly $1000 at that time, the SP-600 was intended for the military and commercial user market. It was a very popular receiver and many thousands were built, especially for military applications. It's likely that some of the SP-600 design input came from the U.S. Army Signal Corps, especially the selectable crystal oscillator and the turret band switching sections. The Signal Corps did have some WWII-version Super Pro receivers modified with three-channel crystal oscillators in 1947-49 (R-270/FRR receivers, also other Super Pro receivers with Improvement Kit MC-531 installed - see R-270/FRR section above.) Also, the Signal Corp accepted a similarly designed receiver from Hallicrafters in 1949. The Hallicrafters receiver was designated R-274, as was the Hammarlund SP-600. The SP-600 used suffixes A, B and C while Hallicrafters used suffix D. Though most versions were built throughout the 1950s, the SP-600 continued to be produced in smaller numbers up into the early 1970s. The standard SP-600 tunes from .54 to 54MC in six bands. A "J" suffix indicates JAN parts were used in the construction and an "X" suffix indicates a selectable crystal oscillator for maximum stability as the LO. Hammarlund also offered a "JL" version with 100-400KC substituted for the .54-1.35MC band and a "VLF" version that covered 10-540KC (details on the SP-600VLF version in photo caption below.) Hammarlund made over 40 variations that were assigned a numerical suffix which identified the particular circuit, mechanical changes or sometimes the end-user. The last in the "time-line" was the model variation SP-600 JX-21A from 1969-1972 which utilized a product detector circuit, two additional tubes, different knobs and some other changes to make it "compatible" with SSB operations.

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

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

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

Shown in the photo to the left is the SP-600VLF-31 receiver from 1958. This version covers 540kc down to 10kc and uses 21 tubes. Unlike the standard SP-600, the VLF version uses a 705kc IF and is a single conversion receiver. Double pre-selection is employed on all bands (two TRF amplifiers.) The "X" option crystal oscillator provides four channels (instead of six) and uses FT-243 type crystals. Five selectivity positions are provided (instead of six.) The SP-600VLF provides tremendous sensitivity and requires a "tuned" antenna for it to operate to its full potential. Generally, a "tuned loop" antenna will greatly reduce the RFI noise that is very high below 500kc and it can also provide directivity that may also help in noise reduction. Nearly all signals in the LF spectrum are now digital signals or beacons with virtually no voice signals at all. Best results using the SP-600VLF will require an "RF-quite" location, a tuned loop antenna of reasonable size (minimum 3' diameter) and using a headset for the audio output since many signals are "in the noise" and barely detectable.

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

For more details on rebuilding the Hammarlund SP-600 receiver, read our article - "Rebuilding the Hammarlund SP-600" - navigation link at the bottom of this page

photo right: 1953 Hammarlund SP-600 JX-21


 the Hallicrafters, Inc.  -  R-274/FRR, R-274D/FRR (aka: SX-73)

In the late-1940s, the U.S. Army Signal Corps needed a source of high-quality receiver that had specific Signal Corps design requirements. The main features were a selectable crystal oscillator to stabilize the LO and BFO drift and a rotating turret band switch. Both Hammarlund and Hallicrafters built successful versions of these types of receivers. Although WWII Hammarlund Super Pro receivers had been modified by the Signal Corps to have the selectable crystal oscillator in 1947 through 1949, the quantity of available receivers was limited. The Signal Corps needed a manufacturer that could deliver a new receiver with the specified options in fairly large quantites. Hallicrafters built their receiver on a 1949 contract and it was assigned the designation of R-274. Hammarlund had been advertising their SP-600 since 1948 (as the SPC-600-X) but apparently the receiver wasn't available (or not ordered by the Signal Corps) until 1950. The Signal Corps didn't want to assign a new designation to a similar receiver so the Hammarlund SP-600 was assigned R-274 with suffixes A, B or C used for specific identification. Future R-274 receivers from Hallicrafters would be designated as R-274D.

The typical post-WWII military contract production quantity was probably relatively small, perhaps 1000 receivers per contract. It doesn't seem likely that Hallicrafters would have gone through the effort for one contract. Hallicrafters probably thought there would be many more contracts in the future but that doesn't seem to be the case. The Signal Corps obviously favored the Hammarlund version and literally tens of thousands of SP-600s were supplied to the Signal Corps over the next decade. Since Hallicrafters had invested in some production line tooling and had obviously set up component suppliers for production, they decided there might also be a commercial or even a ham market for their receiver. The civilian designation assigned was SX-73 and these receivers are virtually identical to the R-274D except for the ID tag, which shows "SX-73" as the receiver type. Some advertising mentions that a cabinet was supplied with the SX-73 though advertising artwork generally shows the receiver in the rack mount configuration. Selling price was quite high at $975 which certainly limited purchases of the SX-73 by the civilian market. The SX-73 version is seldom seen and production must have be very limited. The R-274D and SX-73 were available from late-1951 up to around early-1954.

The R-274/SX73 is generally referred to as "Hallicrafters' version of the SP-600" or the "Hallicrafters' Super-Pro" since there are so many similarities between the two receivers. The similarities are to be expected since Hallicrafter's design had to meet Signal Corps specifications (as did the SP-600.) The most obvious similarity is the turret band switch which, while functionally the same at the SP-600's, is not nearly so robust in construction using fiber board modules while the SP-600's are made of ceramic. The tuning dial provides a main dial and a logging dial as the SP-600 does but behind a single escutcheon rather than separate dials behind two escutcheons as the SP-600 does. There is a selectable six channel Crystal Oscillator that functions like the SP-600 "X" option and provides improved stability for RTTY and other data modes. Like the SP-600, the bandwidth is selectable in six selectivity steps with three of those steps using a Crystal Filter for narrow bandwidth (a front panel Phasing control is also provided.) A 600 ohm balanced audio output is also similar to the SP-600 audio output.

One major difference between the R-274/SX73 and the SP-600 circuit is the conversion frequency of the SP-600 is 3.955mc while the R-274/SX73 uses 6.000mc. Also, the placement of the conversion frequency with reference to tuning range four has the double conversion starting at 7.0mc on the R-274/SX73 while it is 7.4mc on the SP-600. This results in double conversion being used for the 40 meter ham band on the R-274/SX73 but not on the SP-600. The R-274/SX73 frequency coverage of each tuning range is beneficial to the ham user in that 160, 80, 40 and 20 meters are on separate tuning ranges while the SP-600 combines 80 meters at the low end and 40 meters at the high end on tuning range three. In the audio section of the R-274/SX73, the coupling capacitors are .01uf in the R-274/SX73 while the SP-600 uses .0015uf capacitors. This results in the "communications-grade audio" found in the SP-600 while the R-274/SX73 has a more conventional audio response. Additionally, the R-274/SX73 provides an Antenna Trim control while the SP-600 does not. Possibly the most important difference between the R-274/SX73 and the SP-600 is that the former receiver utilizes almost entirely ceramic disk capacitors in the circuit rather than the "leakage-prone" molded capacitors that have negatively influenced the reliability and reputation of the early SP-600 receivers. In considering the restoration of the R-274/SX73, the ceramic capacitors will certainly and positively reduce the amount of rework that is going to be necessary.

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

The R-274D/FRR shown in the photo above is now owned by KØDWC.


1951 Collins R-390/URR in a CY-979/URR cabinet

Collins Radio Company  -  R-390/URR & R-390A/URR

R-390 - Arguably, the R-390/URR and its later kin, the R-390A/URR, are the ultimate tube-type receivers. The first version of this incredible receiver was the R-390 featuring 33 tubes (includes the 3TF7 ballast tube,) double or triple conversion, two RF stages, six IF stages, modular construction, three audio filter settings, six selectivity bandwidths and frequency coverage from 500 kc. to 32.0 mc. in 32 - one megacycle wide - bands. It is a high performance receiver that really "shows its stuff" when conditions are poor but will also provide fairly nice audio quality when receiving conditions allow for it. The most common complaint is the cumbersome tuning that, while "parked" on one frequency is not apparent, shows up when spanning an entire band or changing ranges. Most of the "stiff tuning" complaints can be traced to an over accumulation of grease and dirt in the gear train. When clean and properly (lightly oiled) lubed, the tuning is very light and easy to manipulate. Only Collins or Motorola built the R-390 contracts which ran from 1951 through 1953. The military complained that the R-390 was very difficult to maintain and too expensive. Some of the maintenance issues involve the R-390's elaborate electronically regulated B+ circuit that uses two 6082 tubes along with two 5651 voltage reference tubes and a 6BH6 DC Voltage Amplifier tube. This circuit runs quite hot and accounts for many of the problems that develop in the audio module (where the regulator circuit is located.) Additionally, the R-390's gear train has a moveable "locking gear" that must be installed prior to removing the RF module (if you want to keep everything synchronized.) This gear was painted green and usually mounted with a screw on the front of the gear box. Each time the RF Module is removed and then replaced on an R-390, the KC and MC drive shaft split gears have to be reset for backlash, the Crystal Oscillator module's bandswitch has to be synchronized and the oldham coupler installed. Removal of any of the crystals in the Crystal Oscillator module requires removal of the hard-wired crystal oven. When the military complained about complex maintenance issues, they weren't exaggerating.


photo above: 1955 Collins R-390A. Who knows what the "37.4" stencil means? (I don't.)

R-390A - Collins designed a replacement receiver that was introduced in 1954 with the designation of R-390A/URR. Though the new receiver looked very similar externally to the R-390, inside numerous changes were made to improve cost-to-performance and ease of maintenance. The new receiver's gear box was removable as a unit and synchronization would be maintained, the crystal oven just plugged into the Crystal Oscillator module (it is secured by screws though,) the B+ voltage regulator circuit became a standard 0A2 tube, the crystal calibrator was combined into the RF module (eliminating the separate Crystal Calibrator module of the R-390) and the Crystal Oscillator module was mounted to the RF module so removal of the entire RF deck kept everything synchronized together except the PTO. Most of the maintenance "quirks" of the R-390 were corrected in the R-390A. The major performance change involved the installation of four mechanical filters in the IF section of the receiver. The steep slopes of the mechanical filters gave the R-390A excellent selectivity on 16KC, 8KC (really about 11KC,) 4KC and 2KC bandwidths. The 1KC and .1KC bandwidths are crystal filter derived from the 2KC wide setting. The R-390A uses 26 tubes (including the 3TF7 ballast tube) with one RF stage, four IF stages, mechanical filters on four of the six selectivity positions, plus an 800Hz audio filter. When properly set-up, one can dig right through the QRM while maintaining fantastic sensitivity making the R-390A one of the finest tube-type receivers ever built. However, some ham AM operators find the audio on an R-390A to be a bit harsh due to "ringing" in the mechanical filters. The R-390A was produced in yearly contracts from 1954 up through 1967 (and a very small contract in 1984) with many different contractors building the receivers during those years. Though the R-390A's six modules and redesigned maintenance approach made field repairs easier, it was still a complex receiver. Though the military wanted a less expensive receiver, it certainly wasn't that either.
The R-390 and R-390A receivers have provided reliable communications under adverse conditions for years and even though the designs are over 50 years old, they are still one of the best tube-type receivers around. Many R-390 and R-390A receivers are still being used today, some in professional applications, but also for serious SWLing and, of course, in vintage ham stations around the world. Many AM operators prefer the R-390 version for its better over-all sound quality when listening to SWBC or AM stations in general, however one must consider the maintenance challenges when selecting the R-390 for a station receiver. Nowadays, many R-390 and R-390A receivers are being used in "as purchased" condition - that is, the receiver has not been rebuilt, restored or even thoroughly serviced and properly aligned (or is it "alined?") The performance of a fully functional, rebuilt (restored) and recently aligned R-390/390A is incredible. 

Contemplating rebuilding an R-390A? Go to our web-article "Rebuilding the R-390A Receivers" for easy to follow information and lots of photographs. Link below in the Navigation Index.


photo right: Motorola 1956 contract R-390A. The early  Collins and Motorola panels had silk-screened nomenclature rather than engraved nomenclature. By 1957, all front panels were engraved.

1967 Electronic Assistance Corp. R-390A

The last year of "standard contract production" of R-390A receivers happened in 1967. There are two contacts numbers that are specifically for the 1967 EAC receivers.

FR-36-039-N-6-00189 (E) and DAAG-05-67-C-0016. Each contract was probably for around 5000 receivers.

One of the reasons that the 1967 EAC is sought-after by R-390A enthusiasts is that these are the latest versions that used more modern components. Many ceramic disk capacitors and metal film capacitors replaced the early paper-dielectric types used in the 1950s. Also, some of the 1967 EAC versions were sold directly to civilians so were never in the military repair depots. These receivers are thought to be in better condition since they were generally treated carefully since these civilian owners paid around $2500 for their new R-390A in "1967 dollars."

Shown to the left is a 1967 EAC from contract FR-36-039-N-6-00189 with the serial number of 974. Note that this receiver has the Security Dial Cover installed. These covers were kept "down" after a surveillance radio op had tuned to "the" frequency. It prevented observation (accidental or intentional) of the classified frequency by radio room visitors who didn't have the proper clearance.

1961 contract manufactured by Capehart with OD front panel

R-390A  - End-User Front Panel Repaints

From time to time, R-390A receivers will show up with the front panel sporting a non-standard paint color. The original specification only calls for the panel to be painted gray. Consequently, many shades of gray are found from the many different contractors that built R-390As over the years. Sometimes though, completely non-standard colors will found and these are always "end-user" applied paint jobs. The most commonly known "end-user" paint job is the USAF's flat-black panels (see photo below.)

Several years ago, a 1961 Capehart contract R-390A with an olive drab painted front was found in the Northern Nevada area. It had been supposedly repainted by the USMC. Here is another 1961 Capehart with the OD painted front panel, also found here in Northern Nevada, in 2009. Also, North Korea has made the U.S.S. Pueblo into a museum. The ship had many R-390 and R-390A receivers onboard. There is a color photo on the Internet that shows some of the receivers from the U.S.S Pueblo and two of the R-390A receivers appear to have OD panels. All R-390A receivers left the contractor's facility with gray panels - it was the specification - but the "end-users" were liable to repaint during a rebuild so anything might be possible - just not original.

This is probably the best known photograph of R-390A receivers sporting end-user front panel paint jobs. The installation shows banks of R-390A with dark panels, presumably "flat black" paint. Note that many (not all) of the receivers have severe wear on the Kilocycle Change and Megacycle Change knobs indicating continuous operation of the sets. Most of the racks have Teletype machines associated with them. This photo was of USAF Morse Hall at Clark Air Force Base in the Philippines.    



photo from:


R-390A - "Blue Stripers"

When some of the military R-390A receivers were decommissioned, they were sent to a facility located in Portsmouth, Virginia called St. Julian's Creek Annex. At this repository, thousands of derelict R-390 and R-390A receivers were piled one on top of another and stacked side by side on pallets. The receivers typically had their meters removed (due to the radium used on the needles and the scales) and usually the data plates were also removed. Many times the top and bottom covers were already missing. Sometimes receivers were found that still had their meters installed. The story goes that the meters were checked for radiation levels and removed if the reading exceeded a predetermined level. If the radiation level was below the spec then the meter was usually "tagged" with a stick-on paper dot. The final indication that the receiver was "ready to scrap" was to brand it with a "blue stripe" that was generally applied from a spray paint can. Many times, yellow paint was used but the use of blue paint has accounted for the moniker - "Blue Striper."

These receivers were left out in the weather with no protection whatsoever. The receivers that were in the middle of the vertical stacks generally faired best while the receivers at the top of the pile got all of the rain, snow, sun and dirt. The duration of the storage depended on when certain pallets were sold off. At one time, the receivers sold for as little as $37.50 per receiver, taking into account the entire pallet had to be purchased. Apparently, over the years, R-390As came and went at St. Julian's Creek Annex. Some receivers may have been sold in small lots but the majority were sold by the pallet. It appears that well into the 21st century, R-390A receivers were still being sold from St. Julian's Creek.

Some of the R-390A receivers sold by Fair Radio Sales in Lima, Ohio were "Blue Stripers" from St. Julian's Creek Annex. The Fair Radio Sales' "Blue Stripers" were the ones that were sold as "needing some work" - maybe a bit of an understatement. Selling price was an incredible $330 in the 1990s. It's generally thought that Fair Radio Sales would put together the "needing some work" R-390As from various condition "parts sets" and "used spares." Consequently, most (if not all) Fair Radio "Blue Stripers" are not true St. Julian's Creek R-390As and probably only have some parts that came from the annex.

St. Julian's Creek Annex - piles of R-390As

 The R-390A shown above was sold by Fair Radio Sales many years ago and, after its purchase, it sat for many more years in a garage in the San Francisco Bay Area. It was donated to the museum in 2011 by NU6AM. Note that the panel was repainted a non-original very light grayish-white and the nomenclature has been filled in black. Additionally, this receiver has a Raytheon PTO dated 1977. I would think that this is probably a Raytheon rebuild of a Cosmos PTO. Overall, the condition of this "Blue Striper" is surprising good. Of course, none of the modules match (contractors) which seems to confirm that Fair Radio did "put together" this receiver from parts. What is odd is that they would go through the trouble to assure that relatively good condition modules were provided and then use a "Blue Striper" front panel. At the moment (2011,) the receiver is non-functional but updates on its condition will be forthcoming.

Update 2013: Unfortunately, like a lot of receivers in the condition that this "Blue Striper" is in, this one has become a source of parts to restore other R-390A receivers that have faired better. The RF deck has gone into a 1962 Teledyne R-390A, the IF module has a bad 4kc mechanical filter but may donate the MF tuners to another IF module. The PTO is going into the 1961 Capehart. The ON/OFF microswitch went to repair an Amelco R-390A. It is unfortunate but parts are what keep other R-390A receivers in "top condition" so they can be operated and appreciated for the incredible performance they provide the user/owner.


Collins Radio Company - R-648/ARR-41

The nickname "Airborne R-390A" was certainly appropriate for the R-648. After all, it was for use in Navy aircraft and it uses a lot of the same circuit technology as it's bigger brother, the R-390A. Since the R-648 was going to be airborne, it had to be light-weight - not like the R-390A that weighs-in at around 80 pounds! By reducing the size of the components and mechanics along with eliminating many of the R-390A features, the R-648 only pushes the scale up to 30 pounds. Still, 17 tubes are used in the receiver providing two RF amplifiers, double conversion for most bands, three 500kc fixed-frequency IF amplifiers with two mechanical filters and an audio amplifier with three stages of amplification. When looking at the chassis, one sees the familiar slug racks and slugs, a PTO, modular construction with seven modules and, of course, a mechanical-digital frequency dial.

On board the aircraft, the power (at that time) was usually +28vdc that was provided by the battery-charger buss. This powers the R-648 via an onboard dynamotor that puts out +250vdc at 100mA. The +250vdc is also routed to an 0A2 regulator tube to provide +150vdc. Additionally, a divider network provides about +31vdc for AVC bias. Tube heaters are wired in series-parallel to run on +28vdc and the dial lamps are in parallel on the +28vdc line in the receiver (#327 lamps.)

The GAIN control functions as the RF gain when the receiver is in the CW mode with the AF gain automatically set to maximum. The GAIN control functions as an AF Gain control when the receiver is the VOICE mode with the RF gain controlled by the AVC line. The Sensitivity control is provided to set the maximum available sensitivity and is a slotted-shaft pot behind the "toilet seat" cover marked SENS ADJ." The audio output level is set with a pot adjustment located at the rear of the chassis. This setting was to act as the "maximum" limit so that the headset used would not be over-driven. The output impedance is not critical and anything over 300Z ohms was considered appropriate. The audio output has ample volume if a 600Z ohm load is provided. This can be a loudspeaker with a 600Z ohm matching transformer. 

The sticker on the front panel is a "Narf Norva - Iran - 1st Qtr, 1975" label that indicates that the receiver went through the Naval Air Rework Facility at Norfolk, Virginia and that the receiver was part of a repair contract with Iran (a the time, the Shah of Iran was our ally.) Narf Norva also had repair contracts with the UK and other countries.


The Technical Materiel Corporation - GPR-90RXD

The Technical Materiel Corporation was founded by Ray DePasquale in the mid-1950s to provide commercial and military-grade communications equipment along with high-quality communications receivers for radio amateurs. TMC's most popular product (along with the CV-591 SSB Adapter) was the GPR-90 receiver that was produced from 1955 up to about 1962. It was a $400 to $500 receiver that was primarily designed for use by hams. Within a short time, the military decided that they too wanted some GPR-90 receivers with some variations to the design. The military wanted TMC to add a separate crystal oscillator (HFO) with ten selectable crystal-controlled channels to the GPR-90 along with changing the antenna input impedance from selectable 300Z ohms or 75Z ohms to a fixed 75Z ohm input. The first RF amplifier was also changed to a standard tuned grid input rather than the grounded grid input used in the GPR-90. Additionally, inputs for an external LO and external BFO was provided. These receivers were designated as GPR-90RX. TMC also built a diversity version of this receiver that provided accessible Diode Load and AVC lines for interconnecting two (or more) receivers for diversity reception. These receivers were designated as the GPR-90RXD. There was also a GPR-91RXD that provided a 15kc selectivity bandwidth for four channel independent SSB reception or multiple channel RTTY reception (the GPR-90RX[D] had a 7kc bandwidth.) The military versions also had military designations with R-825/URR used for the GPR-90 and with the RX and RXD(?) versions usually identified as R-840/URR.

All of the GPR-90 receivers are double preselection on all bands (two TRF amplifiers) and dual conversion on the top three bands (5.4mc to 31mc) with the first conversion at 3.995mc and the second conversion at 455kc. Three IF amplifiers are utilized and the Detector is a standard envelope type using a 6AL5 dual diode tube. Audio output uses a single 6V6 tube and the output transformer provides 4, 8, 16 and 600 Z ohm impedances. 15 tubes are used in the GRP-90 while 16 tubes are used in the RX and RXD versions (adding the 6AG5 Selectable Xtal Oscillator.) A matching loud speaker was available for the GPR-90 receiver. All RX and RXD versions were rack-mounted receivers.

The BFO circuit in the GPR-90 receivers is lightly coupled with only a few pico-farads of capacitance to prevent "masking" of weak CW signals. This, in addition to the standard envelope detector, makes reducing the RF gain control necessary for proper signal to BFO injection ratio for either CW or SSB reception. For improved SSB reception, TMC offered a "GPR-90 matching" table-top SSB adapter, the GSB-1. It's also possible to use the rack-mounted CV-591 SSB adapter with the GPR-90 receivers. Some of the rack-mounted SSB adapters were designated as "MSR" units (Mode Selector - Receiving) and these are similar in design, appearance and use to the CV-591. The GPR-90RXD manual specifies that the MSR-6 can be used for enhanced SSB reception. See section below on the TMC SSB Adapters. 

In 1963, the GPR-92 was introduced but only 115 were built before the model was discontinued. TMC continued to expand in the 1960s with several companies located in many different states (Mamaroneck, New York is the headquarters location.) TMC provided the military with several types of transmitters and other types of communications equipment. Eventually, Neil DePasquale (Ray's son) began running the company. While all of their competition either went out of business or were purchased by other companies, The Technical Materiel Corporation is still in business (although sparsely staffed) and is still run by Neil DePasquale.

The GPR-90 has a varied reputation based on the many subjective reviews that abound on the Internet. It's easy to find conflicting reviews about the receiver with some users rating the GPR-90 as an excellent performer while others feel the receiver is not up to TMC quality in either performance or construction. There was a tendency by many to compare the GPR-90 to the Collins R-390A receiver but the two receivers couldn't be more diverse in either design or selling price. Consider that the R-390A sold for over $2000 while the GPR-90 was $495. Even the GPR-90 manuals warn users that if they want "frequency meter" accuracy then they should buy a "frequency meter" and not ask the receiver to "double as both." Certainly a statement that illustrated TMC's frustration with comparisons of the GPR-90 to Collins' "frequency accurate" tuning systems. A fair comparison of the GPR-90 would be to the Hammarlund HQ-180 receiver. Both receivers were basically contemporaries and sold for about the same price for the same intended end-users. There's even a similarity in both receiver's tuning drive systems (rim-drive pinch-wheel) and in their front panel layout of using two dials separated by the S-meter. Interestingly, all reviewers rate the GPR-90 audio as "excellent."

The GPR-90RXD version does perform somewhat differently than the standard GPR-90 since it has a fixed 75Z ohm input and has other changes within the chassis for diversity reception and other various purposes. Still, the RXD is very sensitive and has probably the best Crystal Filter circuit of the time (so does the standard GPR-90.) Dial accuracy is very good considering that it is an analog readout that is limited by its resolution. Proper alignment is critical in many receiver designs and certainly the GPR-90s will perform best when in good condition with no defective components and with a fresh IF/RF alignment. 

Technical Materiel Corp. -  SSB Adapters  -  The Technical Materiel Corporation designed and built several variations of their popular CV-591 SSB Adapter that were compatible with many types of receivers used by the military and commercial users. The receiver used had to provide an IF output that was then connected to the input of the SSB adapter. TMC supplied SSB adapters for the popular IFs used in the 1950s and 60s (455kc, 500kc and 200kc.) Shown in the photo below is the GPR-90RXD set-up with the TMC MSR-6 Mode Selector - Receiving (SSB Adapter.) Both the receiver and the adapter are assigned the same serial number (126) which implies that they were sold, installed and used together as a "set" (although originally "rack mounted.")

All of the TMC SSB adapters work in a similar manner. The adapter circuitry down-converts the incoming IF from the receiver, 455kc for example, to a lower frequency (17kc) and then mixes a BFO to provide product detection. Bandpass and Low Pass LC filters were utilized to increase selectivity. A crystal-controlled oscillator is used for the first conversion (receiver IF to 17kc) and also a VFO called "bandspread" (manual select) is provided. Crystal frequencies are 17kc above and below the IF to provide selectable Upper or Lower sideband. >>>
>>>   Sideband selection uses a push-button that actuates a selector relay which also operates the "U" and "L" lamps accordingly. The AVC circuit operates on the incoming signal from the receiver and provides a fairly constant level of signal and has selectable fast and slow AVC action. Audio output is 8.0Z ohms or 600Z ohms from a single 6AQ5 tube. Although specifically for SSB reception, CW signals can also be received. Also FSK can be received. Additionally, the ability to select the upper or lower sideband of an AM signal or to receive an AM signal in the "exalted-carrier" mode (AM + BFO) is available. Exalted carrier reception usually improves weak, difficult to copy AM signals. There are several remote operation options available including remote sideband selection or detection along with remote BFO operation and tuning. Some versions were specifically designed to utilize remote inputs from specific TMC-built receivers. 

Once connected to the particular receiver, if only the SSB converter audio output is utilized then the user will not have the availability of the receiver's circuitry down-stream from the last IF stage, usually the noise limiter and the receiver's audio output circuitry. Two separate speakers can be used with one connected to the standard receiver audio output and another speaker connected to the SSB adapter's audio output. With the dual speaker set-up, the user can choose the receiver's audio output for AM signals (probably better fidelity) and the SSB adapter's output for SSB or CW signals (virtually distortion-free SSB.) Single speaker operation can be achieved by paralleling the adapter's 600Z output with the receiver's 600Z output (Z is then 300ohms.) Then connect 4.0Z ohm speaker to receiver's 8.0Z ohm tap.

TMC offered the CV-591A and CV-657A (and many other variants) for the military. The CV-591A utilized 455kc IF input while the CV-657A utilized 200kc. The MSR series went from MSR-1 thru MSR-9. MSR-3 utilized 200kc IF input while the MSR-8 utilized 500kc IF input. All other MSR numbers utilized 455kc IF input. The MSR-1 and MSR-4 are also considered CV-591A units.


Racal Electronics plc (UK) - Racal Communications, Inc. (USA) -  RA-6117

Racal Electronics plc was a British company that was founded by Raymond Brown and George Calder Cunningham in 1950. The company name was derived from the names of the founders RAy Brown and Geo.CALder Cunningham. In 1953, the British Royal Navy wanted Racal to build a couple hundred Collins 51J receivers for RN use. Racal wanted to use mostly British parts but Collins insisted on parts from the USA. After an inspection of the (then) small Racal manufacturing facility, Collins refused to license the manufacture of the 51J by Racal. Left with designing their own receiver for the contract, Racal contacted Dr. Trevor Wadley to help with the project. Using a circuit that Wadley had developed in the 1940s for test equipment (and that Dr. Wadley was incorporating into a receiver design of his own,) the "Wadley Loop" was incorporated into the Racal receiver design. The Wadley Loop virtually eliminated frequency drift by using a 1mc crystal oscillator combining with a MC/VFO and also simultaneously having the 1mc crystal oscillator feeding a harmonic "comb" filter to produce multiple 1 mc harmonics that are used to produce thirty 1mc wide tuning ranges. The "drift cancelling loop" uses the same "comb filter" output but running it through a 37.5mc amplifier, a bandpass filter, then another 37.5mc amplifier and then into a second Mixer. Since Mixer 1 and Mixer 2 outputs are inverse from each other any drift in MC/VFO is cancelled. Racal's first receiver was designated the RA-17 and it was the first successful receiver to employ the "Wadley Loop" system for oscillator and conversion stability. The RA-17 was produced from 1953 up to around 1967. As the Racal receivers became more and more popular with commercial and military users, a manufacturing facility was opened in the USA (one of many around the world.) This company was called Racal Communications, Inc. and it was located in Silver Spring, Maryland. Many of the receivers produced in the USA will have either an "A," a "C" or "6" (or combinations) added to the standard model number thus the RA-6117 is the USA version of the RA-117 receiver (the RA-117 was an updated RA-17.) At one time, Racal employed 30,000 workers, was the third largest electronics firm in Britain and had facilities in 110 different countries. After 1966, Ernest Harrison was in charge of Racal. The company also owned Decca, Chubb and Vodafone to name a few of their holdings. Several reorganizations occurred in the 1990s and, in 2000, Thomson-CSF (aka Thalen Group) purchased Racal.

The RA-6117 is a 25 tube, triple-conversion superheterodyne using all standard USA manufactured tubes. Most of the hardware is also standard US threads and sizes. US style knobs are used on the RA-6117. The kilocycle tuning dial is a film-strip type that is six feet long and spans 1000kc with a resolution of 1kc. When tuning the receiver, the dial index remains stationary while the numerical dial scale moves behind the index in a linear fashion. The megacycle dial is circular and is read thru the lower window of the dial escutcheon. To readout the tuned frequency one has to add the megacycle dial setting to the kilocycle dial reading. If the megacycle dial is set to 3 and the kilocycle dial reads 868 then the tuned frequency is 3.868mc (as shown in the photo above.) The tuning is super-smooth and very light feeling. The film strip is easy to read to better than 1kc accuracy. The RF amplifier can be switched out if necessary but when in operation it must be tuned to the correct frequency. There is also a step-attenuator provided for coping with very strong signals but still being able to retain the tuned selectivity that the RF amplifier provides. The step-attenuator is very effective at reducing static noise while retaining an intelligible signal. The Noise Limiter is also effective if the static is more severe. Several bandwidths are provided with 13kc being the widest and 100hz being the narrowest. Fast and slow AVC is provided. A three position switch allows the meter to act as an RF signal level, S-meter or Audio level meter. The RA-6117 uses a standard envelop detector - no product detector. Several outputs are provided for External VFO, Oscillator Outputs, 100kc IF Output, several 600Z ohm outputs and a 1W 3.0Z ohm output. A separate audio output (600Z ohm) with an Audio Level control on the front panel that has its own output transformer and operates separate from the standard receiver audio output. This output was for driving a data device, RTTY converter, for example. The small built-in speaker can be switched off if a larger speaker is desired (connecting to the 1W 3.0Z ohm output works best.) Audio reproduction is excellent (with a good external speaker) and Shortwave BC stations that are transmitting good quality audio sound incredible. Sensitivity is first rate but one has to be sure to "peak" the RF amplifier tuning for best results. The receiver is built with a combination of modular and fixed circuit construction all mounted on a large cast-aluminum chassis that has various compartments underneath for circuit isolation and shielding. Some modules are interconnected using coaxial cables with BNC connectors. The RA-6117's superior performance demonstrates why the RA-17 family of receivers were continuously produced from 1953 up to 1967 with only minor changes.

Although the RA-6117 panel is standard 10.5" x 19.0" the receiver's chassis is quite deep at 21.5" which, of course, won't fit into any standard American cabinets. The receiver usually will have its dust covers and, like the R-390A, looks quite nice setting on a table "sans cabinet." Weight is around 67 lbs. The particular receiver shown above was built in 1966. 

Meter Notes: The AF meter only works on the 600Z ohm 3mw line that has its own individual output transformer. This line gain is controlled by the Audio Level control. If you have the Audio Level control set to minimum you will not see any indication on the meter. Also, with the meter in the RF Level position, if the BFO is turned on, it will show a constant level on the meter. RF Level only shows signal levels in the AM mode. The S-meter position provides a relative indication of signal strength but the readings will depend on the setting of the RF Attenuator.

Adding the Optional Remote Standby - The receiver muting, aka remote standby, was a factory option that wasn't installed on this receiver (sn: 193.) There isn't any actual specific data on the standby circuit in the manual other than a basic block drawing that is part of the block diagram circuit flow drawing. The block drawing shows that the B+ going to the Stand By position on the function switch is disconnected at the filter choke and routed to a relay that disconnects B+ and alternately connects a 10K 10W load resistor to the B+ when the remote standby relay is actuated. The voltage for the relay coil isn't specified on the drawing or the receiver schematic but does show that the relay coil is connected directly to the "Receiver Mute" terminals. Apparently, it was up to the end-user to specify what voltage coil the relay would be used. Since I didn't want to have to supply a separate voltage to "mute" the receiver, I chose a very small Potter-Brumfield sealed relay that was 4PDT and had a 115vac coil that only needed 10mA to actuate the relay. I tied all of the four NC, NO and ARM contacts together for maximum current carrying capability. I made a small bracket to mount the relay at the top-rear of the chassis near the terminal strip (making use of exiting holes.) The wires from the relay were routed thru an existing chassis hole (that was located directly under the relay) to the receiver power supply section. I desoldered the wire (B+) connected to pin 2 on L80, the choke. I added an extension wire and this was connected to the NC contact of the relay. A wire from the ARM of the relay was then soldered to pin 2 of L80. Then a wire from the NO contact was connected to the junction of the 10K 10W load resistor and white-yellow wire from S5 (standby position.) One Mute terminal was connected to one relay coil terminal. The other relay coil terminal was wired to 120vac at the downstream side of the fuse. The second Mute terminal was wired to the 120vac neutral at the power transformer terminal. Shorting the two Mute terminals completes the 120vac circuit and turns on the relay which disconnects B+ and connects it to the 10K 10W load resistor which puts the receiver in standby. The manual selection of "Standby" with the function switch is not affected by this addition. This remote standby actuation is somewhat different than the standard NC-receive/NO-transmit action with a NC action on the Mute terminals required during transmit. This is somewhat like the R-390A "Break-in" function which also requires NC on transmit. Most T-R relays provide contacts for both NC or NO on transmit functions so no problems should arise with this remote standby addition. The Remote Standby addition makes the RA-6117A an "easy to use" station receiver that provides excellent signal reception along with an impressive bench presence.


Siemens AG  -  E-311-E1

Siemens is a very old and very large German conglomerate that manufactures many different types of products. The company was founded in 1847 by Werner von Siemens and Johann Halske to sell their non-Morse-type telegraph system. The Siemens telegraph was installed in many locations throughout Europe. Siemens continued to grow through the nineteenth century adding electric trains and light bulbs to name just a few types of products offered. Many varied companies were added to the Siemens conglomerate during the twentieth century. During WWII, Siemens produced equipment for the Nazi military and utilized forced-labor from concentration camps in some facilities. After WWII, Siemens continued to add new products and added into medical equipment to the conglomerate. Today, Siemens employs over 300,000 people world-wide and is a major supplier of medical equipment and many other types of products.

The Siemens E-311-E1 was produced from 1959 to 1961. It's a seventeen-tube, triple-conversion superheterodyne that tunes from 1.5mc up to 30.0mc in five tuning ranges. The IF conversions are a tunable 1300kc to 1400kc IF followed by a 370kc IF that can be tuned by an interpolator and finally a fixed 30kc IF. The receiver is comprised of seven modules that interconnect although some hard-wiring is also used. The modes receivable are A1(CW,) A3(AM) and A3a(SSB.) When AM is selected the receiver automatically shuts off the BFO and uses different filters for 6kc-wide DSB-AM reception. When SSB is selected the BFO is automatically turned on and the Sideband selection switch enabled allowing the user to select USB or LSB. The meter can read either audio output or RF signal voltage. The selectable IF bandwidths are only available on CW or AM and are 3.1kc, 1.5kc, 0.5kc and 0.15kc for CW and 2X for AM. SSB bandwidth is fixed at 3.5kc. Two time-constants of AGC are provided, 2.0 sec decay and 0.2 sec decay. The built-in speaker can be turned off by pushing in the AF Gain control to actuate the switch. An external loudspeaker (5Z ohms) can be connected via the front panel phone jack. 600Z ohms is also available via a front panel phone jack as is the headphones jack. A Noise Limiter control is also provided. The front panel nomenclature is bi-lingual in that both German and English indicate control function.

Extreme dial accuracy is provided by dual frequency indicators. The arc-dial reads megacycles while the vertical dial reads kilocycles. If the 100kc synchronizing circuit is turned off then the receiver can be coarse tuned using the left-hand tuning knob. With the 100kc oscillator set to SYNC then the coarse tuning has to be set to a 100kc position where the 100kc "lock" lamp indicator will stop blinking and turns on continuously indicating a 100kc frequency "lock." Then the Fine Tuning knob will tune that 100kc bandwidth via an interpolator circuit tuning the second IF providing an accuracy of 100hz on the KC dial. For example, if 14.100mc was set on the Coarse tuning (and in lock) then the Fine tuning would tune from 14.100mc up to 14.200mc. Each 100kc part of the spectrum selected by the Coarse tuning will be tuned by the Fine tuning when in 100KC SYNC. If extreme accuracy isn't necessary or if wide coverage of the spectrum is desired then the Sync can be set to OFF and then tuning can be accomplished with the Coarse tuning with the Fine tuning set to 00. Since extreme accuracy was necessary in some applications, the 100kc oscillator and the Interpolator oscillator operate in special oven temperature-controlled ovens. Green indicator lamps show if the ovens are powered. When the ovens are at temperature the green lamps cycle "on and off" as the temperature is maintained.  The Calibrator provides a 400kc marker that corresponds to special markers on the megacycle dial. There was an optional 1kc spectrum oscillator that functioned like the 100kc SYNC and provided a 1kc marker for extreme accuracy in determining actual frequency of a received transmitted signal. The 1kc spectrum oscillator is not installed in the "E1" version of the receiver.

The seventeen tubes used in the E-311-E1 are made up of only four different types of tubes, 3 - EF93 (6BA6,) 5 - ECH81 (6AJ8) and 8 - EC88CC (6922.) The 85A2 (0G3) regulator tube accounts for the seventeenth tube. All inputs and outputs are located on the front panel. The rear of the receiver is entirely enclosed when mounted in its cabinet. The receiver shown is mounted in the standard desk top case. There was also a shock-mount type case available. The original selling price of the E-311 was 15,000dm (approximately $3750 in 1960.)

Performance Notes:  An impressive receiver that has ample sensitivity and excellent dial accuracy. Random tuning can be cumbersome due to the "Locked" 100kc limitations but one can always "scan" the band with the "Unlocked" Coarse tuning. SSB reception is very good although the fixed 3.5kc bandwidth might be a problem in a crowded band. CW has bandwidths available down to 150 cycles, so no problems there. AM is actually quite good since the bandwidth "doubles" so the 3.1kc bandwidth is actually 6.2kc. AM signals sound very good. There is no remote standby provided and the operation of the E-311-E1 as a station receiver requires a good isolation within the antenna relay. No muting is provided although it's possible to use the Squelch control if the signal leakage is low enough.


ITT Mackay Marine  - Type 3010C

International Telephone and Telegraph Corporation, aka ITT, had controlling interest in Mackay Radio and Telegraph Company since the late twenties. ITT also owned Federal Telegraph Company since 1931 although the name was changed to Federal Telephone and Radio Corp. in 1940 (see the Mackay Type 105-A at the top of this page for a more detailed history of ITT and Mackay.) It wasn't until the 1960s that the "ITT Mackay Marine" brand name started to be used. The ITT Mackay Marine Type 3010C was just about the last of the vacuum tube-based, shipboard receivers produced. They were generally installed into a Mackay MRU-14 up to MRU-23 consoles as the ship's receiver along with ship's transmitter and all of the other gear the radioman needed for ship-to-ship or ship-to-shore communications. The 3010 was also available in a table model configuration for other end-users. The front panel on the receivers used in the MRU consoles are standard 19" rack size (as shown above) but the table cabinet receivers use a slightly smaller front panel.

The Type 3010C is a 17 tube, triple conversion superheterodyne receiver that tunes from 70kc up to 30mc. It will actually tune from 70kc down to 10kc at a somewhat reduced sensitivity. The tuning dial is a "film strip" type of dial similar to the type used on earlier Racal receivers (although the "film" is actually a fiberglass material.) The film strip is 90" long and covers 2000kc that is divided into a 1000kc yellow band and a 1000kc green band. The strip is further divided with 2kc index markers. The megacycle dial shows two one megacycle bands at a time with color codes of yellow (even) and green (odd.) Reading the tuned frequency requires matching the colors and adding the numbers. As shown in the photo above, the receiver is tuned to yellow 320 and therefore yellow 14mc plus yellow 320 equals 14.320mc.

The first conversion results in an IF of 37mc to 39mc. The second conversion is a product of mixing the VFO tuning 42.94mc to 44.94mc and the first IF of 37mc to 39mc resulting in a 5.94mc IF. The third conversion mixes a 5.485mc xtal osc with the second IF to produce a 455kc third IF. The signal then goes to a standard envelop detector for AM or it will be routed to a Product Detector for CW or SSB. Interestingly, the 3010C provides two plug-in Collins mechanical filters (same type as used in the 75A-4) for the selectable bandwidths. The IF passband is around 6kc wide in "Broad" but one can select a 3.1kc mechanical filter in the "Med." position or a "Narrow" filter which is usually a 500hz MF. The narrow filter was a customer option while the 3.1kc filter was provided with the receiver. Audio output is 1 watt into 600Z ohms and the audio bandwidth is 300hz to 2500hz at 6db down. Images are at least 80db down at any tuned frequency. Sensitivity is < 1uv if the antenna is matched to the input impedance of 75 ohms. The "Antenna Filter" attenuates the AM BC band by about 30db and was intended to be used if a strong local AM BC station was causing cross-modulation while the ship was in port or near a coastline.

The Mackay Marine 3010C has a easily adjustable front end with a tunable RF stage that also has a step-attenuator that allows quick reduction of the input signal level. An adjustable IF gain control is also provided. This combination allows the user to cope with just about any signal problems such as QRN or elevated static levels. By reducing the input signal level and balancing it to the IF gain and the AF gain one can reduce static to a minimum while actually enhancing the desired signal. Additionally, the mechanical filters add the ability to combat QRM in a "Collins-like" manner. Audio reproduction is pretty good and although the specs "look" like the receiver wouldn't reproduce AM signals very well, it actually sounds okay (some absence of bass, of course) when a quality speaker enclosure is used. The dial-strip readout is highly visible and does allow about 1kc accuracy although you have to "split" the 2kc index marks to read 1kc. A 100kc calibrator is provided. A very nice receiver with an outstanding ability to cope with poor receiving conditions.


General Dynamics Corporation - R-1051/URR

General Dynamics began as an early submarine building company started by John Holland in 1899. Holland had the intention of selling submarines to an Irish faction that wanted to use them to sink British ships (but, mostly the subs sank.) Holland also sold subs to both the Russians and the Japanese during the Russo-Japanese War (1904-5.) Holland eventually lost control of the company which was known then as "Electric Boat" to an investor and both men died before WWI. The Electric Boat Company continued on and was eventually officially formed in 1925 as a submarine builder. Although usually sold to the U.S. Navy, Electric Boat also sold subs to the Imperial Japanese Navy before WWII. During WWII, Electric Boat also produced PT boats for the USN. Post-WWII, Electric Boat changed their name to General Dynamics Corporation in 1952 and began acquiring many different companies, such as Canadair and Convair, to expand their business into a major defense contractor that also supplied military aircraft. One of the major electronics companies acquired was Stromberg-Carlson (1955.) Aerospace was added in the 1960s and the company continued to expand to become one of the largest defense contractors, aerospace suppliers and aviation contractors. Along the line, the Electronics Division of General Dynamics (who developed the R-1051) was sold off in the early 1990s (although Stromberg-Carlson had been sold off in the 1970s.) General Dynamics continues today as one of the major defense contractors

The R-1051/URR is a triple conversion, synthesized-tuned superheterodyne that covers 2.0mc up to 30mc. Rather than individual tuning ranges, the operator selects the desired frequency using five rotary switches that represent the each decade of the tuned frequency - two "MCs" and three "KCs." The receiver uses circuitry that is contained in various modules that employ discrete transistor designs for all functions except the RF input module that utilizes two vacuum tubes. The receiver uses a synthesizer that has a 5.0mc crystal controlled oscillator as its core. There are several other frequencies that are derived from the 5.0mc oscillator to allow the synthesizer to tune the receiver from 2.0mc up to 30mc. As a frequency is "dialed in" with the decade switches, the MC decades actuate the motor-driven chain-drive in the RF and Synthesizer sections. The KC decade digits don't actuate the motor-drive since the frequency change is not as significant. Since the receiver is tuned to the "kilocycle" by the decade switches a vernier control is also supplied for "CPS" tuning (essentially a fine tuning control.) The first versions of the R-1051 used a three position switch (0-500-Vernier) and 100 cycle vernier. The "B" versions (and later) used an 11-position switch that selected 100 cycle changes (000 thru 900) and then a "V" (vernier) 1000 cycle (in 270º rotation) vernier control. Dual IF and Audio sections are employed for USB and LSB. Actually, CW, AM, FSK, ISB (Independent Sideband) and USB are routed through one IF/AF section while LSB and ISB are routed through the other IF/AF section. ISB allowed the operator to simultaneously receive independent information on each sideband of an ISB transmitted signal. Product detectors are used for all receiving functions except AM. The two meters monitor audio output levels for each IF/AF section. The rear audio connections for USB and LSB are 600Z ohm outputs and are active depending on reception function selection and on the Rear Output Level control. Front panel Phone Outputs are also 600Z ohm and have their own output level controls. Many of the functions of the receiver circuitry are determined by the reception function selected. If LSB is selected then the signal is routed through that IF/AF circuit and the USB IF/AF is not active. Tunable BFO is only active when CW is selected. The operator has to determine what type of signal is going to listened to and must know the specific transmitter frequency before actually tuning in the signal. For the military this wasn't a problem since all operations were on specific predetermined frequencies and modes. Audio levels are adequate but since the receiver was supposed to be set-up to have the audio output routed to a switchbox that was further routed as necessary within the ship or shore facility, the audio output level direct from the receiver is not robust. For headset monitoring the audio output is quite loud and since many R-1051s were used for RTTY (FSK) where constant audio monitoring was usually not necessary the use of phones for initial comm set-up was standard procedure. 

The R-1051/URR was developed in the early-1960s and was being produced for the USN by 1964. The first versions (non-suffix) were produced up to the late-sixties when the "B" version started to be produced. There was an "A" version that was not produced in any quantity and had unique mods to just that version. The "B" versions were built by Bendix Corporation. Suffixes run up to "H" and production continued into the 1990s. Contractor-manufacturers were General Dynamics, Bendix or Stewart-Warner. The price to the USN was around $25,000 per receiver but the latest "H" version was priced at over $50K per receiver. The R-1051B/URR receiver shown in the photo above was produced in 1968. It is mounted on one of the original types of shock mounts that were designed for "stand-alone" receiver set-ups. The side mounts shown allow for "stacking" two receivers. This particular shock mount was built by Barry Controls.

Using the R-1051B/URR takes a little getting used to. One can no longer just "band cruise" looking for signals. You have to plan in advance what type of signal and on what frequency you are going to be listening for. The R-1051 works really well for net operations where the mode and frequency are known in advance. Stability is so good you never have to retune the receiver (that doesn't mean that all of the stations on the net will be on frequency though.) For shortwave listening, one can tune via the "tens-KC" decade and pretty much "hit" most of the SW BC stations within a SW band. AM doesn't require specifically accurate tuning but in the AM mode any "fine tuning" can be accomplished with the "ones-KC" decade. Just "looking around the band" for signals doesn't work too well. The R-1051 was designed for a different operation methodology other than band scanning but the receiver can still provide excellent sensitivity, stability and frequency accurate tuning. You just have to do a little pre-planning before listening.


Commercial-Military VHF Receivers

Nems-Clarke, Inc.  -  1302 Special Purpose Receiver

Allen Clarke started in the electronics design business in the 1940s and by 1951 had a small electronics design business. NEMS was an acronym for National Electric Machine Shops, a name chosen by NESCO, National Electric Supply Company, when they incorporated in 1937. NESCO goes back to 1899 and the company was involved in radio manufacturing very early with many contracts assigned to them in WWI and after. NEMS and Clarke merged in 1955 as Nems-Clarke - specializing in high-end commercial-military radio equipment. All (?) Nems-Clarke receivers operate in the VHF and UHF part of the spectrum that was then being used in part for telemetry from some kinds of missiles and for other military and quasi-military purposes. Later Nems-Clarke receivers monitored Russian missile launch telemetry. The 1302 Special Purpose Receiver is a VHF AM/CW/FM receiver that utilizes a Western Electric 416A Planar Triode tube in the front end. The 416A operates at a very high temperature and is cooled by a small forced-air blower. The receiver tunes from 53mc up to 262mc in one continuous coverage tuning range with a 0 to 35 scale for logging. The tuning dial is not illuminated. Behind the grille on the left side of the panel is the built-in four inch speaker. The 1302 was designed to operate with a matching Spectrum Display Unit, (SDU) or Panadaptor.  The upper zero-center meter is for tuning FM and the lower meter shows relative signal strength and can be used for tuning AM signals or measuring relative FM signal strength. The 1302 was primarily used for surveillance by several different government users. Later versions of the 1302 use a different front-end tube (7007) and are styled more like the Nems-Clarke receiver shown below.


Nems-Clarke (Vitro Electronics) - 1306-B Special Purpose Receiver

Nems-Clarke was purchased by Vitro Electronics in 1957. Nems-Clarke/Vitro continued to produce Special Purpose Receivers for surveillance and telemetry that were used throughout the late fifties and sixties. The 1306-B Special Purpose Receiver is a 29 tube AM-FM-CW receiver that was usually operated with a SDU-200-6 Spectrum Display Unit (panadaptor.) The receiver tunes 30mc to 60mc and 55mc to 260mc with separate dials for each band that are only illuminated when in use. Selectable IF bandwidths, selectable IF AGC/Manual Gain controls, Squelch and a BFO with variable Pitch Control are provided. The zero-center meter provides accurate tuning for FM signals while the right hand meter measures signal strength. The built-in speaker is a very small "communications quality" unit that is located behind the screened cover. A 600 ohm audio line is provided on the rear panel can provide excellent audio quality to a matched loud speaker. The "SPEAKER" switch is a factory modification that replaced a PHONE jack with a switch to silence the panel speaker. Many of the Nems-Clarke surveillance receivers were used to monitor Russian missile launches and analyze data transmissions along with any voice traffic. The 1306-B is a great performer with an impressive appearance.


Federal Telephone & Radio Corp. for U.S. Navy - R-482/URR-35  a.k.a.  AN/URR-35

The AN/URR-35 is a VHF-UHF receiver that covers 225mc up to 400mc in one continuous tuning range. The circuit is double conversion with the first conversion at 18.6mc and the second conversion at 1.775mc. The URR-35 is an AM only receiver but it can also receive MCW signals (modulated CW.) It is a manually tuned vacuum tube receiver that also has provisions for a single frequency, crystal controlled fixed-frequency operation. Squelch is adjustable with the controls underneath the right side door. The left side door provides access to the crystal and selector switch. The left meter reads signal level and the right meter reads audio output level. Controls (l. to r.) are Manual Tuning, Dial Lock, Dial Lamp Dimmer, Noise Limiter, Output Gain, Power ON, Audio Output (standard phone jack under the toilet set cover.) Power input, Antenna Input, Audio Output are accessible via a plug-in rear module that is generally mounted to the rear of the cabinet. Shock mounts are integral to the cabinet and the receiver was usually supplied with a kit for rack mounting if desired. The contract number on the receiver shown in the photo is NObsr57142 probably dating from 1957. The receiver shown was rebuilt at the Philadelphia Shipyard in 1966. Serial number is 792 although the field change record tag on the front panel indicates serial number 1600 (which indicates that the receiver was installed into a different case at sometime.) Other contractors built the URR-35 besides Federal T & R.

Although this is a working URR-35, the 225mc to 400mc region of the RF spectrum is not overly-populated with AM signals. I was of the opinion that the URR-35 was only good for listening to static or maybe a VHF RF signal generator connected to the antenna input of the receiver. Luckily, fellow URR-35 owner Rex, KE7MFW (QTH - Yerington,NV,) discovered that the Fallon NAS uses AM on 318.50 mc for their pilot training exercises. Rex also reports that the receiver is very specific about the antenna used which must present a fairly good 50 Z ohm match. I use an 8" piece of 14 gauge wire inserted into the SO-239 antenna input and this seems to work well on 318 mc allowing me to copy the NAS training exercises quite well (although transmissions are typically sporadic and quite short.) Both Rex and I  have also heard some voice transmissions in the 270mc to 285mc region. Transmissions are brief and the length of time "on the air" minimal which means you have to keep tuning around until you happen to find a transmission in progress. Activity is sparse but with patience you'll hear some voice transmissions. For checking reception, there are a number of unmodulated carrier beacons in the spectrum covered by the URR-35 that are "on the air" continuously but without a BFO these are somewhat difficult to hear. I usually can tell they are present by hearing the background noise drop as the carrier is tuned and watching the input or output meters for an indication of tuning through the signal. As to the purpose of the beacons,...unknown.


Military Morale Receivers

Zenith Radio Corp.
for U.S. Army Signal Corps

R-520/URR  &  R-520A/URR


photo left: R-520/URR (H-500 Series TO)

photo right: R-520A/URR (600 Series TO)



Morale radios, that is radios specifically made for entertainment reception by military personnel, have been around since before WWII. It's not surprising that the Signal Corps wanted something for the soldiers to listen on during the Korean War. The Zenith Transoceanic was a very popular portable AM-BC and SW radio with Zenith cranking out well over one hundred thousand units by 1952. It seemed like a natural choice for a morale radio. >>>

>>>  A few changes had to be incorporated into the basic H-500 style, five tube TO. A neon pilot lamp (a power on indicator) was added. The band selector switch information was color coded. Increased shielding was added. The black stag covering was replaced with a brown vinyl oil-cloth covering. Additionally, "USA" was stamped in Signal Corps orange paint on all sides of the cabinet and a metal data plate attached to the front below the latch. This was the R-520/URR "Transoceanic" that was produced for the Signal Corps. About 5000 units were built but by the time Zenith finished the contract and had shipped the R-520s out, the Korean War was over. The Army decided to give a majority of the R-520s to the PXs at various bases around the country. The idea was to rent the TOs out to soldiers for use at their base housing or in barracks. Many were rented and never returned. Some made it the surplus market. Some had the orange "USA" removed along with the military data plate to make the TO look more civilian - especially if it was one that had been rented and never returned.

Around 1961, the Army was planning the Bay of Pigs invasion. A contract for special "air drop" cases for the R-520 was issued with a quantity of between 250 to 500 units. These were designated for the R-520 but it's probable by that time the R-520 had been replaced by the R-520A version which was a "militarized" 600 Series TO. The same sort of changes were incorporated into the new R-520A military version with a neon bulb pilot lamp, tube shields, removal of the fold-down log book, no front headphone jack, no dial lamp switch and "USA" was stamped into the back of the chassis. A schematic and instructions were glued to the inside back cover along with two fuse boxes and a Signal Corps TM manual. The black stag of the civilian model was replaced with an olive drab colored oil-cloth type covering and the orange "USA" stamped on the left side of the cabinet. Some will have an orange Signal Corps acceptance stamp on top of the cabinet. This model was designated as the R-520A/URR. It was still the basic five tube TO but, as with all 600 models, this one also had a ballast tube to compensate for low battery voltages. Zenith produced around 3000 units, making the R520A the lowest production TO. The need for more morale radios in the early 1960s was dubious so its likely that the Army went through the same procedure to make use of the R-520As with the same end results. Many are found today with the orange "USA" removed along with the military data plate, for obvious reasons. There is speculation that some of the R-520A TOs were used in Vietnam but this is only speculation. Certainly, both of the military versions of the Zenith Transoceanic were very low production with only one contract for each model.


Military Radio Transmitting-Receiving Equipment

Collins Radio Co. - AN/GRC-19

GENERAL INFORMATION - Around 1951, the U.S. Army Signal Corps needed a portable transmitter-receiver combination that was modern, operated at a moderate power level and could be used outdoors or even deployed via parachute to remote locations. The result was the GRC-19, a set-up that consisted of the T-195 transmitter - an autotune unit capable of around 100+ watts of carrier power - and the R-392 receiver that was based on Collins' the highly successful R-390 receiver. Since the transmitter-receiver had to be portable, it was designed to operate exclusively on +24vdc to +28vdc. Since the GRC-19 was going to be exposed to the weather in many types of open vehicles, the entire system had to be somewhat "weather proof." To allow the receiver to be completely sealed with no ventilation and thus, to have the receiver run as cool as possible, no voltages higher than +28vdc are used in the R-392. The T-195 transmitter used forced-air cooling for the three external-anode tubes used in the PA and Modulator so an air filter was provided for the intake but this did not "water proof" the transmitter while it was in operation. It was possible to seal the intake and exhaust ports when the transmitter was not in use to aid in the weather-proofing. Additionally, many times the military was going to have to "drop" communications gear from the air, so the R-392 and T-195 had to be "ruggedized" to be able to survive this type of deployment for portable field use (the usual drop was a fully-equipped Jeep that included the radio gear.) The GRC-19 was commonly used on Jeep-type vehicles up to larger "command car" types. A whip antenna was used if operation was going to be mobile but, if the vehicle was going to be in one location for longer than an hour and a half, a dipole antenna was usually erected since power output was much better with this type of antenna. Although generally thought of as an "army radio," the "AN" designation implies "ARMY-NAVY."  Additionally, the Air Force had its own technical manual designation (TO 31R2-2GRC19-11) so the GRC-19 was used by all branches of the military for various purposes from the early 1950s up into the late 1970s.

 T-195 (GRC-19) - Details

The transmitter for the GRC-19 set-up was the T-195, designed by Collins Radio Co. around 1951. The T-195 is a 100+ watt carrier output transmitter capable of AM, CW or FSK transmission on frequencies from 1.5mc up to 20.0mc. Seven preset frequency channels are available along with a Manual Tuning position that can also serve as an eighth preset channel. The transmitter is built onto a main frame with modules that plug into various Amphenol-type sockets and inter-module contacts to provide power and signal routing. The circuit uses a PTO to generate an oscillating signal that is fed into an Exciter-Multiplier module that mixes the signal to the correct output frequency. As expected from Collins, the PTO and the Exciter-Multiplier are permeability tuned with the Exciter-Multiplier using a slug rack and RF transformers that are very similar in appearance to the R-390A RF transformers. The Exciter-Multiplier has a 5763 output tube that drives the PA module that has a single 4X150D external anode type RF amplifier output tube along with a built-in discriminator circuit to control the automatic Plate tuning of the PA tube. The Modulator module contains the speech amplifier and the push-pull 4X150D modulator tubes and the modulation transformer. The output loading and matching is all accomplished automatically by using a Discriminator module, a Servo Amplifier module, an Antenna Capacitor module, an Antenna Inductor module and the Output Capacitor section that is part of the Main Frame. A total of 21 tubes are used in the T-195 which features complete autotune capabilities and uses a Veeder-Root type of mechanical-digital readout for the transmitter frequency. Modes available were AM VOICE, FSK or CW. Additionally, the T-195 can be set-up as a RELAY station by using the R-392 receiver audio to drive the T-195 audio input and thus "relay" an incoming signal.

1952 T-195 Transmitter built by Stewart-Warner for Collins Radio Company. This particular T-195 has been upgraded to have the +HV Solid State power supply rather than the original dynamotor. After this modification the transmitter was usually designated as T-195A.

T-195 Circuit Details - Inside the T-195 are several small DC motors. Two motors operate blowers to provide forced air-cooling for the three 4X150D external anode tubes used in the PA and Modulator (4X150D - the "D" version must be used since it has a 26.5vdc filament.) Another DC motor is the autotune motor which operates the channel preset frequency selection. The Output Capacitor selection uses a DC motor. There are three AC servo motors (operating on 115vac 400~) to control the loading and tuning operation which is also part of "autotune" in that the T-195 does "automatically tune" itself to whatever antenna load is connected. This "tuning" is accomplished by using a discriminator module that creates error voltages based on phase and load sampling (of the PA) which are then amplified to drive the servo motors. When the error voltages are zero then the transmitter is "tuned" to the antenna load. On early T-195s, two dynamotors are internal to the transmitter with a HV dynamotor providing +1000vdc and a LV dynamotor that supplies three voltages, +250vdc B+ along with -45vdc for bias requirements and 400 cycle 115vac. Additionally, there are several thermo-switches to prevent overheating along with relays and interlocks galore (there are over 15 relays used in the transmitter.) The T-195 is very complex because of its autotune capabilities and because it was designed to essentially be used by operators with no particular skill or training, thus the transmitter had to basically "take care of itself." Several of the thermo-switches will shut down the transmitter if things get too hot. A few minutes "cool down" is normally required to let the thermo-switch reset (you should also correct the problem that caused the over-heating in the first place.) The T-195 is robustly built so reliability was usually pretty good. The photos below show the densely-packed modular design and the complexity of the T-195.

T-195A and B Versions - Late in the military's use of the T-195 (just before the Vietnam War era) there was a retrofit to convert many of the T-195 transmitters to use a solid-state power supply to replace the +HV dynamotor. The replacement power supply is shaped somewhat like the original dynamotor but has no vent holes or any moving parts. This reduced the total transmitter current required down to about 35amps but the big reduction was in the +HV dynamotor "starting current" that no longer was required. Later, a solid-state power supply for the +LV dynamotor was also available. Still later, these SS power supplies were installed from the factory and these models will be designated as the T-195A or T-195B. Although it's likely that the A version has just the +HV PS and the B version has both, I can't find any documentation that states this is the case.

A look at the T-195 upper deck. On the left side is the PTO and Exciter-Mulitiplier. Clustered in the center are the Discriminators, Antenna Capacitor and inter-connecting sockets. On the right side is the Antenna Inductance module. Note the white ceramic form for the motor-driven ribbon-wire variable inductor that is part of the autotune antenna tuning system. To the front of it is one of the blowers and the  Output Capacitor selector. Just visible on the lower deck is the back of the LV dynamotor (with the vents) and below the Antenna Inductance module is the +HV SS PS. The +HV PS is built into a case that is the same size and shape as the original dynamotor.

Looking at the underside of the T-195. At the upper left is the Modulator module. Note the yellow captive screws that indicate that this small chassis is removable to allow installation of new modulator tubes. The black cylinder is the +HV SS PS. Since the +HV PS had to fit in a specific space that was limited it had to mount like the original dynamotor and be the same shape. The module in the center is the Servo Amplifier. The RF PA module takes up most of the right side of the bottom deck. The air variable is the plate tuning capacitor that is servo motor driven during the autotune cycle. Upper right just below the Amphenol socket is the RF PA blower housing.

R-392/URR (GRC-19) - Details

Circuit - A stout, small and fairly lightweight receiver, the R-392 still has a lot of the features found on it's big brother, the R-390. Frequency coverage is .5mc to 32mc in 32 tuning ranges each with 1mc of coverage. Permeability tuning using slug racks driven by a complex gear train with a PTO, variable tuned IF and fixed Crystal Oscillator providing double and triple conversion is very similar to the R-390 receiver's front end as is the frequency read out provided by a Veeder-Root digital counter. 25 tubes are used in the double and triple conversion circuit that also provides 2 RF amplifiers and 6 IF amplifiers. Also, the IF stages are similar to the R-390 in that mechanical filters are not used for the selectable 8kc, 4kc and 2kc bandwidths. Data modes, e.g., portable RTTY, could be received via the IF output connector (the T-195 was capable of FSK transmission.) The Audio Output is 600 Z ohms and accessed from either of two twist-lock type connectors marked AUDIO or it can also be accessed from the POWER INPUT-TRANS CONT (PI-TC) connector. There is no phone jack on the R-392 because in the GRC-19 configuration the audio was routed to the T-195 (via the PI-TC connector) where typically a telephone handset, the H-33, was used for both transmit (microphone) and receive (earpiece.) The typical field speaker, if used, was the weather-proof LS-166. A Noise Limiter circuit is activated with the Function switch and a Squelch function is also available. When operated as the GRC-19 there is a short interconnecting cable between the T-195 transmitter and the R-392 receiver using the PI-TC connector that allows the two units to function together with the T-195 providing Break-in or Stand-by functions along with receiver to transmitter Signal Relay capabilities.

Variations in the R-392 Receivers - The initial contract in 1951 was from Collins Radio Co. but soon, just like the R-390 and R390A, many other contractors built the R-392 receivers. There are some variations from early production to the later receivers. Early receivers will use 26A6 tubes for the RF amplifiers while later production used an improved version of that tube, the 26FZ6. The change to the 26ZF6 was to help with cross-modulation problems when using the receiver near operating transmitters. Most of the later manuals specify that either the 26A6 or the 26ZF6 can be used as RF amplifiers. Early panels have silk-screened nomenclature while later panels are engraved. The 2kc-4kc-8kc BANDWIDTH nomenclature layout is closer together on early panels but spaced at 90º on later panels. Cabinets on early models have large flutes that run front to back while later cabinets have five "ribs" that entirely encircle the cabinet running parallel with the front panel. These "ribs" strengthened the cabinet significantly. Like many contractor-built items, the color tint of the olive drab paint used varies from contract to contract with some receivers appearing very light brownish-OD while others appear dark greenish-OD. R-392 production ended in the mid-1960s.

1963 Western Electric R-392/URR

More Information on the GRC-19? For the ultimate in details on repairing, rebuilding and operating the AN/GRC-19 go to our web-article "Rebuilding and Operating the AN/GRC-19" - navigation link below in the Index


Various Contractors for the U.S. Army Signal Corps

By the early 1950s, the widely-used Signal Corps version of Hallicrafters' HT-4, the BC-610, was rapidly becoming outdated and a modern replacement was needed. The T-368 transmitter was designed to be an updated replacement that would fill similar needs for a medium power, continuous duty transmitter that could be placed in communication huts, set up for mobile operation from a truck or set up for stationary operation. The design allowed for 400 watts to 450 watts of RF power to be delivered to a full-size resonant antenna or, if the BC-939 tuner was employed, a random length end-fed wire or a mobile whip antenna could be used. The RF output tube is a 4-400A that is modulated by two 4-125A tubes. It appears that Collins Radio had some engineering input (or influence anyway) on the T-368 as the design uses a Collins' style PTO to drive a permeability-tuned Multiplier/Exciter module which in turn drives the RF output tube. Also, a Veeder-Root type of mechanical digital counter is used for the transmitter's frequency readout. The similarity to other Collins' exciter-transmitter designs is apparent. The output section of the transmitter uses vacuum variable capacitors in the Pi-network since the +HV is over +2500vdc. There were several versions produced with minor differences that required different letter suffixes for identification. Several different contractors built T-368 transmitters throughout its production history and while Barker & Williamson (B&W) built some of the first contracts other companies that built later T-368s included Stromberg-Carlson, TRW, Bendix and others. Besides the "Basic" or non-lettered version, there are lettered versions from A through F. The T-2368 was a SSB transmitter based on the T-368 using the same basic three deck construction. The T-368/URT shown in the photos left and below is the "Basic" or "non-lettered" version with B & W as the contractor and with serial number 29 assigned to it. The contract is from 1952.

The T-368 will transmit in AM voice, CW or FSK modes. It can also be used as an RF amplifier using an external low power exciter via the External Input. For FSK RTTY operation, a proper FSK driver is necessary and is connected to the FSK input on the front panel. Many T-368 transmitters were set up in mobile RTTY communications huts and designated as the GRC-26. These huts contained two Collins R-390 receivers and a CV-116 diversity RTTY converter. Two TTY printers and one TTY transmitter/tape perforator were also used. The T-368 was set-up with the BC-939-B (designated as TN-339/GR) antenna tuner and the ME-165/G SWR bridge along with the MD-239 RTTY Modulator. Three vertical whips were used, two for diversity reception and one for transmitting. Also, a tapped doublet could be used for transmitting if the BC-939B was bypassed. Additionally, 1000 feet of copper wire, several masts and guy wires were in the spares kit for building various antennae for either receiving or transmitting, as needed. The GRC-26 hut was mounted to a 12 ft by 7ft trailer and towed to field sites. Also, a trailer-AC generator was part of the equipment. With the GRC-26, Voice or CW communications were only used in emergencies. However, it was common practice to establish comms using CW and then switch-over to RTTY.

 photo left: 1952 B&W T-368/URT Basic SN:29. On top right is the BC-939A tuner, left is the ME-165-G SWR Bridge/Dummy Load

To transmit voice generally required the use of the M-29 (or similar) carbon microphone connected to the CARBON MICROPHONE input but this isn't strictly the only way to voice modulate the transmitter. You can also run audio into the 600 Z OHM input via the rear Remote Input connector. Though intended for use with telephone line inputs, the 600 Z OHM input is almost exactly the same electronically as the CARBON MICROPHONE input with the exception that 600 Z OHM 1st Audio Amp has slightly higher gain than the CARBON MICROPHONE 1st Audio Amp circuit and there is no bias voltage or coupling capacitor in the input circuit. The disadvantage to using the 600 Z OHM input is that the mike cable must be routed to the back of the transmitter. Also, sometimes the 600Z OHM line picks up hum since the shielded cables can develop ground loops depending on the version of the transmitter (usually only on early versions.) Push-to-Talk is also available at the Remote Input. If you switch between the Carbon input and 600Z ohm input, be sure to reduce the Gain on the unused audio input to zero. This grounds the grid on that audio stage preventing noise from entering the Speech Amp.

The T-368 is heavy,...a brute that weighs-in at around 650 lbs. It's physically large - around 30"W x 40"T x 18"D. Generally, the T-368 will be found mounted on the military base plate which may (or may not) have large casters installed. If casters are installed it allows for easy "rolling" moving of the transmitter once it is within the room where it is going to be used. To remove any of the three decks is arduous work due to their weight (the PS section alone weighs well over 200 lbs!) Fortunately, almost all troubleshooting and certainly all routine maintenance can be performed by removing the D-zus fastened rear cover and, in some instances, extending a particular deck out enough to access underneath that chassis. If more extensive rework is necessary, the T-368 usually requires two men to disassemble or reassemble due to the massive size and weight of the individual decks and their component parts.

The "heavy-duty" components make the T-368 nearly "bullet-proof." It was designed for continuous operation and is generally only "coasting" in amateur service. Audio frequency response is controlled by internal hi-pass and lo-pass filters within the Speech Amplifier section (separate chassis mounted on the Modulator Deck) and limits the low end response to around 300hz (1.0db down) and rolls-off the upper end at 3500hz (1.0db down.) However, the PA load on the modulation transformer (which is around 7000pf) will also limit the upper end of the audio response with 3 or 4db roll off at 3500hz being typical. This assured the T-368 bandwidth in the AM mode was reasonable - around 6kc. This load is somewhat different in later versions that use two series-connected RF plate chokes.  There is also an adjustable Clipper circuit in the Speech Amplifier that limits modulation "peaks" and can provide some increase in average modulation levels (at the expense of audio quality when adjusted to high levels of clipping.)

The T-368 will provide a powerful signal that can dominate the frequency but the transmitter shouldn't be modified from its basic design as a military "communications" transmitter. Modifications installed to make the T-368 sound like an AM Broadcast transmitter will defeat the original design intent and go against the whole idea of collecting, restoring and operating vintage military radio transmitters in the first place. It is acceptable to disconnect the carbon mike bias wire and tape its end. Then move the mike input wire to the opposite end of the coupling capacitor. This allows a crystal or dynamic mike to be connected to the CARBON MICROPHONE connector on the front panel. An Astatic TUG-8 amplified base will allow several different types of mike heads to be used. This is about all that a T-368 needs to produce good quality AM audio on the ham bands.

photos: 1952 B&W T-368 sn:29 showing the three decks and the cabinet during its "tear-down" to inspect, repair, clean and "de-mod" the transmitter.

photo far left: PS in front. Modulator behind. Note the SS rectifiers. These have been replaced with original rectifier tubes, 3B28 types.

photo left: Cabinet which does contain all of the interconnects and harnesses. There was an abrasion on one harness that cut one of the wires. This happened because of broken cable mounting straps that allowed an incorrect position of the harness in relationship to the Modulator Deck.

photo right: The RF deck during repair of the Output Network Band Switch micro-switch interlock. The interlock switches off +HV if the band switch is moved with +HV on.

More information on T-368 transmitters?  For the ultimate, detailed information source, go to our web-article "T-368/URT Military Transmitter - Repair, Rebuilding and Operation" for history, circuit details, repairing and restoring, performance and more, with lots of photos. Navigation link below in the Index.



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