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:   Artwork from TDP-1 Direction Finder Manual

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

Pre-WWII Gear - 1932 - 1942


Airport Communication Receivers

National Company, Inc. - Airport Receivers 

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

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

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

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

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

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

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

photo above: AGS-X SN: F-151


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

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

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

RHM in Top Photo - I've owned this particular RHM receiver since early 1991. It came with all fifteen original coils and the original wood and metal, rack mount spare coil holder. This receiver was missing its original ID plate but the dust cover was marked "RHM #4" in black grease pencil. The ID plate shown in the photo is a reproduction (not a very good one.) This RHM is functional on "all original parts." This only means that the receiver barely functions. It does receive stations and it seems to work okay. However, I recently obtained RHM SN: 8. This receiver was rebuilt (by me in the late-1990s) for NU6AM whom I purchased it from in May 2018. This RHM is a phenomenal performer with excellent sensitivity and stability. This shows that, when new, the RHM receivers could certainly have met the demands of airport and airway communications service. >>>
>>>  Today however, the performance of any of the RHM/AGS family of receivers, even rebuilt ones, will seem somewhat antiquated and crude. In 1932 they were "state-of -the-art" but, within just a few years, National had completely surpassed these receivers in design and performance with their HRO receiver. Although the RHM/AGS performance might be considered "dated," the fact that many of these receivers are still operational and are still fairly accurate in their dial readout is testament to National's build quality and Herbert Hoover Jr. and James Millen's design capabilities. This same design team again worked together in 1934, producing the famous HRO receiver.

NOTE: In the 1930s, Airports were municipal or government facilities that provided runways for take-off and landing, tower to pilot communications (larger airports,) radio "beam" navigation and weather information (larger airports.) Airway was a defined route, generally within a specified altitude range, that a pilot flew within from one airport to another. Airway Range Beacon stations provided radio "beam" navigation, weather reports and ground to pilot communication if necessary. In areas of the USA where large airports were separated by great distances that prevented reliable Range Beacon reception by the aircraft in flight, remote Airway Range Beacon stations provided navigation radio "beam" signals.

* 600kc IF for the RHM? - For years, since 1990, I'd assumed that the IF for the RHM was 500kc, just like the AGS. I probably came to that assumption because of the copy of the RHM Operating Instructions I had obtained from somewhere. Within this copy, in the Intermediate Frequency Amplifier section, someone had crossed out (to the point of illegibility) the original indicated IF and had handwritten in "500kc." I never questioned it and had aligned my first RHM to 500kc. It seemed to work fine so I figured that the correction was valid.

Fast-forward to 2018 when I obtained RHM serial number 8. I had restored this receiver in the late-1990s for NU6AM who owned the receiver then. Of course, during the restoration, I aligned the IF to 500kc. When I acquired SN: 8 from NU6AM, I was given another copy of the RHM Operating Instructions but this copy didn't have the "handwritten" IF. It showed, in original print, "600kc." I thought that must have been a misprint and that's why the other instructions had "500kc" handwritten in. However, I had to verify if indeed the RHM IF was 600kc.

I aligned SN: 8 to 600kc and to my surprise it worked. It wasn't a dramatic performance change but there were certainly some subtle differences. I'd almost have thought that 600kc might have been the correct IF since the calibration charts seemed more accurate. However, the gain was somewhat down a very small amount but this receiver had sensitivity and gain to spare. I recalibrated SN: 8 back to 500kc IF and the change was a little more obvious then. Better gain overall. The real test was to recalibrate my other RHM receiver, serial number unknown. These two RHM receivers are identical and obviously from the same run. Both receivers had "RHM #4" written in black grease pencil on the dust covers. The difference was that SN:8 was rebuilt with new replica capacitors and a few replica resistors (in the late-1990s) and the other RHM was all original.

I then aligned this original RHM to a 600kc IF. The gain was way down. So far down that I couldn't pick up any signals even with a 135' tuned Inv Vee antenna. I recalibrated the IF back to 500kc and there were the signals. Since this RHM is "limping along" with all original parts, I wouldn't expect it to have nearly the performance of the SN: 8 and, consequently, since the original RHM wouldn't even work with a 600kc IF but it does work with a 500kc IF, the correct IF must be 500kc. This also seems to indicate that the original operating instructions had a glaring error when 600kc was shown as the IF. 

I don't know if any other RHM owners have tried both 500kc and 600kc for the IF to compare performance. If you have, let me know your results and thoughts.

You can use this e-mail link:  RHM IF TEST RESULTS  


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

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

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

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

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


National RCE, ca: 1937

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

National Co., Inc. had been supplying receivers for airport and airway communications since the RHM receiver in 1932. The mid-thirties HRO also was used in some airports. The most popular airport receiver by far was National's Airport and Airway Communication Receivers that were based on their NC-100 receiver. The continuing upgrading of communication and navigation equipment was initially the responsibility of the Department of Commerce and the Bureau of Air Commerce with its various branches in charge of airports and navigation. The new and developing radio navigation systems provided a pilot the means to fly an airplane to an airport using a radio range beacon called a "beam." The navigational radio beam allowed that pilot to follow a predetermined altitude and route called an "Airway." The navigational beam and various beacon stations also provided two-way communications with pilots to report weather conditions and other information that might be needed by pilots. National's history of providing top quality receivers for airport use (RHM, the fore-runner to the AGS, and the HRO) practically assured them of continuing contracts for airport-specific receivers. The first "moving coil" receivers supplied to airports were standard NC-100X receivers. Some of these early receiver used the "art deco" overlay panel while others used a black wrinkle finish rack mount panel. In 1937, National began supplying the Department of Commerce, Bureau of Air Commerce, Dept. of Navigation (still in charge of airport communications at that time) with the RCD receiver, a slightly modified NC-100X receiver that had the AM BC coils replaced with 200kc to 400kc coils. At the end of 1937, the RCE was introduced and it became the "standard" Airport-Airway Communication receiver from National. When the U.S. Civil Aeronautics Authority (CAA) was created in 1938, National then supplied the CAA with airport receivers. These new CAA receivers were continually being upgraded as new contracts were issued. Some receivers were even produced during WWII for use at both military and civilian airports. The RCK tuned 200kc to 800kc in the two lowest frequency bands and 2.5mc to 23.5mc in the three highest frequency bands. The RCK was supplied to the U.S. Navy during WWII. Also during WWII, the RCL was introduced and it featured a two position bandwidth switch. After WWII, early versions of these receivers (RCK and RCL receivers) were upgraded 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 Airport-Airway Communication Receivers are based on the NC-100 chassis, there are some significant additions to the circuit. The most obvious is that the receiver is rack mounted and has a very different front panel when compared to the striking "art deco" panel of the standard NC-100 receiver. These receivers use a 3/16" thick aluminum front panel that is black wrinkle finished. All early Airport-Airway receivers were equipped with an I.N.S. control. I.N.S stood for "Interchannel Noise Suppressor," which was actually a "squelch" control. 

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

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

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

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

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

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

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

For the ultimate information source on the AR-60 receiver, including history, performance comparisons, restoration information, serial number analysis and more, including the frequency to dial readout chart, go to our web-article "RCA's Legendary AR-60 Receiver." 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 most successful communications receiver was the AR-88. Designed in 1940-41 by Lester Fowler and George Blaker (and rushed into production because of WWII requirements) 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 were 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. Since the AR-8506-B is a "transformerless" AC-DC circuit, always operate the receiver using a 1:1 isolation transformer.

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


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" (though late 51J-1 receivers will have "CALIBRATE" on the front panel.) 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 well 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' production assigned number that usually was sequential for the entire production however the ID tag's serial number was sequentially assigned for the specific contract only. These two serial numbers never match and considering how they were assigned, shouldn't.

Sometimes an R-388 will be found with both USN Anchor acceptance stamps and Signal Corps acceptance stamps. 

photo above: 1951 contract R-388/URR

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. The civilian 51J-3 is seldom seen. These receivers shouldn't have the MFP coating applied since they were going to be in commercial or amateur use. The Collins serial number tag will identify the receiver as "51J-3" and the serial number assigned should be numerically very low.

Be aware that many so-called 51J-3 receivers are actually R-388 receivers with their ID tags removed. Also, be aware that if the 51J-3 data plate has a serial number in the 12000 range, it is a reproduction data plate and the receiver is likely not an authentic 51J-3. If there's no ID tag or a bogus serial number, check the receiver to see if there are any Signal Corps acceptance stamps to verify the model. A 51J-3 receiver shouldn't have Signal Corps inspection stamps or a MFP coating.

The 51J-3 was introduced in 1951 and produced in very limited numbers up to around the introduction of the 51J-4 in 1955.

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.

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

For the ultimate in detailed information on the entire 51J Series including history, rebuilding and restoration with lots of photographs, 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 with production starting in late-1951, the SP-600 was intended for the military and commercial user market. It was a very popular receiver even though the selling price was nearly $1000, but many thousands were built, especially for military applications. It's 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.) The Signal Corps must have been working with other companies to find acceptable receivers designs for their requirements because, in 1949, the Signal Corps contracted with Hallicrafters to build a "Super Pro-type" receiver that met the design requirements. The Hallicrafters receiver was designated R-274/FRR. The SP-600 was first ordered in a 1950 contract that was for a R-483 receiver (JX-5.) The first R-274A Hammarlund version was on a 1951 contract (JX-1) and this was the first SP-600 officially produced in November 1951. Hammarlund used R-274A and C designations for Signal Corps receivers and R-274B for USN receivers. Hallicrafters versions was assigned the suffix D on the next contract (1952.) Eventually, Hammarlund SP-600s were the choice of the military who ordered tens of thousands of them over the next decade while Hallicrafters' version was not ordered after only two contracts (1949 and 1952.)

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

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

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

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

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

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

Best results using the SP-600VLF will require an "RF-quite" location, a tuned loop antenna 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


Hallicrafters R-274/FRR sn: 762

 The Hallicrafters Co.  -  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 quantities. 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 heavy-duty plastic module bases with stub pins pressing against flex contacts. The SP-600 coils are on a ceramic base with longer pins passing thru dual pinch contacts. The tuning dial provides a main dial and a logging dial as the SP-600 does but behind a single escutcheon rather than separate dials behind two escutcheons as the SP-600 does. There is a selectable six-channel Crystal Oscillator that functions like the SP-600 "X" option and provides improved stability for RTTY and other data modes. Like the SP-600, the bandwidth is selectable in six selectivity steps with three of those steps using a Crystal Filter for narrow bandwidth (a front panel Phasing control is also provided.) A 600 ohm balanced audio output is also similar to the SP-600 audio output.

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

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

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

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


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 very small contracts in 1968 and 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 

There was a four year gap between the Imperial Electronics 1963 R-390A contract and the last year of "standard contract production" that was given to Electronic Assistance Corp. in 1967. There are two identification numbers used for the 1967 EAC receivers - a contract number or a order number. FR-36-039-N-6-00189 (E) is the 1967 EAC order number and DAAG-05-67-C-0016 is the 1967 EAC contract number. Both numbers will be on all of the individual modules and on the main frame. The data plate will be found with either the order number or the contract number shown but not both. It's unknown why different data plates were used on the '67 EAC receivers. Civilian sales receivers may have a data plate with an EAC "rocket" logo and no order or contract number.

One of the reasons that the 1967 EAC is sought-after by R-390A enthusiasts is that these were the last versions built, using the newest components available. These receivers should require the least amount of rework. However, there are some quality concerns for some of the EAC production due to a supposedly high reject-rate from the military. These rejected R-390As were reportedly returned to EAC who then sold the rejects to the civilian public (ca. 1968) as new production. Urban legend?

photo left:  '67 EAC SN:974 - with fold-down frequency security cover

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 are 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 black anodized finish. 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. Note that the R-390A receivers in the foreground are also equipped with the vernier control on the BFO that was installed to allow fine tuning of FSK signals for RTTY use. Most of the racks do 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.

For more details on rebuilding R-390A receivers, with lots of photos of all modules, go to "Rebuilding the R-390A Receviers" - navigation link below in the Index.


Collins Radio Company - R-389/URR - LF Receiver

Basic Description - Electronics - Built along some of the same lines as the famous R-390 receiver, the Collins R-389 is essentially the LF companion receiver of the R-390 covering 15kc to 500kc in one tuning range and 500kc to 1500kc in the second tuning range.

The R-389 uses very complex methods, both electronic and mechanical, to achieve its complete MW, LF and VLF coverage while still utilizing a 455kc IF. The receiver uses 36 tubes within five modules that interconnect and are mounted within the main frame. The 15kc to 500kc tuning range utilizes five permeability-tuned RF bands. The 500kc to 1500kc tuning range utilizes two permeability-tuned RF bands. The motorized band switching occurs seamlessly as the receiver is tuned from the lowest to the highest frequency within the two tuning ranges.

Two RF amplifiers are used and the first conversion mixes the incoming RF signal frequency with the VFO (470kc to 1955kc output f) plus the 10.455mc Crystal Oscillator (8.5mc to 9.985mc resulting f) to achieve a 10mc IF. The second conversion mixes the 10mc IF with the same 10.455mc Crystal Oscillator to achieve the 455kc IF. This double conversion scheme was to allow complete coverage from 15kc to 1500kc with no gaps in the frequency coverage. Additionally, since the two mixer stages are 180 degrees out of phase, any drift within the conversion mixers is cancelled leaving only the VFO drift. This is similar to how the "drift-cancelling" Wadley Loop operates.

From the second mixer circuit on, the R-389 utilizes the same modules that are found in the R-390. That would be the six-stage IF module, the two channel audio and electronic voltage regulator circuit module and the power supply module. Although the PTO (VFO) looks exactly like that found in the R-390, it's very different inside and tunes from 470kc to 1955kc.

Mechanical Details - The manual tuning of the receiver RF front end uses clutch-coupled gears to rotate the main RF tuning shaft that has worm gears that perpendicularly engage and rotate the gear-driven front and rear line shafts that have worm gears that in turn engage gear-driven vertical screw-shafts. The RF shafts are cut with forward and reverse threads while the VFO and Mixer shafts are fine cut threads. All of the vertical shafts raise and lower the various slug racks as the receiver is tuned. This gear-driven system is very easy to tune and feels about the same as tuning a good condition, properly adjusted R-390A gearbox. Since the amount of spectrum covered in the two tuning ranges is so large and requires so many turns of the tuning knob, a clutch-coupled, motor-drive tuning system is provided. A separate motor-driven bandswitch is actuated a specific frequencies and allows seamless band changing as the receiver is being tuned.

The Veeder-Root counter is somewhat different than that used in the R-390 and provides two sets of digits, one for 15kc to 500kc (lower set) and the other for 500kc to 1500kc (upper set.) The resolution of the digits (tuned f) is to the tenth of a kilocycle (which are the red background digit wheels.) Neither a calibration oscillator or an antenna trimmer are provided (or needed.)

Most of the controls are the same as those found on the R-390. The BFO controls, the Noise Limiter, the Local Gain, Line Gain, Line meter range switch, RF Gain, AGC switch, Break-in switch, Audio Response switch and Function switch. The controls that are unique to the R-389 are Motor Drive, IF Bandwidth (five ranges instead of six,) RF bandwidth KC indicator and the single tuning knob. The two meters perform the same functions as the R-390 meters, that is, Carrier Level and Line Level.

More Details - Physically, the R-389 is the same dimensions as the R-390 and will fit into the CY-917 or CY-979 table cabinets. If installed into a table cabinet, the top and bottom covers should be removed. The receiver weighs 82 pounds but, for easier moving (e.g., up or down stairs,) the power supply and AF module can easily be removed and then the receiver weighs around 65 pounds.

Two antenna connectors are available. Balanced input for 125 ohms input impedance from dipoles or other balanced antennae. Balanced is connected to the primary winding of each antenna coil. Unbalanced input is for random length wire antennae. This input is capacitively-coupled through a .01uf capacitor to the RF amplifier coils. The Unbalanced input impedance is not specified but is probably fairly high assuming that end-fed wires were probably the design target Z. The Balanced input utilizes a "Twin-ax" two-pin coaxial connector and the Unbalanced input utilizes a "C-type" coaxial connector. As mentioned, due to the low frequencies, no antenna trimmer is provided so the antenna impedance should be somewhat matched to the particular antenna input used.

Both audio outputs, Local Audio and Line Audio, are 600 Z ohm outputs and can provide about 500mW on Local and about 10mW on Line. The phone jack doesn't disconnect the audio output (LOCAL) from its respective load. There is a series resistor and a load resistor to the PHONES jack to keep the audio level (5mW) from over-driving the headset if the proper 600 Z phones are used.  

The AC power connector is a four-pin military connector that is keyed and held in place with a central screw that has a fold-down, wing-type handle. There are at least two different types that fit,...sort of. The original (CX-1358/U cable + connector PN) connector has a small round cylinder-shaped housing with a cable exit tube on the side. This type will fit in almost any orientation and must be used if the receiver is installed into a table cabinet.   >>>

photo above: Top of the R-389 showing the RF module and IF module mounted in the receiver. The RF module has the slug racks located under the cover. The two tubes showing thru the opening are the two RF amplifiers. The Crystal Oscillator and first mixer is the to the left and the second mixer is to the right. The IF module is to the left side frame.

>>>  Unlike most other LF and VLF receivers, the R-389 doesn't have any fixed-circuit audio restrictions within the audio module other than the switch-selected Broad-Medium-Narrow. Selecting Broad results in a fairly wide audio bandwidth. Medium is shaped for voice with noisy conditions and Narrow is a bandpass filter at 800hz for CW. The IF bandwidth can be restricted down to 100hz. Both 100hz and 1000hz IF bandwidths use a crystal filter that's onboard the IF module. The 2kc, 4kc and 8kc IF bandwidths are determined by the IF transformers and Q-resistor set-up. For static bursts and other types of atmospheric noise, the dual positive-negative noise limiter is available. When tuning in the AM BC range, the receiver's bandwidth can be increased to 8kc and BROAD and, with no other specific audio restrictions, the resulting audio isn't too bad. However, the audio is more-or-less communications-grade audio so don't expect high fidelity because it isn't. Most listening on LW will usually be using a headset. Most listening on the AM-BC band will be on loudspeaker.

For best results on longwave always use a 600Z ohm headset connected directly to the LOCAL audio output terminals on the rear panel. Connecting the headset to the PHONES jack will introduce a resistor network that reduces the audio level to 5mW to prevent over-driving the headset. However, since the audio level has to be increased to drive the phones adequately, high noise levels result. By driving just the headset only with the LOCAL output, the noise is greatly reduced allowing better copy of LW DX.

Only 856 R-389/URR receivers were built on the single contract 14214-PH-51 (that contract was also used for R-390, R391 and early R-390-A receivers.) Although the contract is from 1951, the build-date on SN: 268 was 1955 (based on component date codes.)

Go to "Vintage Long Wave Receivers" for more details on the rebuilding of this R-389 and a write-up on its performance. Navigation Index is at the bottom of page.

photo left
: Bottom of the R-389 showing the Power Supply module on top, the VFO in the center and the Audio and Electronic Voltage Regulator module on the bottom. Motor drive system is directly behind the front panel. The long rectifier (green with fins) is part of the motor drive power supply. The bent, metal arm directly behind the tuning knob is the motor-drive clutch engagement arm that is cam-driven from the MOTOR TUNE control. The three coaxial cables behind the VFO are from the Antenna Relay box and connect the Balanced Antenna input and the Unbalanced Antenna input to the RF Module.


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 (doing RTTY in some airborne installations) and it uses a lot of the same mechanical design approach as it's bigger brother, the R-390A. Both receivers are modular and employ a mechanical digital frequency dial. However, much of the circuit design borrows heavily from Collins' 51J series of receivers, especially the dual variable IF with reduced number of crystals in the Crystal Oscillator and the fixed 500kc IF. Since the R-648 was going to be airborne, it had to be light-weight unlike the R-390A that weighs-in at around 80 pounds! By reducing the size of the components and mechanics along with eliminating most of the R-390A similarities, 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. One feature the R-648 has that neither the R-390A or 51J series has is tuning from 190kc up to 550kc. The remaining frequency coverage is 2.0mc up to 25.0mc.

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.

In actual use the R-648 is a very sensitive receiver that provides an accurate frequency readout and excellent stability. The selectivity is steep-sided as expected with mechanical filters but the early receivers had a 9.4kc MF for VOICE reception and many times that's a little too wide. The audio filter can be switched but it's easier and more effective to just tune up or down a couple of kc to reduce QRM. CW is fine with the 1.4kc filter and SSB also sounds just fine with the narrow bandwidth. The CW audio filter is very narrow. There is some vibration from the dynamotor and although it can be felt it doesn't seem to affect the receiver in any way. Noise from the dynamotor might seem a loud at first but, after the audio comes up, the dynamotor noise is pretty much masked by the receiver audio. A nice performing, small, light-weight receiver that's easy to find room for in any vintage ham shack.  

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. IRAN doesn't indicate the country, Iran. It's an acronym for the process "Inspect, Repair As Needed."

The shock mount shown is homebrew. It does have the correct shock feet but the rest of the mount is not like the original (but it does function fine.)


Arvin Industries, Inc. - R-725/URR

The R-725/URR is a 1967 Electronic Assistance Corporation-built R-390A receiver that was modified (in 1967) by Arvin Industries, Inc. for the USAF to use in semi-portable radio direction finding systems. Each R-725 receiver had the following modifications installed. First, the standard R-390A IF module was replaced with a new manufacture Series 500 IF module built by Arvin Industries or Servo Electronics. The Series 500 IF module was essentially a R-390 IF module (six IF amplifiers with no mechanical filters) that had minor updates to coax connectors to allow the Series 500 IF module to be installed with no modifications to the R-390A circuitry. However, further design development for the R-725 modification turned up a 60hz modulation problem that required additional modifications. A small chassis is mounted in the main frame space directly in front of the power supply module. This chassis has a 25vac transformer, two resistors and a connector-harness. This was a "hum-bucking" transformer that basically disconnected the VFO tube, the BFO tube and the ballast tube and powered the tube heaters with a "floating" 25vac (not referenced to chassis) and then used the resistive divider connected to B+ to "swamp" the AC with DC. The result was these tube heaters and ballast tube series string operated on +25vdc. To further protect the PTO from 60hz hum pickup, the entire PTO case had a grounded ferrous metal shield installed. The final modification was to the IF Output connector. The larger Series 500 IF module prevented connecting the IF Output cable to the back connector due to lack of clearance. A special "low profile" right-angle coax fitting was installed that allowed the IF Output to be available at the back panel. The contract number for the R-725/URR was DAAB05-67-C-2338 with a total number of receivers modified being less than 300.

The Non-Secret R-725 Story - The purpose of the R-725 mods was for compatibility with military portable direction finders that used four vertical antennae per installation along with three receivers. The DF system used went back to the Bellini-Tosi type of DF set-up that used two crossed loop antennae with a rotating loop inside to create a radio-goniometer. Bellini and Tosi had discovered that crossed loop antennae would "re-radiate" the signal they were receiving within the small field inside the antenna's space. The "re-radiated" signal retained all of the directional properties of the original signal and could be measured for varying signal intensity dependent on direction. The crossed loop antenna size didn't affect the frequency of operation allowing for reduction in the size of DF loops on LW. The original Bellini-Tosi system dated from around 1900 and the system was sold to the Marconi Company around 1907. By the early twenties, vacuum tube amplifiers were being added to increase performance capabilities of the DF antennae systems. The most common B-T DF systems used the crossed loops but some larger systems used the four-square vertical antenna system. This system was developed by Adcock during WWI and because the connections to and from the four square verticals were underground it didn't respond to skywave propagation and allowed ground wave DFing over long distances. The B-T DF and Adcock systems continued to evolve and improve with the systems being used throughout WWII. During WWII, oscilloscope displays began to be used for direction indications. After WWII, larger DF systems continued to be developed up to the mammoth "elephant cage" antennae ("Wullenweber" was the actual name) that were over a thousand feet in diameter and consisted of several "rings" of circular antennae all working to provide accurate DFing over great distances and wide frequency spans. By the 1990s, most of these large arrays were becoming obsolete and nowadays most have been dismantled.

photo right: Top chassis of the R-725 showing the Series 500 IF module on the left side of the main frame.

photo above: The underside chassis showing the "hum bucker" located in front of the power supply module and the ferrous metal shielding on the PTO.

The mechanical filters used in the R-390A resulted in signal path phase shifts that caused errors to show up in the DFing electronics. When used with the four square antennas, the low frequency modulation added via the radio-goniometer interacted with the mechanical filters creating the error. Early versions of this DF set-up had used R-390 receivers and the radio-goniometer was located quite a distance from the receivers to reduce any interference. In the 1960s, the USAF wanted to reduce the size of the entire DF system so it could be towed around on a trailered hut. This meant the radio-goniometer had to be in the same room as the receivers. This was going to require some protection to certain receiver circuits. The R-390 had been out of production for several years, so the solution was to design the new portable system to use modified R-390A receivers that could be easily purchased. Arvin Industries was the main contractor with Servo also doing some rework. The modified receivers would have the Series 500 IF module, essentially a R-390 IF module that was slightly updated to not require any rework to the R-390A receiver it was installed into. That eliminated the mechanical filter phase shift problem. Additionally, with the close proximity to the radio-goniometer, a 60hz hum appeared on the PTO tube filament  and that also interfered with the LF modulation of the DF system. A special "hum bucker" chassis was added to the receiver that essentially operated the VFO tube, the BFO tube and the 3TF7 Ballast tube on +25vdc. Also, a grounded ferrous metal shield was added to the PTO housing to prevent hum "pick up." Arvin bought new R-390A receivers in 1967 direct from Electronic Assistance Corporation and the modifications were installed at Arvin. When complete, the receiver was tagged as "R-725/URR." The tags will generally show Arvin Industries as the contractor but sometimes Servo Electronics will be encountered. Arvin ink-stamped a serial number on each Series 500 IF module and when that module was installed into the receiver that same serial number was stamped onto the front panel data plate.

The Secret Project - Was there another purpose that was the "real" reason that the R-725 was created? According to an article that appeared in Electric Radio in January 2006 by Chuck Teeters, there was a "top secret" purpose for the R-725 and the receiver "mods" were primarily created for that "secret" project. The R-725 was a product that resulted from the Cold War jamming that was common between the USA and the USSR. In the mid-to-late 1960s, the NSA, the USAF and the Signal Corps were developing a new system called "Tropicom" that was an upgrade to the antennas and transmitters to improve HF communications for the military. The Tropicom upgrades also included the incorporation of the "F9c" anti-jamming/crypto system. The F9c system used a spread spectrum transmission of digital noise and signal that ran through a digital encrypo-key generator that had 144 stages of looped-feedback that also fed through phase modulators to maintain proper phase relationships of the signal and noise. When recombined at the receive end the signal to noise extracted the signal and left the noise and any jamming attempts far below the signal level. Since the system used spread spectrum, the signal couldn't be detected without the proper combination of equipment and decryption and that left any jamming attempts at just "blind" shots. However, when the F9c was used with a R-390A on the receive end, the phase changes in the mechanical filters interfered with the recombination process and the system didn't work. When used with R-390s with a standard IF amplifier circuit, the F9c system worked fine.

Since the R-390 receivers dated from the early-1950s, there were only a limited supply of those receivers still available and those that were available needed constant maintenance. The ultimate solution was to have new R-390A receivers with newly-built R-390 IF modules installed available for the Tropicom system.

In order to keep the F9c project "secret," the actual use of the R-725 couldn't be known to those outside the Tropicom project. Since there really was the Adcock DF system upgrades that really did need a non-mechanical filter type R-390A, the R-725 was directed to be built for the DF purpose only. However, those running the F9c project had the R-725 order quantity doubled and half of the R-725 receivers were procured for F9c use while the other half went to the DF systems. The secret classification stayed on with the F9c system and it was used for quite a long period with many upgrades over the years. So, even though half of the R-725 receivers were used in direction finders, the other half had a "secret life" used in the anti-jamming/crypto communications world of the NSA, the USAF and the Signal Corps.

Performance - The R-725/URR is very much like listening to a R-390 receiver. The modifications to the VFO-BFO heaters using the "hum-bucker" are not audible. The big change is the Series 500 IF module. With six IF amplifiers, the R-725 has plenty of gain. So much, that most strong signals will push the Carrier Level meter to 70db or 80db and then if the receiver is tuned off of the signal, the meter drops to 20db or less. I have the IF gain reduced by 40%. Audio sounds slightly different than the R-390A with mechanical filters but still there is lots of selectivity and QRM is not a problem. The R-725 is basically like having an R-390 without all of  the maintenance headaches.


U.S. Army Signal Corps - AN/PRD-1
Direction Finder Set

Andrea Radio Corp,  Parkchester Machine Corp

The AN/PRD-1 was a portable direction finder set that consisted of a R-395 receiver, a DY-79 dynamotor, a CY-947 battery box, a MT-870 tripod mount, an AS-536 combination loop and sense antenna and an AT-301 sense antenna extension. The entire set could be packed into four wooden crates for transportation. The PRD-1 could be mounted to a Jeep for portable operation or it could be transported to and used "in the field" mounted on its tripod. When mounted in a Jeep the +24vdc battery system of the Jeep powered the DY-79 that then powered the R-395. When the PRD-1 was operated in the field there were two power options available. The provided 50 ft. power cable allowed connection to the Jeep's +24vdc power with the PRD-1 set-up in the field. The other method was to not use the DY-79 but to instead use the CY-947 battery box that provided the necessary R-395 voltages via dry cells. The voltages necessary were +1.5vdc filaments, +6.0vdc tube heaters, -6.0vdc bias voltage and +87vdc plate voltage. Most of the tubes used in the R-395 are low current battery receiver tubes, e.g., 1U4, 1U5, 3Q4, but there are a couple of six volt cathode tubes used also, 6AK5 and 6C4. The CY-79 dynamotor box also has some electronic circuitry that uses two ballast tubes, two 12AU7 tubes and one 6AK5. When using the CY-79 to operate the R-395, a 6 volt "lantern battery" has to be installed on the dynamotor chassis. This battery supplies the -6.0vdc bias voltages. When the complete PRD-1 "in the crates" was available then cables, headsets, spares, set-up compass, locating stake and many other odds and ends were included. Initial contracts were in 1951, 1954 and 1955.

The R-395 is a fifteen tube, double preselection, single-conversion superheterodyne that tunes 100kc to 30mc in seven over-lapping bands. It was designed to receive CW, AM or MCW (or ICW) modes on all seven bands and to also receive FM signals on Band 7 (12.5mc up to 30mc.) The circuit utilizes two IF sections, a 455kc IF for all bands except Band 2 which uses 1610kc for the IF. FM IF is 455kc but uses different 455kc IF transformers that utilize the 1610kc IF tubes. A dial mask provides "band in use" viewing along with an opening for viewing the logging dial. The meter will read the various battery levels and also signal strength in the IND position. When MONITOR is selected, the loop antenna is disconnected and only the vertical sense antenna is used since it is omni-directional. The DF position connects the loop to the input of the first RF amplifier and the sense vertical to the input of the sense amplifier stage. The sense amp output is then routed back to the first RF amplifier input. The first RF amp output is combination of the two antenna responses. ANT TRIMMER and DIAL ADJ (index) controls are provided. The SENSITIVITY control also has an AVC (on) position that can be utilized for general listening in the MONITOR mode (non-DF.) AVC should be turned off for DF purposes and only the minimum amount of RF gain used for accurate bearing indications. Strong signals tend to be rather broad and are more difficult to DF accurately. Audio output is 600Z ohms and designed for headset although the R-395 will drive a 600Z ohm speaker quite well. When operating on the DY-79 dynamotor, the current required is around 7 amps at +24vdc.

photo above: Close up of the R-395 receiver and the DY-79 dynamotor box. Note the "red" and "white" scales on the loop compass.

photo above
: The AN/PRD-1 set up on the MT-870 tripod with the DY-79 dynamotor and the R-395 on top. The AS-536 loop antenna only requires the loop extension AT-301 on Band 7, so it isn't installed. The shop ceiling is nine feet high and there is about one inch of clearance with the top of the loop (without AT-301 installed.)

DFing Objectives - The object of using the PRD-1 was to determine a true bearing of a signal from an unknown location. That signal could be friendly but more often it was from an enemy transmitter. Generally, these signals were not very strong and were only transmitted for very short time periods. The R-395 has ample sensitivity to detect very, very weak signals, not for DX purposes, but for locating nearby, weak, enemy signals. It wouldn't do much good to determine the bearing of a strong signal located 1000 miles away. The PRD-1 was for finding enemy transmitter locations that were nearby. In fact, it's direction finding (DF) works best with ground wave signals. Sky wave propagation tends to adversely affect DF accuracy. By adding a second DF location (some distance from the first) allowed for "triangulation" to determine the exact location of the enemy transmitter. The direction bearing had to be determined quickly since most enemy transmissions were brief. Additionally, sometimes enemy transmissions were simultaneously sent from two different locations to confuse the DF process. Also, high angle-of-radiation antennae were sometimes used to "force" skywave propagation in an effort to thwart DF accuracy.

Determining True Bearing of a Received Signal - The receiver has to be set-up in the field so that it is oriented to North and South correctly so that the degrees indicated on the loop compass accurately represent azimuth positions. The tripod has to be leveled using the bubble-level in the accessory compass. Then the compass (indicating magnetic N) and stake allowed operators to "sight" the stake driven in the ground 150 feet from the receiver. The compass was then removed and the receiver and dynamotor were installed onto the tripod. The loop was rotated to read 90 degrees and then "sighted" to the stake out 150ft. The loop compass could be adjusted by loosening the locking thumb screws and rotating just the degree wheel to the correct indication. If "true North" was required for bearings then the magnetic deviation for the particular area had to be known. Magnetic deviation varies all over the earth and can be a number of degrees east or west of the theoretical magnetic North. Charts provided the users with the correct deviation for the area of use. From this figure and theoretical magnetic North, "true" North could be calculated and adjusted into the loop compass. At this point, the PRD-1 was ready to accurately measure true direction of a received transmission.

The loop antenna in combination with the vertical sense antenna allows the PRD-1 to determine "true direction" of a tuned signal. The loop antenna alone will respond to a tuned signal in a "figure 8" pattern that is in-line with the axis of the loop to provide the strongest response. By adding a vertical "sense" antenna within the field of the loop, the "figure 8" pattern is modified to be a cardioid pattern with the strongest response being from one lobe of the loop in the direction of the signal's location. However, a more accurate method used by the PRD-1 was for the operator to first rotate the loop for a null in the signal. Then, the indication in degrees on the white scale of the antenna compass is noted and the loop rotated +90 degrees (with the white scale numbers increasing.) This will result in a stronger signal response. Then, while noting the meter reading, the operator would switch to WHITE and then switch to RED while noting the meter reading changes. Which ever reading was the lower then the true bearing would be indicated on that color scale on the loop compass.

Loop Antenna - The AS-536 is a combination loop antenna that is a "diamond" shape and a vertical sense antenna that is in the center of the loop field. The sense antenna has an adjustable section that allows for completely "folding up" the loop assembly for packing and portability. When setting up the AS-536, the height of the sense antenna is adjusted for best fit of the loop ends into the antenna compass receptacles. Each side of the loop is three feet long. The AT-301 is a three foot extension for the top of the vertical sense antenna. AT-301 should only be installed if the intended operation is on Band 7, that is, 12.5mc to 30mc.

Tripod - MT-870 was used when the PRD-1 was set-up in the field. Its height is adjustable by lengthening each leg or by the angle of the three legs. When in dirt, the pointed ends will "dig in" and prevent movement of the legs. On floors or hard surfaces, the pointed ends will slip and the tripod won't stay at its correct height. Note in the photo to the left, I've installed some 14 gauge wires that allow the legs to only spread out to 24 inches. This provides a positive limit to the "leg-spread" and prevents the tripod from moving once the DY-79 and R-395 are installed. The DY-79 is mounted to the top of the tripod using a threaded rod with attached handle. The R-395 is clamped to the top of the DY-79 with four bale clamps.

Performance - I'm operating this PRD-1 with a +25.5vdc 10A power supply via an original power cable (two 14ga and two 10ga with shield.) Set-up is in the shop. The first test was during the day and consisted of tuning in local AM BC stations, 40 meter hams (on SSB) and some Shortwave BC stations. In MONITOR position, all stations were received well. In DF, the position of the loop could enhance or reduce tuned signals as desired. On medium wave, FCH 342kc, CC 335kc and MOG 405kc were tuned in. DF provided lower noise than MONITOR since the sense vertical alone responded to noise. During the night, I tuned in 23 NDBs in about 25 minutes of listening. Best DX was DDP 391kc in San Juan, Puerto Rico and QD 284kc The Pas, MB, Canada. The advantage of the PDR-1 was the ability to immediately adjust the loop to null noise or to enhance signals. Also, "blowtorch" NDBs could be nulled to allow copy of other weaker signals. This really isn't what the PRD-1 was designed for but it does show that the R-395 is sensitive and that its DF capabilities do work, even with sky wave propagation.


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 well-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.


National Company, Inc. - NC-400

The NC-400 was introduced in 1959. It was obvious from the astounding list price of $895 that the intended customers were going to be commercial users along with military and government purchasers. Many of the features of the NC-400 were necessary for commercial-military applications but weren't required by the ham users. The five, selectable, fixed-frequency, crystal-controlled receiving channels or the diversity options were certainly for military or commercial use. The crystal-controlled BFO option or the IF output (both for RTTY) were other features not normally found in a ham receiver. Production estimates vary widely - from a low of several hundred receivers up to perhaps as high as 1000 receivers. Probably less than ten percent of production went to the "well-to-do" hams and SWLs who could afford the "stratospheric" price. The remaining receivers were sold to commercial and government users. Among the more commonly mentioned government users is the FBI where the receivers were installed in some field offices for various purposes. Certainly other government users and the commercial users found the NC-400 filled their requirements and the exorbitant price was paid either by tax-payer dollars or company requisition funds. The NC-400 was available up to about 1963.

The NC-400 is a double pre-selection receiver (two TRF amplifiers) and is also dual conversion above 7.0mc. Seven tuning ranges cover from .54mc up to 31mc. The band spread dial is calibrated for the HF ham bands with a logging scale provided for general coverage. The conversions take place at 1720kc and 455kc of which the latter is derived from a crystal-controlled oscillator. There is a "plug-in" feature for the crystal filter assembly which can be removed and replaced with an optional mechanical filter assembly that allowed the user switchable access to a maximum of three different mechanical filters for bandwidth selectivity. The LO tube heater is operated from a higher heater voltage winding on the power transformer that has the voltage dropped thru a 4H4C current regulator to provide stable heater voltage regulation. A dual triode 6BZ7 is used for the LO and for the switchable Crystal Oscillator. The three IF amplifier stages actually are dual IF strips that utilize the same amplifier tubes. One IF strip is for SSB and provides 14 tuned circuits to achieve a fixed 3.5kc bandwidth. Upper or lower sideband can be selected with the bandwidth switch as "SB1" or "SB2." The bandwidth switch selects various capacitors to shift the IF slightly for proper sideband reception. The operator can also select other bandwidths for the SSB mode but the BFO may require a slight adjustment other than using the SSB arrow on the BFO knob. The Crystal Filter can't be selected in SSB. The fixed 3.5kc bandwidth was thought at the time to be ideal for SSB and the switch sideband selectability was to ease tuning difficulties in the SSB mode of reception. In CW, the crystal filter is switched into the circuit instead of the first set of SSB IF transformers after which the CW signal continues thru the first IF amplifier tube and thru the remaining SSB IF strip. In the CW mode, AVC is not utilized and the RF must be manually adjusted. It is, of course, possible to tune in SSB signals in the CW mode to take advantage of the Crystal Filter and manual RF gain control, if desired. AM signals are switched to the second IF strip that utilizes six tuned circuits and provides switchable bandwidths of 16kc, 8kc and 4kc. The VS position selects the crystal filter to provide another fives steps of selectivity down to 150hz bandwidth. SSB and CW signals are routed to a Heterodyne Detector and BFO that is essentially a product detector. AM signals are routed to a diode detector. Tone control is a two-position switch that was only intended for noise reduction. The noise limiter is also a basic clipper type. The audio is 1 watt output from a single 6AQ5 tube with either 600 Z or 3.2 Z available. The matching loud speaker was specified as the NTS-2 (also specified for other National receivers of the time.)

Today, many vintage ham gear enthusiasts tend to believe that the NC-400 was "over-priced, over-rated and used cheap parts." Another common opinion is that the NC-400 is one of National's worst-looking creations (if you think the NC-400 is ugly, you should see the matching loudspeaker, the NTS-2.) However, the NC-400 can perform quite well if it's in good condition and aligned. It has sensitivity and stability that was competitive with any of the communication receivers of the time. However, sensitivity isn't all that's required for reliable radio communication. National wasn't alone in producing a tuning dial system that couldn't resolve the tuned frequency "to the kilocycle" as Collins equipment could. The NC-400 dial resolution is vague and if you have the optional crystal calibrator you can then find markers on 1000kc or 100kc but most of the scale index markers are in 10kc increments so anything more accurate than 10kc is just guesswork. Some of the parts used for the NC-400 seem inconsistent with the high selling price. Most of the knobs appear to have been "leftovers" from old National televisions with these knobs pushing onto half-shaft controls. The knob nomenclature is so miniscule as to be unreadable except under magnification. Sometimes the S-meter scales will have a "spotty" appearance when illuminated. Out of the cabinet, the chassis seems somewhat flexible and that can't be good for alignment - which has to be performed out of the cabinet. The front panel and cabinet design must have been conceived by an interior decorator since the paint scheme uses three different shades of gray in both smooth and wrinkle along with black winkle finish with chrome trim strips on the sides of the cabinet. Then there are both gray knobs and black knobs along with four small aluminum knobs. National's advertising artwork never flattered the NC-400 and, for some reason, the receiver is just not photogenic. It will always look better "in person" than it does in photographs. If you are an owner who finds the NC-400 is just too homely to look at, you can always listen to the receiver with the "lights out" - just kidding!

If you're planning on rebuilding a NC-400, luckily most of the capacitors are ceramic disks which don't require replacement. There are a handful of plastic molded paper dielectric capacitors. These are the later Sprague capacitors with a black body and yellow nomenclature. These tend to be more reliable than "black beauties" but still, they are paper dielectric and probably should be replaced. The multi-section electrolytic uses four capacitors. Three are power supply filters and one is the cathode bypass for the 6AQ5. There are new-manufacture multi-section capacitors available that have the correct values (made by CE using original Mallory machinery - available from and are an exact fit mechanically. If the 4H4C current regulator is open, it can temporarily be replaced with a 6V6 tube. The 6V6 heater provides the correct voltage drop for the 6BZ7 LO tube and since only the heater is required, the 6V6 doesn't even have to test good, just have a good heater. Although using the 6V6 as a 4H4C substitute will allow the receiver to function you'll probably notice that the receiver drifts more, especially noticeable in CW or SSB. The 6V6 heater can't respond to slight voltage changes as fast as the 4H4C does, thus the noticeable drift with the 6V6 installed. 4H4C regulators are not hard to find and the price is around $15 to $25. Carbon resistors should be checked for value although this isn't usually a problem. Test all tubes on a quality tube checker and replace any that don't check "as new." Do not substitute any other type of tube for the 6BZ7. Lab-quality test equipment is required for a proper alignment after which your NC-400 should be performing its best.

When it comes to the alignment,...well, it's not technically difficult but it is cumbersome and awkward to have to perform the alignment with the receiver on its side. You'll find the upper end trimmers will be under the chassis on one side and lower end inductance adjustments are on top of the chassis on the opposite side. Several of the inductance adjustments are located under the tuning condenser assembly. National did provide access to these adjustments by locating them in the gaps between the condenser sections. When all of the adjustments for RF tracking are completed, then the receiver is returned to its right side-up position, which tends to flex the chassis a bit. You'll generally find that the alignment will have changed slightly with this position manipulation. Operation of the band switch seems to also affect alignment on the upper bands. Repeating the alignment of the LO on the upper bands several times seems to settle-out the change and the alignment will generally stay pretty close after that. Why National didn't provide for all alignment adjustments to be accessed from the top (like their competition was doing) is a mystery, especially since National wanted commercial and military users to purchase the NC-400. Also, there are two alignment errors in the National NC-400 manual. The location of L-11 is not shown anywhere (bottom of the crystal filter) and when performing the 1720kc conversion adjustment the instructions don't tell you to change bands up to the dual conversion range. 

As expected, ownership and actually operating the NC-400 goes a long way toward overall appreciation with its performance at the forefront. The NC-400 can pull in signals quite well, especially after a careful alignment. The audio quality is a pleasant surprise considering it's a single-ended 6AQ5 that's producing the very natural sounding reproduction, however, the Tone switch is not very useful. When you get used to its appearance and its quirks, the NC-400 becomes a fine station receiver.

Although many might consider the NC-400 a "ham receiver" because of National's advertising, I have it listed here in the commercial equipment page because most of these receivers were sold to government and commercial users. The particular NC-400 (SN: 543 0006) shown above was a commercial receiver that was owned by radio-tv station WHAS in Louisville, KY. Many of the large radio and tv stations would have their own repair facilities and technician staff. It's likely that the NC-400 was in that environment at WHAS since it was obviously used very little.


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 mixing 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 (VFO Mixer) and Mixer 2 (Comb filter mixer) 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 carrier levels in the AM mode. The S-meter position provides a relative indication of signal strength but the readings will depend on several factors including antenna type, the setting of the RF Attenuator or the tuning of the preselector (unless it's bypassed.)

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 existing 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 and, even today, Siemens continues to add new products, including medical equipment, to the conglomerate. 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. A Squelch control is also provided but, since it's a carrier-operated circuit, it only functions in the AM mode. 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 SYNC 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:  The E-311-E1 is an unusual receiver that has impressive sensitivity and excellent dial accuracy. "Locked" tuning can be cumbersome due to the 100kc tuning range limitations but one can always "random scan" the entire selected tuning range using "Unlocked" Coarse tuning. I find that "unlocked" is best for SW BC listening since I can cover the entire SW band quickly. SSB reception is very good although the fixed 3.5kc bandwidth might be a problem in a crowded band. I've only experienced SSB QRM that made copy difficult once, so 3.5kc works fine most of the time. CW has bandwidths available down to 150 cycles, so no problems there. 1.5kc bandwidth works best on CW most of the time. AM is actually quite good since the bandwidth "doubles" so the 3.1kc bandwidth is actually 6.2kc. AM signals sound very good. I am using a separate loudspeaker that is 10" diameter (4Z ohms) and the reproduction is better than the panel speaker for most reception. Unfortunately, there's no remote standby provided and using the E-311-E1 as a station receiver requires good antenna input isolation during transmit (like a Dow-Key) and manually reducing the RF gain.


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


Order No. 3472-PHILA-52   Serial No. 29

Barker & Williamson

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 many 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.

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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Rebuilding the R-390A Receiver
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Rebuilding the ART-13 Transmitter
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Rebuilding the Hammarlund SP-600
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           T-368 Military Transmitter                    
Detailed Information on Reworking, Testing and
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Rebuilding and Operating the AN/GRC-19
T-195 XMTR & R-392 RCVR

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Successfully Operating the BC-375 on the Ham Bands Today
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Rebuilding the Collins 51J Series Receivers
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