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COMMERCIAL & MILITARY  COMMUNICATIONS  EQUIPMENT

1932-1941   &   1946-1959

(For WWII gear go to our "WWII Communications Equipment" section)

 Illustration:  From Signal Corps TM11-688 showing the operation of the TRD-4 Direction Finder that utilized three R-390/URR receivers

1932 - 1942

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 here in Virginia City, Nevada. John Mackay initially made his fortune in Comstock silver but he later (1883) moved into telegraphic communications. Mackay, along with J. Gordon Bennett Jr., formed several telegraph communications companies to compete with Jay Gould's Western Union. Postal Telegraph Company (1886) was the best known, along with Commercial Cable Company (1884). Eventually, these companies, along with other Mackay-Bennett telegraph companies, had transoceanic cables across both major oceans. When John Mackay died in 1902, Clarence inherited the businesses. Clarence Mackay saw to the completion of the transpacific cable in 1904. Radio was added to the business end of things in 1925 to provide "radiogram" service to every area of the world. Mackay Radio was mainly interested in maritime communications which went along with the maritime radio-telegraph business. By 1928, ITT had merged with most of Mackay's business interests but the Mackay name continued on for several decades. 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. For awhile ITT tried the consumer radio market with Kolster International but it was a short-lived venture. The name of Federal Telegraph Co. was changed to Federal Telephone and Radio Company around 1940.

The Type 105-A is actually a pre-WWII commercial shipboard receiver that dates from sometime after the Federal Telegraph move to New Jersey since the ID tag lists Newark, N.J. as FTC's location. It is a four tube receiver using five-pin cathode-type tubes. It is possible to use type 27 or type 56 tubes and with an increase in the filament voltage, type 76 tubes could also be used. It is possible that this Type 105-A was updated either at the factory or by a professional radio work shop for the cathode type tubes since there are some indications that the original design may have used direct-heated filament type triodes. The frequency coverage is 1500kc down to 15kc in seven tuning ranges. Power is supplied by batteries. Like earlier designs for shipboard receivers, e.g. the IP-501-A, the Mackay 105-A utilizes an LC Antenna tuner ahead of the regenerative detector to increase gain and selectivity. An Antenna Series Condenser switch selects various value capacitors to match the ship antenna to the receiver input and a stepped Tone control provides some relief from static. The panel meter is a dual meter that normally reads filament voltage but B+ voltage can also be monitored by activating a panel switch. The left large tuning knob tunes the Antenna Condenser, the middle large knob controls the Regeneration Condenser and the right large knob tunes the Detector Condenser. The Mackay 105-A is built for shipboard use being physically stout and very heavy. Originally the receiver was panel mounted in one of the Mackay Marine Radio Units that housed the majority of the radio gear for the ship. (See our "Vintage Longwave Receivers" webpage for an in depth article about this receiver.)

Mackay Radio & Telegraph Co. - Type 102-B Frequency Monitor   Shown in the photo to the right is an accessory piece of test-monitoring equipment for shipboard operation, the Type 102-B Frequency Monitor. This device works something like a 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 operating frequency can be determined as a heterodyne beating with the internal oscillator heard on 'phones plugged into the phone jack. Since most shipboard transmitters in the thirties operated CW or MCW, the operator could also monitor the transmitted signal for other characteristics or suspected problems. 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 the actual received frequency. Frequency coverage is 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.

National RCF-2 ca: 1940

National Co., Inc. - Airway Communication Receivers (Modified NC-100 Receivers)

National Co., Inc. had been supplying receivers for airports since the RHM receiver in 1932. The mid-thirties HRO also was used in some airports. The most popular airport receiver by far was National's Airway Communication Receivers that were based on their NC-100 receiver. The continuing upgrading of communication and navigation equipment was initially the responsibility of the Department of Commerce and the Bureau of Air Commerce with its various branches in charge of airports and navigation. With the developing navigation systems that allowed flying an airplane to an airport via the "beam" and also to allow two-way communications with pilots in addition to weather reports, newer and airway-specific equipment was going to be required. National's history of providing top quality receivers for airport use (RHM, the fore-runner to the AGS, and the HRO) practically assured them of continuing contracts for airport-specific receivers. In 1937, National began supplying the Department of Commerce (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. 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. After WWII, early versions of these receivers were again 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. 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 CAA Airway Communication Receiver is based on the NC-100 chassis, there are some significant additions to the circuit. The most obvious is that the receiver is rack mounted and has a very different front panel when compared to the striking "art deco" panel of the standard NC-100 receiver. The Airway receivers use a 3/16" thick aluminum front panel that is black wrinkle finished. All Airway receivers were equipped with an I.N.S. control. I.N.S stood for "Interchannel Noise Suppressor," which was actually a "squelch" control. Additionally, the push-pull audio was changed to single-ended and the output transformer was internal to the receiver. The audio output impedance was 600 Z ohms and 20K Z ohms. Another addition was a relay that could remotely quite the speaker without affecting headset reception. Most Airway Communication Receivers came with two speakers, a single table top speaker box and a rack mount dual speaker. No carrier level device (meter or eye-tube) was used on the majority of the CAA receivers although it is possible that a few were so equipped. The chassis is covered top and bottom with a "slide on" dust cover. 12 tubes are usually used in the circuit which utilizes a 457kc IF. Some later receivers may be found modified with Non-OEM Crystal Filters or variable coupled IF transformers. Shown in the photo above is a 1940 Airway Communication Receiver Type RCF-2 S/N 13.

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

Radio Corporation of America (RCA) - AR-60

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

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

Perhaps the most famous use of the AR-60 receiver is aboard the USCG Cutter ITASCA in its assistance to Amelia Earhart's ill-fated flight in July 1937. The ITASCA was equipped with two CGR-32-1 receivers. During the late thirties, many USCG Cutters were equipped with CGR-32-1 versions of the AR-60-R that were specifically built for the Coast Guard. In Nov.1939, a contact was issued for approximately 30 CGR-32-1 receivers for U.S. Coast Guard installation on the ten Lake Class Cutters that were being rebuilt and refitted at the time. This was probably the last contract for the CGR-32-1 receivers. Soon after that, the ten Lake Class Cutters were loaned to England as part of Lend-Lease for the duration of WWII. However, according to the USCG website much of the sensitive equipment was removed prior to delivery of the Cutters. It's likely that the CGR-32-1 receivers were used elsewhere during WWII, either in other USCG facilities or other military uses.   photo left: Radio Room on the USCG Cutter TANEY ca: 1938 showing the two CGR-32-1 (or CGR-32-2) receivers. USCGC TANEY was a Treasury Class Cutter with a spacious radio room when compared to the cramped quarters of a Lake Class Cutter's radio room.                            photo from: http://oldcoasty.info

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 until a tuning chart is made or the dial readings learned for specific frequencies. It is awkward that the 80 meter band is split with the section below 3.8mc tuned on range 2 and the section above 3.8mc tuned on range 3, however, the AR-60/CGR-32-1 was really never designed as a ham receiver. 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, go to our web-article "RCA's Legendary AR-60 Receiver." Link below in the Navigation Index.
 

RCA AR-88D ca.1944

Radio Corporation of America  -  AR-88 Series

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

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

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.

1946 - 1959

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. The purple (when illuminated) dial provides a unique visual experience when tuning in the various signals. 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.)

1950 Collins 51J-2

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, commercial or individual/enthusiast use providing accurate frequency readout and great stability. The receiver utilized a permeability tuned, double conversion circuit using the 70E-7 PTO in a dual, tuned-IF system and a multiple frequency crystal oscillator to cover .5mc to 30.5mc in thirty (1mc wide) bands. Dial accuracy is maintained at 1.0kc throughout the tuning ranges by limiting the coverage of each band to 1.0mc. Band 1 actually is triple conversion but only to allow coverage of the AM BC band. Bands 2 and 3 are single conversion while all of the remaining bands are double conversion. 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 Carrier Level meter that was actuated by a toggle switch next to the meter and the 100kc Crystal Calibrator nomenclature was changed from "100 KC OSCILLATOR" to "CALIBRATE." Some later production 51J-2 receivers may be found with the 70E-15 PTO installed but whether this was a post-sale retrofit or a Collins engineering-production upgrade is unknown at this time.

In 1952, the 51J-3 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 at the request of the Signal Corps, who wanted to use the 51J receiver for their teletype installations but found the fixed 300 ohm Z antenna input requirements difficult to work with as most of their installations used either dipoles or vertical whip antennas. The military had their version of the 51J-3 designated as the R-388/URR and it was built from 1952 through about 1955. With the R-388 and 51J-3, grab handles were added to the front panel along with a high quality Burlington Co. sealed meter. 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. Also, 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.

Reworking the early 51J Receivers- If you are working on a 51J-2 receiver be aware that the 1950 version of the 51J-2 manual is fraught with errors in every section, almost on every page. Errors abound especially in the receiver alignment section, circuit description section, the component designations and component identification layouts. One should consider that most of the earlier 51J-1 and 51J-2 receivers have had thousands of hours put on them by former commercial/military users. These receivers were sometimes roughly treated and are usually well-worn, needing much more than the usual re-cap and alignment to function at their design level of performance. The early 51J-1 & 2 gear boxes have brass drive gears where the later R-388/51J3,4 versions have steel drive gears. Sometimes the wear on early gearboxes is so severe that binding and jamming are experienced when changing the Megacycle position. The IF transformers often seem to sustain internal damage with rough handling of the receiver requiring disassembly and re-gluing of the coils and ferrite shields back into their proper position for correct operation. The R-388 and 51J-4 receivers seem to be in much better overall condition and usually don't require extensive rebuilding. Additionally, the manuals are accurate in their descriptions and procedures. There are some component quality issues that affect the AVC circuit performance in some receivers. NOTE on later 51J Receivers: On R-388 receivers, the PTO end-points on nearly every R-388 PTO are excessive and beyond the range of the adjustment capacitor. The end-point issues on R-388 receivers (and to some extent on 51J-3 and 51J-4 receivers) are related to the poor quality of the ferrite slug material used in most of the 70E-15 PTOs. To increase the range of the adjustment capacitor requires compensation coil trimming to correct. You will also find that a separate voltage source must be provided to remotely switch the receiver to stand-by on all R-388, 51J-3 and 51J-4 versions. The earlier 51J-1 and 51J-2 receivers have a conventional remote stand-by circuit that parallels the front panel "STAND BY" and "ON" functions.

Performance - As to the 51J/R-388 performance overall, most of the military 51J/R-388 receivers were used for data reception such as RTTY where the severe audio limitations made sense. For AM BC reception and SW BC reception the receivers might be disappointing to some ears because of their limited audio reproduction of 200 to 2500Hz. Stock 51J receivers do sound a bit "muffled" in AM reception but changing the value of the capacitor that is across the audio output transformer primary from .01uf to .001uf will increase the audio highs somewhat. There are also issues with the AVC circuit which never seems to function very well however the components used the AVC circuit are chronic problems that always need to be replaced for proper operation. Product Detector mods are most common and involve changing the BFO from a 6BA6 to a 6BE6 mixer tube to accomplish the circuit modification. There are mods galore to the 51J/R-388 AVC and BFO circuits however the usual modification caveats that should be carefully considered before hacking up the receiver. If a new owner/user is aware of the 51J/R-388 intended use and will learn how to operate the receiver correctly, mods are not necessary.

Dial accuracy and stability - both necessary for RTTY work - were the 51J selling points to the Signal Corps and still are the 51J's primary attributes today. Although the stock audio is somewhat restricted, the receivers are great performers providing they have been thoroughly rebuilt and fully aligned. Additionally, the 51J/R-388 receivers are the best looking of the Collins receivers with a beautiful layout of the two dial system that allows separate megacycle and kilocycle readouts direct along with the added benefit of general coverage reception from .5mc to 30.5mc.

1953 Hammarlund SP-600-25C

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

Introduced in 1950 and selling for nearly $1000 at that time, the SP-600 was intended for the military and commercial user market. It was a very popular receiver and many thousands were built, especially for military applications. Though most versions were built throughout the 1950s, the SP-600 continued to be produced in smaller numbers up into the early 1970s. The standard SP-600 tunes from .54 to 54MC in six bands. A "J" suffix indicates JAN parts were used in the construction and an "X" suffix indicates a switchable crystal 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. 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 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 switchable 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 user could switch to any of the crystal LO frequencies for increased stability for that particular frequency - however, 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 impressive 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 great 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.

It is interesting to note that Hammarlund did not refer to the SP-600 as a "Super-Pro" receiver in their manuals and only occasionally is its "Super-Pro" lineage mentioned in Hammarlund advertising. The inset photo shows the SP-600 JX-21 version from 1953. The JX-21 was generally used by the USAF. This is the early version of the JX-21 - so it does not have the product detector SSB changes. The header photo shows the SP-600-25C built for the US Army Signal Corps and it is housed in an original SP-600 cabinet. The SP-600-25C version has the large square 25-60 cycle power transformer and does not have the switchable crystal oscillator ("X" option.) SP-600 cabinet colors vary from dark charcoal to light gray depending on when they were manufactured. A rebuilt SP-600 is a pleasure to operate with incredibly smooth tuning, competitive sensitivity, a great selectivity set-up featuring six positions that include three crystal filter positions and, of course, its massive controls and domination of the ham station landscape.

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

Shortly after Hammarlund began building the SP-600 for various military contracts, the military began accepting alternative designs for high-performance receivers that could replace the SP-600, if necessary. Hallicrafters is the only company that was successful in submitting an alternative design that was accepted by the military. The Hallicrafters receiver was produced with the R-274 designation. Interestingly, Hammarlund also built SP-600 receivers with designations of R-274. With the contracts that followed, Hammarlund was assigned the suffixes of A, B and C to specifically identify their versions. With their second contract, Hallicrafters was assigned R-274D, adding the D suffix to specifically identify their version. The early versions of both Hammarlund and Hallicrafters receivers will be found with just the R-274 designation. Apparently, only two contracts of the Hallicrafters receivers were produced for the military. The typical military contract production quantity was for around 5000 receivers and it doesn't seem likely that Hallicrafters would have gone through the effort for a smaller contract. It's also possible that Hallicrafters thought there would be more contracts in the future but that doesn't seem to be the case. 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 that essentially described the "basic" SP-600. The most obvious similarity is the turret band switch which, while functionally the same at the SP-600's, is not nearly so robust in construction using fiber board modules while the SP-600's are made of ceramic. The tuning dial provides a main dial and a logging dial as the SP-600 does but behind a single escutcheon rather than separate dials behind two escutcheons. There is a selectable six channel Crystal Oscillator that functions like the SP-600 "X" option and provides improved stability for RTTY and other data modes. Like the SP-600, the bandwidth is selectable in six selectivity steps with three of those steps using a Crystal Filter for narrow bandwidth (a front panel Phasing control is also provided.) A 600 ohm balanced audio output is also similar to the SP-600 audio output.

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

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

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

Collins Radio Co.  -  R-390/URR & R-390A/URR

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.

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.

1961 contract manufactured by Capehart with OD front panel

R-390A  - End-User Front Panel Repaints From time to time, R-390A receivers will show up with the front panel sporting a non-standard paint color. The original specification only calls for the panel to be painted gray. Consequently, many shades of gray are found from the many different contractors that built R-390As over the years. Sometimes though, completely non-standard colors will found and these are always "end-user" applied paint jobs. The most commonly known "end-user" paint job is the USAF's flat-black panels (see photo below.) Several years ago, a 1961 Capehart contract R-390A with an olive drab painted front was found in the Northern Nevada area. It had been supposedly repainted by the USMC. Here is another 1961 Capehart with the OD painted front panel, also found here in Northern Nevada, in 2009. Also, North Korea has made the U.S.S. Pueblo into a museum. The ship had many R-390 and R-390A receivers onboard. There is a color photo on the Internet that shows some of the receivers from the U.S.S Pueblo and two of the R-390A receivers appear to have OD panels. All R-390A receivers left the contractor's facility with gray panels - it was the specification - but the "end-users" were liable to repaint during a rebuild so anything might be possible - just not original. This is probably the best known photograph of R-390A receivers sporting end-user front panel paint jobs. The installation shows banks of R-390A with dark panels, presumably "flat black" paint. Note that many (not all) of the receivers have severe wear on the Kilocycle Change and Megacycle Change knobs indicating continuous operation of the sets. Most of the racks have Teletype machines associated with them. This photo was of USAF Morse Hall at Clark Air Force Base in the Philippines.         photo from: sites.google.com/site/cobradensite/usafss-history-by-ron

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. 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.

1963 Western Electric R-392/URR

Collins Radio Co. - R-392/URR (GRC-19)

Around 1950, the U.S. Army Signal Corps needed a small but high-performance receiver that could be paired with a similar design transmitter with all of the equipment mounted for portable operation on a Jeep or similar vehicle. 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. Since the 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 vehicles, the entire system including the receiver had to be "water 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. Additionally, many times the military was going to have to "drop" communications gear from the air, so the R-392 had to be "ruggedized" to be able to survive this type of deployment for portable field use.

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 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 connecting cable between the T-195 transmitter and the R-392 receiver using the PI-TC connector that allows the two units to function together.

OPERATION - Though some collectors find the R-392 a bit "too military" in appearance, its performance certainly wins over many prospective users. A separate +25vdc to +28vdc power supply will be required and it should be capable of at least 3 or 4 amps. The typical +24vdc computer-type power supply with the voltage adjusted up to +26.5vdc will work fine.  BE SURE TO USE AT LEAST +25VDC - - - +26.5VDC IS BETTER!  Receiver performance will begin to drop off as the supply voltage is reduced below +25vdc and the R-392 will barely function below +24vdc. The GRC-19 system was designed to operate with the vehicle running or with some sort of charging system used with a 24vdc battery set-up. The typical battery-charging system voltage would have been around +28vdc (although this depends on the engine RPM and the condition of the batteries.) The GRC-19 will only operate marginally at +24vdc since the T-195 is spec'd at +28.5vdc input. When operating the R-392 as a "stand alone receiver" the operating voltage is applied directly to the receiver rather than through the transmitter resulting in higher actual voltage at the receiver than when operated as the GRC-19. Although the apparent gain of the receiver can be increased by running the supply voltage at +28vdc, most of the tubes utilized in the R-392 are 26.5 volt filaments and a properly operating R-392 will function great at +26.5vdc supply voltage. This assumes that only one power supply is going to be used and LINE and PLATE are connected together. Increased performance is possible by operating the LINE at +26.5vdc and operating the PLATE at a slightly higher voltage, up to maybe +30vdc. This requires two power supplies and separate voltage wires in the power cable going to the PI-TC connector.   IMPORTANT NOTE: When testing the 26.5 volt tubes, be suspicious of readings using a typical mutual-conductance tube tester, e.g., the TV-7, etc. Some tubes will show very little gain, perhaps as much as 60% lower than minimum specified useable test level, and yet these tubes will function fine in the R-392. This is probably due to the tube tester's method of powering the tube versus the R-392 circuit's application of DC voltage on the heaters with +28vdc plate voltage. The best indicator of the tube's usability is by substitution in an operative receiver. Naturally, tubes that read high on a tube tester are going to work best but don't necessarily discard the 26.5 volt tubes just because they show "bad" in a tube tester. Try them in the receiver, you might be surprised.

VARIATIONS - 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. Ample sensitivity, super-accurate frequency readout and decent audio (from a good speaker - not the LS-166) not to mention the "extreme" military looks along with a small and lightweight package (well,...52 lbs) have made the R-392 a popular receiver with many military collectors and even some BA collectors.

Collins Radio Co. - T-195 (GRC-19)

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. The T-195 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 performance was much better with this type of antenna. The transmitter is basically modular in construction and features autotune capabilities on 7 channels. Inside the T-195 are three motors, two of which operate blowers to provide forced air cooling for the three 4X150 external anode tubes used in the PA and Modulator. The remaining motor is the autotune motor. On early T-195s, two dynamotors provide +1000vdc with one dynamotor and dual lower voltage B+ from a dual output dynamotor. +28vdc at 40amps is required to run the T-195 and this was generally provided by vehicular battery-charger systems. Though the power input is marked "+24vdc" the T-195 doesn't run very well at that low of input voltage and prefers at least +28vdc (and the data plate is marked "+28.5vdc @ 40A.")  Late in the military's use of the T-195 (early Vietnam War era) there was a retrofit to convert many of the T-195 transmitters to use solid-state power supplies to replace the two dynamotors. This reduced the current required and slightly reduced operational noise. Still later, these SS power supplies were installed from the factory and these models were designed as the T-195A or T-195B. Modular construction is used throughout the T-195 to ease in-field repairs. Still, the transmitter is a complex unit that, while robustly built, can prove difficult to troubleshoot if the technician is inexperienced. Weight can also be an issue. At 125 lbs, the T-195 is a challenge to move around. If you add the R-392 and the dual shock mount, making the combo a GRC-19, the weight is up to around 200 lbs! Additionally, noise from the blowers will assure you that the aural experience of "on the air" operation is equivalent to vacuuming with an industrial shop vac. The GRC-19 was designed to be used in an open vehicle and generally outside where the operational noise wasn't much of a problem. Inside the shack the din will at least let everyone else in the house know that you're "on the air."

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.