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(c) 1999,2017 Peter McCollum

Mystery Radios

The equipment presented in this section has the "look and feel" of clandestine radio gear, but only very limited information is available at this time.


Set #1

[Additional info on this set can be found in "Wireless for the Warrior, Vol 4", by Meulstee and Staritz.]

This set was found in London, and has no markings other than a U.S. inspection stamp. The transmitter uses three 3S4 tubes (one osc., two power amps in parallel). The receiver tubes are five of the 1T4, 1U5, etc., series of battery tubes.

It was described in an Electronic Industries magazine article in October 1944, as a briefcase transceiver. The set was developed by Crosley Corp. under a "development order" by the U.S. Signal Corps. The specifications were almost entirely related to physical size and frequency coverage.

The set appears to include an alternate packaging of the SCR-295, pictured in “Signal Corps Radio Sets, Front Views, Receivers and Transmitters”, dated January 1943. Shown are versions by Crosley and General Electric, with dates of December 1941. The SCR-295 appears to be a body-worn receiver only (no transmitter).


A front view, with the receiver stacked on top of the transmitter. Photo courtesy of Robin Greenwood.

The Crosley version of the SCR-295. This is presumed to be a receiver and battery box only (no transmitter).


A view showing the insides of the low, flat cases. Photo courtesy of Robin Greenwood.


Set #2 - CMS

This Navy set has certain features that are consistent with a 'clandestine radio', such as the wood suitcase-style case. However, the technology is outdated (given late WWII), and some features are inappropriate to clandestine use, such as the simple regenerative receiver design, and the AM-voice transmitter capability. Use by 'Coast Watchers' is one possibility that has been suggested. Another suggestion is that it is the result of a contract that pre-dates the war, and for some reason its introduction was delayed.

The CMS tunes 3.1 to 13.5 MC. The xmtr can operate with either 6V6 or 6L6 tubes.

More specific historical info is needed.

Info from Keith Melton, via an e-mail from Gary Sharp:

"I researched this set about 30 years ago. It is not a clandestine radio, but rather a special purpose set intended to provide for communication from a member of the crew on shore back to the ship. The radio, circa WW2, is sometimes referenced during an amphibious invasion where it was sometimes necessary to coordinate the waves of material and personnel landing on the beach head with the vessels off shore. The USN did not operate agents unilaterally to require clandestine radios."

Comments/rebuttal from Hue Miller:

The Navy ran clandestine operations in the Philippines, as far as supplying the guerrilla operations, and was involved in China before the Army with the SACO organization. I take the CMS to be an expedient radio intended for the Philippines – perhaps its use was supplanted by the Australian products ATR-4 and 3BZ. The Navy already had plenty of capable shore party radios in the TBX series. Why use a comparatively crude and hard-to-operate CW set for this purpose?” Hue points out that the tube sockets are wired so it can use a variety of 6 and 1.5 volt tubes, both pentodes and triodes, but operates them all in triode mode. This implies that flexibility and simplicity (minimum parts count) were a design goal, although the User Instructions does not mention the possibility of using battery-filament tubes in the transmitter.

Here is the CMS Operating Instructions, courtesy of Dennis Monticelli.

Set #3

The designation of this set is unknown, and no other examples have been seen to date. Perhaps it is a manufacturing prototype, from a company bidding on a contract?

Additional info from Bill Howard:

This set came in a canvas bag, similar to a mechanics tool bag. Covers fold over each other and had two snaps to hold it closed. It had a carry sling for over the shoulder carry. In side were 9 compartments. Two small compartments held the receivers, and smaller pockets held the accessories. There was a small compartment which looked to be made for a manual or message pad. A large center compartment held the transmitter. Two smaller compartments held accessories. One had a wooden box, assume there was a box for the other compartment. Set had a transmitter and two receivers but no transmitter power supply. Assume it was lost along the way. Think there was space in the bag for a power supply. Two more compartments were there and it appeared that one could have held the power supply and the other held the key. The larger compartment had an antenna of copper wire that we did not think was original.

The cases reminded me of the SSTR-5 set and they were metal, of the same shape and design. there the similarity ended. The transmitter has a dove tail U shaped bracket on one end into which one of the receivers fits. The cases were painted black. No effort was made to treat the metal so the paint was flaking off.

The receivers are about 4 inches by 4 inches by 3 inches. The receiver is mounted on a metal panel which is held in place by 6 screws. Once the panel is removed the battery compartment is accessible The batteries are BA-231/U for filaments, and 2 45 volt B batteries made by NIHON SEKISO with brand name "Flattery". Batteries are hard to change, require a screwdriver, removal of the set from the case. I do not think these sets were made for extended use. Probably one time use only (short term mission??). Receivers are 5 tube sets, tubes are the submini as used in PRC-6 and PRC-10 radios. In sockets, and covered with a tar like substance. They can not be removed in the field. Front panel controls are the headphone socket (1/8" plug), the On/Off switch, a Ground connection, an antenna connection, a crystal socket and a BFO tuning control. Set is tuned by a crystal to a pre-set frequency. Underside the BFO is a small capacitor, the crystal socket a "module" made of a phenolic material, shaped like an H with the tube sockets mounted three on one side, two on the other. A tar like substance was dripped on the tubes to hold them in place. Resistors and capacitors were mounted on the ends of the "H". The capacitors were the more modern type, like the orange drop capacitors. No schematics were available so I can not comment on the circuit. There were no IF transformers.

The transmitter was about twice as long as the receiver. The transmitter was mounted on the front panel, again held in place by six screws. Across the top were the power socket, 4 pin connections; a similar socket for the key to be plugged in. Think these are called amphenol connectors. The next row down had the connection for the receiver antenna, the crystal socket, the Ground connection, a tuning lamp, and the Antenna connection. Bottom row had a meter, a rotary control, connected to a small variable capacitor and marked 1 to 10; a band switch marked 5-10 and 10-20, an antenna loading control and a transmit/receive switch. Transmitter components on the under side revealed one 6AG7 tube, held in place by a clamp. The transmitter had a tank coil and antenna loading coil wound on a fibreboard(?) form. Had a small capacitor about 0-300 pf (1 inch long) older US style resistors and modern capacitors. The antenna and ground connectors appear to have been made for the WWII EE-8 field telephone. They were smaller than the standard line terminals. The antenna loading coil had multiple taps connected to the rotary switch which was very small. Definite post war production. There was a small trimmer capacitor wired to the tube socket. This was not user adjustable. There was one four-winding RF choke. Visible in the photograph are two capacitors and 7 resistors ranging from 1/4 watt to a 1 watt resistor, of WWII vintage. The accessories were the post war earphones, 2 sets, indicating that both receivers were expected to be in use at the same time, several crystal socket extenders; 2 jump cables for connecting receiver antenna to transmitter; some wire which I suspect was the "as issued" antenna and a hank of string, probably for getting the antenna hung in a tree. A spare 6AG7 tube was included. Battery for receiver filament was dated 1953. My impression was that this was a prototype set and made in limited quantities.




Set #4

This set is Japanese, and multiple examples are known to exist. It uses components that appear to be the same as WWII Japanese military parts; although the tubes are U.S. types. The transmitter tubes are a 6J5 and a 6L6.

My observations from studying these and other photos:

Receiver: The schematic says "RSK-253 SW Receiver". At the top of the schematic it says "Type A". Audio output impedance is 10K ohms. Tuning knob is a wartime Japanese version of a National-type vernier dial. Dial is marked 0-100 (not frequency), but there is a graphical tuning chart included so that the operator can guess the operating frequency. This scheme was used on many WWII Japanese receivers. Ground post is marked "E" ('Earth'). Tuning cap is branded "Cosmos" in Roman letters, while the IF cans are the same brand, but in Japanese katakana characters. Chassis says MFP treatment applied in 1953. Coil L7 has another Japanese brand logo - this brand made many crystals during the war.

Transmitter: The schematic says "SM-1[?] SW Transmitter". At the top of the schematic it says "Type A". The design is very similar to the RT-3, but without the neon tuning indicators for the oscillator and final tanks. Bill Howard has examined the key, and says it is clearly a wartime Japanese military key; the same type as used with the type 94-3 A and C transmitters (or the same as a type 94-5 key, but with a different connector). Some of the caps visible in the photos are marked "Mica Condenser", and made by Sanko Denki. Tubes are U.S. made (GE and Ken-Rad brands). Transmitter includes a tuning chart (filled out by hand), which indicates the Plate Tuning and Antenna Tuning settings for a few sample frequencies and for antenna lengths of 18, 24, and 31 feet. The chart has 10 frequencies, but only 3 have been filled in.

Power supply: The meter indicates input voltage, and has a red mark at 100V. The label says "Input Voltage, set to 100V". Note that 100V is standard AC power in Japan. The voltage-selector knob is marked for -20, -10, 0, 10, and 20 volts; relative to 100V. The output connectors are very similar to those on the RS-1 power supplies. All of the major components are Japanese brands. The transformer was made by "Taiyo Denki" (lit. means 'Sun Electric'), and is dated Showa 27.7 using the Japanese calendar (this date is July 1952). One of the selenium stacks has the same date. The transformer is rated for 50/60 Hz only.


Additional info from Bill Howard, who has studied the radio in person:

The Ammo can set transmitter made use of a 6J5 as the oscillator. Crystal connected between grid and plate [Pierce]. Plate output of 6L6 to tank coil/tuning capacitors has a coil L2 of 25mh between antenna and ground, (assume this is tank coil), L1 is a 2 mH coil with a 10 ohm resistor across it in the plate lead from the 6L6 tube to the antenna line. L3 seems to be a 2 mh coil, (I can not find it in the picture) in the antenna line between the two variable capacitors, and a 6 volt 100 ma lamp in the antenna lead. L4 is the open air coil which is connected across the 6 volt lamp. Power input is 6.3 volts and 400 volts. Capacitors are bolted to the front panel and have a metal panel bolted to their rears. On this panel are two angle brackets mounting the tube sockets so tubes are horizontal mounted. Saves space. The resistors look like WWII Japanese mfg [and the mica caps are marked with Japanese brand names]. This is high quality construction and is not a lab prototype set as is the bag radio set [Set #5].

The receiver is the same quality construction as is the power supply. The receiver is a 6 tube set with the following tube line up: 1T4 (three), 1R5, 1S5, 1L4. L4 and L5 are 455 IF transformers. L6 is a 455 IF transformer in the BFO circuit. L7 looks like an IF transformer but seems to be an output transformer. L1, L2 are open air coils wound on coil forms about 1/2 inch diameter, L3/L4 seem to be together, again wound on open air coil forms, same size as L1, L2. All the major components are labeled with a part number from the schematic, R1, L4, etc., and tubes are marked on the chassis. The main tuning capacitor is a three gang capacitor, mounted on top of chassis, and there are three trimmer capacitors mounted under the chassis, but I assume connected to each gang. There is a small capacitor mounted on the front panel which seem to correspond to the control marked BFO. Next to it is R1, a potentiometer which is labeled Volume control. The main tuning capacitor is driven by a vernier which was typical of late war Japanese sets (same as used on the WWII 1568 set). Main tuning capacitor has patent number 363480 stamped on the rear. There is a minimal amount of wire used. It is mostly red wire and blue wire. Resistor and capacitor ends have yellow spaghetti covering to provide insulation. Front panel, underside has the phone jack (standard 1/4 inch), the power cord and the on/off switch (toggle switch). Power requirements were 1.5 volts and 90 volts.

Both receiver and transmitter have spaces in the ammo can case for storage of headphones, antennas, key etc. The power supply has a large transformer, two large electrolytic capacitors, two large selenium rectifiers, two smaller selenium rectifiers and a small OB2 tube. Front panel controls are a voltage selector switch (also used as On/Off switch), an input volt meter, two fuses, 100 volt and 200 volt, the tip of the OB2 tube and the female sockets for power connection to the receiver and transmitter. The wire is post WW II wire with plastic/rubber(?) insulation and looks to be about 12 gauge wire.




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