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A look at unique WW2 Japanese sets | Model 94-6 | Type 53C | Type 97 | Special Receiver

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Bernard Nock G4BXD introduces some unusual Japanese sets that have found their way to the Museum.

Japanese Miniatures Model 94-6 Type 53CType 97Special Receiver

A big hello from the Military Wireless Museum here in Worcestershire. Most of the time has been spent checking over various sets, making the odd repair or restoration and building those little units I never had the time to do before. Though, I did manage to secure a few new items. The postman and delivery companies have trodden a steady trail to the museum’s door. In addition to reviewing some old timers from the shelves there is a strange but exciting additions. 

Japanese Miniatures

I have been very fortunate over the years to acquire quite a few Japanese WW2 sets. The first I ever got came from the shed of an amateur just a few hundred yards away. Not long after arriving at this location I was talking to local amateur on 80m and I mentioned my interest in old military sets. “Ah” he said, “I have some old bits in my shed”. Anyway, it tuned out there were a couple of WW2 Japanese sets there, which he kindly gave me, starting an interest in that particular country’s equipment. 

The Japanese had several wartime miniature, battery powered sets and I’ll mention a few in the collection. These sets were used by the standard foot soldier, sometimes carried by individuals or supplied along with bigger sets and used as monitoring devices.

Model 94-6

This amazing little transmitter receiver, although you can call it a transceiver as all the parts are common to both transmit and receive, uses just a single valve, a double-triode type, yet receives and transmits AM or CW.

Fig. 1: The 94-6 receivers

A very small VHF set, Model 94 Mk6, manpack, single valve rx/tx. This, Fig. 1, is my current collection of 94-6 sets, three of the usually seen three-band sets and top right the rarer single-band set, an early production version. The data on my sets: the single-band set, no s/n but 196 stamped in base, meter is dated 1937 and transformer is dated 1937. The three-band sets: s/n 4626, dated 1939, s/n 10081, dated 1941 and s/n 102554, dated 1943.

The double triode acts as, regenerative detector and audio amplifier on receive and on transmit, as a self-oscillator power amplifier (PA) and modulator for AM or, strapped together, as a self-oscillating PA with slightly more RF output. Ingenious. 

Type 53C

Type 53C receiver, Fig. 2, in its transit box, with plug-in coil packs and spare tubes. 0.4 to 5.754MHz using five plug in-coils. CW, MCW and voice reception possible. It uses four miniature tubes, weight given as 11lb, including batteries. Made by the Tokyo Instrument Co, a very compact regenerative receiver. It was issued with the Type 94 No.3 station and used as a monitoring receiver, termed sub-receiver. It is 1-V-2 autodyne method using four valves, a UY14M and three UY11M types. 

Fig. 2: The 53C receiver

Type 97

This is a portable radio, Fig.3, for the Imperial Japanese Navy Land Force. The basic facts from the Yokohama Military Radio Communication Museum’s website (below). It has the same function as the Army Type 94 No.6 radio, but there is some consideration for waterproofing. Communication distance: 1km. Frequency: 23,000-31,000kHz. Radio format: A2 (modulated telegraph), A3 (telephone). Transmission output: 0.5W. Equipment overview Vacuum tube used: 1 31MC. Transmitter: Self-oscillation, anode modulation. Receiver: Super regenerative detection, low frequency one stage. Power supply: Batteries or hand-held generator. Antenna: L-shaped, vertical 140cm, horizontal 65cm.  

Fig. 3: The 97 Type receiver

Special Receiver

Now we come to a very interesting receiver, one where very little information is available and one which is thought to have been provided to agents or spies. I was asked via email if I had any information on this unknown set. I made a few enquiries but discovered very little about it. I relayed this to the owner mentioning that if he was interested in selling, I would be interested in buying. 

Luckily for me he was and I did. Once again, I consulted my friend Mr Doi at the Yokohama Museum to see what info they had. Mr Doi states: “Mukinanu 610 type receiver, for intelligence personnel? The structure of this receiver is similar to the regenerative receiver called ‘1568 type receiver’ said to be a special piece of equipment made by the Army for intelligence personnel”.

Fig. 4: The Special set in its box

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The wooden storage case, Fig. 4, for this receiver, and the front of it was labeled ‘Mukinanu No. 610’. This unit is a regenerative detection (autodyne) type receiver consisting of three direct-heat type pentodes manufactured by Shinagawa Electric, and the operating frequency is four bands covering 3-16MHz. 

I have never seen this one before, so it is not clear about its model number and purpose. However, according to the attached paper, this receiver uses a loop antenna. And made up with three B-03 type small MT tubes. This tube is considered to be similar to 1U5 and 1T4.

Fig. 5: The special receiver

From the above, this receiver, Fig. 5, can be considered as a regenerative portable direction finder (SW receiver in use as well) that receives 3-16MHz. However, its structure is special and cannot be considered a normal army field radio. A similar set, with number 1568, was detailed on the website of the late American collector William L Howard. That one was a regenerative SW receiver (0-V-2) consisting of three B-03 type vacuum tubes. 

The configurations of both units are similar, but unlike the 1568 type, the Mukinanu type receiver has a transformer coupling method that takes into consideration the improvement in gain in the low frequency amplification stage. Compared to the 1568 type, the Mukinanu type receiver has a sophisticated structure and a high degree of perfection, so it can be considered as an improved version of the 1568 type receiver. 

The Valve Museum (URL below) has data on the 1U5. “The 1U5 is a diode-pentode designed for detection and first audio voltage amplification in lightweight portable radio equipment and the sensitivity of the pentode allows for a low anode voltage thus saving weight on the HT battery. Above the top mica is the diode anode and the filament tension spring is hidden by the gettering. The filament is a single-coated inverted-V. The diode anode connection passes inside the main anode cylinder. The pentode section with three wire grids. The thin glass tube envelope is 18mm in diameter, and excluding the B7G base pins is 47mm tall. Type 1U5 was first introduced in 1945.”

The Radio Museum website has info on the maker: “Shinagawa Denki K.K. Office and Plant: 1-429 Gotanda, Shinagawa-Ku, Tokyo. Tube brand: Kotron/K.O. Tron. Small manufacturer listed on page 2 of Hisashi Ohtsuka’s ‘Dawn of Tube Production in Japan’, section 007L, with a founding date of 1941. The company has existed previously however: there is an article ‘On the New Tubes’, by Hisao Maeda of Shinagawa Denki, published in 1936 in the Journal of the Institute of Electrical Engineers of Japan. Founded: 1931. History: Founded as Horikawa Industrial Co in 1931. Company renamed to KO Vacuum Tube Mfg. Co., later to Shinagawa Electric Co. Ltd. In 1942, the tube brand Tou was introduced.

Mr Doi added: “The developer and application of the Mukinanu type and 1568 type receivers are unknown. However, the 1568 receiver is reportedly manufactured by the Army for intelligence personnel. If this is the case, the developer is Army Technology Headquarters 9th Research Institute (former Army Science Research Institute Todo Branch Office) Department 1 (radio weapons, balloon bombs, radios, bacterial weapons, cattle epidemics virus (Research and development), but unfortunately the museum does not have the materials to support it.

The receiver, Fig. 6, is very well made, the main tuning has a very smooth slow-motion tuning control. It’s difficult to trace the wiring as it’s all the same brown-covered type of wire, but I assume that we have the first valve as a tuning detector stage followed by two stages of audio amplification, though this has yet to be confirmed. 

There is a five-position bandswitch but only four bands on the chart. Again, due to the compact nature of the internals it’s hard to figure out if the fifth position is unused. The variable resistor seems to be in the heater line because I noticed when testing the set that the heater current varies with this control’s rotation.

On top of the set is a five-pin socket and there is a little sub-unit housing a switch and what I thought at first was a small lamp, which has a five-pin plug and lead, going to the switch and then to flying leads to the batteries. On closer inspection it seems the small bulb is actually in the HT positive supply to the set so must be there to act as a fuse. I could not see where the headset plugged into; there are no other sockets save for the antenna ones on the set. 

I then noticed that on top of the five-pin plug coming from the switch box there was a two-pin socket into which the headset’s two-pin plug could be inserted. I fitted three 1L34 types, a 1.2V heater pentode, into each socket in turn at first to ensure the valve bases were OK and applied about 60V of HT to the set. At first there was no output in the headset. I measured what should be the anode and screen 2 of the valves. Valves 1 and 2 had HT to both, the third valve had HT only to g2. There was no HT on the output anode. 

I was unable to see just where the anode load was. The output capacitor can be seen but the load was either an open resistor or it might have been one of the two wound components, chokes or transformers, but hard to see. I fitted a suitable anode load resistor from HT to pin 2 and there was plenty of audio out when g1 was touched. Unfortunately, there was no received signal or a sign of reaction or regeneration so obviously there are other faults. 

When in use the receiver plugged into a frame or loop antenna, Fig. 7, which is housed around the inside of the carrying box. There is space in the box for the two batteries, 1.2 or 1.5V and probably something like 45V or 67V types. A small blue label in the lid details the rest of the boxes contents, spare valves (6), 3Ω variable resistor (1), 10µf capacitor (1), low frequency transformer (1), blocking coil (1), bulb  (2), headset(1), connecting leads.

I shall persevere with this little set in the hope of getting it fully functioning again but it was certainly nice see such a clever design and an example of engineering as compact as this set. Now if we could only find out how it ended up in an attic in New England. 
 

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