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Burst encoders
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R-350M   Orel - Орел
Russian spy radio set (Eagle) Mark II

The R-350M was a Russian spy radio set that was developed in the former USSR around 1957. It was the successor to the earlier R-350 and was developed at the KGB Research Institue, probably in Kuchino (near Moscow) [1]. It was not only used by the USSR, but also by the other countries of the Warsaw Pact, such as East-Germany, which is why it was produced in rather large quantities.
Like its predecessor, the R-350, the R-350M was available with a Russian or English front panel. The latter was used for foreign 'activities'. The image on the right shows a typical R-350M with Russian front panel, ready for use.

Whilst the R-350 contained large valves (tubes) that could be accessed from the front panel, the R-350M is completely built around miniature valves with wire-terminals. These can no longer be swapped easily in the field. Furthermore, the rectangular transmitter filter (coil) is replaced by a cylindrical one. In total, 11 coils are supplied, one of which is installed in the transmitter.

The R-350M uses the same burst encoder and burst transmitter as the R-350. When unused, these are stored inside the top lid of the case, together with the spare transmitter coils, a film container, the antenna, the headphones, an overhead light and some spares. The burst transmitter is attached to the right of the case.
R-350M ready for use

The R-350M is a completely self-contained transceiver, with built-in battery and power supply unit (PSU). The latter produces all the voltages necessary to drive the transmitter and receiver valves. The battery compartment has a built-in heater, to allow the battery to be used at extremely low temperatures. The heater it powered by the battery itself. The radio can also be driven from an external 6V power source, which is connected to a socket at the left side.

In the early 1960s, the R-350/R-350M was replaced by the improved R-354 (Schmel). It was smaller and had a motor-driven burst transmitter. Furthermore it only used half the amount of film compared to the R-350. Like the R-350 it was powered by an internal 6V battery.
R-350M case R-350M (Eagle) with its lid open R-350M ready for use R-350M ready for use R-350M with tuning charts Tuning charts Accessories inside the top lid

Audio samples
Below are some audio samples of the R-350M, recorded by karsten Hansky in Germany [5] in June 2014. The radio was connected to a dummy load and an ELAD FDM-S1 was used to receive and record the signal. Further sound processing was done with Audacity.
The R-350M is wired in such a way, that the transmitter, receiver, battery and power supply can easily be removed. Once the bolts at the edges of each block are removed, the module can be lifted out. The modules are connected together by means of sockets that are present inside the pre-wired case. The image below gives a good view at the sockets.
The radio consists of the following functional blocks:
  1. Transmitter (TX)
  2. Receiver (RX)
  3. Power Supply (PSU)
  4. Battery
  5. Burst transmitter
Empty case, pre-wired for the modules (blocks)

R-350M case R-350M case R-350M case Empty case, pre-wired for the modules (blocks) Transmitter socket Empty case, pre-wired for the modules (blocks) PSU inside the case

The transmitter can be adjusted to any frequency between 1.8 and 12MHz, divided over 11 ranges that are selected by means of plug-in coils. The circuit is built around five 1SH29B (1Ж29Б) and three 1P24B (1П24Б) valves [4]. The latter are connected in parallel in order to produce an output power of 6W, which is suitable for a maximum range of 1000 km [3].
For each range and frequency, the antenna must be tuned for maximum performance. A small hand-written booklet with suitable tuning charts is mounted on top of the power supply unit.

The image on the right shows the transmitter once it is removed from the case. At the top left is the removable filter (coil). The connection for the morse keyer is at the bottom right.

The transmitter is capable of sending a 70Hz 'homing signal' that is used by the home office to automatically start a disk-recorder.
Transmitter (TX)

The transmitter is capable of sending data (CW) at 100 to 150 groups per minute (each group is 5 digits). At the top right, just below the antenna tuning lamp, is a black push button that is used to completely shut down the radio when the top lid of the case is closed. The buttons at the bottom are used to select the mode of operation (send, receive, homing, etc.).
Transmitter (TX) Transmitter (TX) Transmitter interior Transmitter interior Transmitter interior Miniature valves in the transmitter Miniature valves in the transmitter Transmitter coil socket

The receiver is a superheterodine design with an intermediate frequency of 750kHz [4], built around five 1SH29B (1Ж29Б) miniature valves.

It can be adjusted to any frequency between 1.8 and 7MHz, divided over two ranges (1.8-2.52 and 3.52-7MHz). The range is selected with two push-buttons at the top left.
Receiver (RX)

Receiver (RX) Receiver (RX) Receiver interior Receiver interior Miniature valves in the receiver Miniature valves in the receiver Filters inside the receiver Receiver connections

Power Supply
The power supply unit (PSU) is completely transistorized, which is quite unique for its age (late 1950s). The aluminium case is designed in such a way that it acts as a heatsink for the 10 power transistors that are mounted on one side. The image below shows the side with the transistors.
When powered by a 6V source, the transistor-based inverter produces 50V, 170V and 270V which are necessary to feed the valve-based transmitter and receiver.

The PSU fits inside a 'slot' at the rear right and connects to the rest of the radio by means of a connector at the bottom that mates with a socket of the pre-wired case. The PSU can be powered by the internal battery, or by an external 6V source connected at the left.
Power transistors in the PSU

Power Supply Unit (PSU) PSU bottom view Power transistors in the PSU PSU inside the case R-350M case Power connector (closed) Power connector

The R-350M can be powered by a built-in 6V battery, which is located at the rear left. The battery itself is housed inside an isolated box. In order to allow the radio to be used at extremely low temperatures, the battery box has a built-in heater that is powered from the same battery.

At the bottom of the battery box is a large socket that connects it to the rest of the radio via the pre-wired case.

The white plastic plate on the top lid of the box, is a notepad that can be used, for example, to write down call signs and frequencies.
Battery box

Battery box Battery box Battery box open Battery box open Battery layout inside the lid of the battery box Battery box interior Battery box connection for lamp

In the image below, most (but not all) accessories are present. At the top left are the 10 removable coils used for selecting the frequency range of the transmitter. At the bottom right is the burst encoder (puncher) which allows a (coded) message to be recorded onto photo film. To the left of the puncher is a circular container with a fresh supply of unused film. Above the container is the morse key with built-in hand-operated burst encoder.
The following accessories are usually stored inside the top lid:   
Accessories inside the top lid

At the far left is a small metal cabinet in which spare light bulbs and fuses are stored. The wire antenna is not shown here. It is normally stored at the left, taking up all space from top to bottom. The headphones are not shown either. They are usually stored in between the container and the puncher.
Message puncher
Each message is first translated into a series of numbers. This is usually done with some kind of cipher system, such as a simple matrix or the unbreakable One Time Pad (OTP). The numerical message is then stored on a a standard 35 mm photo film by punching a series of holes in it, using the device shown here.

 More information
Click for more information about the burst encoder

Burst transmitter
The R-350M is supplied with an external morse key with integrated burst transmitter. Photo film with the punched numerical message can be fed into a narrow slot at the front of the morse key.

A small crank is then used to feed the film through the transmitter at a constant speed between 100 and 150 groups per minute.

 More information
Click for more information about the burst encoder

Almost any type of headset can be used with the R-350M. In most cases, a common USSR military headset was supplied, with rubber ear pads and elastic head bands. Such headsets are commonly used with military radio sets in tanks etc. Headsets are connected to the two-pin socket on the left of the front panel of the radio.   

Help required
We are still looking for the circuit diagram, the complete user manual and the technical manual of the R-350M. If you have any of these, or if you have additional information about this radio, please constact us.
  1. Radioscanner website, R-350 'Eagle'
    Brief description of R-350 (Russian). 2006. Retrieved March 2012.

  2. Ivan Petrov, Radio Communications Facilities in Service with Military Intelligence
    Major General (Retired), Military Parade JSC website, 1998.

  3. Günter Fietsch, Nachrichtentechnik der Nationalen Volksarmee
    Part 2. With Heinz Kösling. 1996. German. ISBN 3-88180-340-8. pp. 278-285.

  4. Louis Meulstee, Wireless for the Warrior, volume 4
    September 2004. ISBN 0952063-36-0.

  5. Karsten Hansky, Sound samples of R-350M transmitted signals
    Germany, June 2014.

Further information

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Crypto Museum. Created: Wednesday 06 June 2012. Last changed: Sunday, 02 October 2016 - 08:35 CET.
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