Spy radio
Burst encoders
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Spy radio transmitter

P-57 is a small valve-based transmitter, developed in 1957 in the former Soviet Union as part of the RION spy radio set, alongside the PR-56 receiver. The transmitter covers a frequency range from 2.5 to 10 MHz, both VFO and crystal operated, with an output power of 10W (CW). The radio set was intended for international espionage and clandestine operations by the Soviet Union.
It has the same form factor as the PR-56 receiver and does not seem to have a model number of its own. The serial number starts with P570xxx, which suggests that is was purpose built for the P-57 RION set and that, unlike the receiver, it was not used as part of another spy radio set.

The transmitter's frequency range is divided over four bands, identified with the colours white, yellow, red and green. The frequency can be determined by a crystal operating in the 2nd or 3rd overtone, or by the Variable Frequency Oscillator (VFO), selectable by a slide-switch.
P-57 transmitter

A suitable wire antenna should be connected to the antenna socket, to the left of the frequency scale. At the top left is an antenna output that should be looped to the receiver. When enabling the transmitter, an internal relay switches the antenna from the receiver to the transmitter. A suitable counterpoise should be connected to the ground socket at the left centre. The antenna tuner at the top left should be used to tune for optimum performance, using the table on the top lid of the accessory box. A morse key is connected to the 2-pin socket at the front centre.
P-57 transmitter Control panel Power indicator Crystal socket and VFO/crystal selector Band selector Power socket, power switch and filament voltage control Antenna matcher Checking the HT voltage

All controls and connections are located at the top surface of the transmitter. At the bottom left is a 4-pin socket for connection of the LT and HT voltages. Immediately to the right of this socket are the ON/OFF switch, the adjustment knob for the filament voltage and a socket for the morse key. At the centre of the control panel is a large frequency scale that can be operated at its left. A fine tuning knob is available at the bottom right of the scale. A 4-position switch allows selection of the desired frequency band, each of which has its own colour that corresponds with the scale.

The meter at the top right can be used to check the filament voltage (adjustable with the back known at the bottom edge), the HT voltage (240V) or the anode current (which is an indication of the output power). When transmitting, the large black double rotary knob at the top left can be used to tune for maximum power, using the tuning lamp and the current meter as indicators.
The interior of the P-57 transmitter can be accessed by loosening the large screws at the sides, after which the case shell can be removed. This reveals the interior as shown in the image below. All parts and subassemblies are mounted to a subframe that is fitted to the radio's front panel.
The image on the right shows the interior after the case shell has been removed, seen from the top right. The circuit has three identical Russian 4P1L (4П1Л) valves at its heart, one of which is used as the oscillator. The other two are used in parallel as the transmitter's Power Amplifier (PA).

The most complex part of the transmitter is the Variable Frequency Oscillator (VFO) that is used when crystals are not available. It allows the transmission frequency to be adjusted freely over each of its bands. The VFO and the band selector are housed in a shielded enclosure.
P-57 transmitter interior

The image on the right shows the interior of the transmitter as seen from the bottom left, after the protective VFO shield has been removed.

Note that even when a crystal is used to select the transmission frequency, the band selector should be set to the required frequency range and the tuning dial should be adjusted close to the actual frequency in order to obtain the best result. This is necessary because it affects the tuned circuit and, hence, the optimum coupling, between the oscillator and the Power Amplifier.
Interior (metal shield removed)

The brown oval block contains the antenna matching unit, which is in fact a tuned circuit between the PA and the antenna, that consists of a multi-tap inductor and an adjustable capacitor. This arrangement allows a wide variety of antenna impedances to be matched. Note that the antenna matching unit has to knobs: the outer ring, which selects a tap on the inductor, and the inner part which adjusts the capacitor. When tuning, a small 2.5V/75mA glow lamp is used as an indicator.
P-57 transmitter interior Interior Interior (metal shield removed) Transmitter interior Interior Top view Bottom view Antenna relay

Frequency bands
  • 2.5 - 3.5 MHz (white)
  • 3.5 - 5 MHz (yellow)
  • 5 - 7 MHz (red)
  • 7 - 10 MHz (green)
  • Frequency
    2.5 - 10 MHz 1
  • 4
  • Output
    10 W
  • Power
    LT: +2.1V, HT: +240V
  1. Note that the frequency range of the receiver is slightly wider: 2 - 12 MHz.

Power connector
  1. LT (-4.5V)
  2. not connected
  3. LT (0V)
  4. HT (270V)
Valves   Tubes

  1. RION Maintenance Manual
    A.08 TO. 1957. Serial number 120. 66 pages.

  2. P-57, Transmitter Circuit Diagram
    A145. 1956. Retrieved October 2009 from [1].

  3. P-57, Transmitter Component List
    A145. 1956. Retrieved October 2009 from [1].

  4. P-57, Transmitter, table of valves (tubes)
    1957. Retrieved October 2009 from [1].

  1. scAvenger, Technical description of the RION spy set
    Website with many photographs. Riga, Latvia. 21 January 2005. Retrieved October 2009. 1

  2. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN 0952063-36-0, September 2004. 2

  3. Radio Scanner, Radio Station 'RION'
    Website (Russian). Retrieved May 2016.

  4. KGB Bunker Museum, Lithuania
    Personal correspondence, February 2017.

  1. Website no longer available in 2016.

Further information

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Crypto Museum. Created: Thursday 23 February 2017. Last changed: Tuesday, 28 February 2017 - 18:25 CET.
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