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Mk. 122
Clandestine transceiver

The Mk.122 was a compact valve-based self-contained transceiver, for use by Special Forces, Stay Behind operations, Agents and the Diplomatic Wireless Service (DWS). It was developed in the UK by HMGCC (now CGHQ) in the early 1950s as the successor to the earlier Mk.121.
 
The earlier Mk.121 was considered not very practicle, as there were 5 different versions; one for each frequency range. The Mk.122 however, combines all frequencies between 2.5 and 20MHz in a single device, divided over 3 ranges:
  • 2.5-5MHz
  • 5-10MHz
  • 10-20MHz
The transmitter is crystal-operated. Frequency range for transmitter and receiver can be set independently, allowing cross-band operation.
  
Mk.122 spy radio set

The physical shape and design of the set is similar to the Mk.121, but many improvements have been implemented. Apart from the selectable frequency ranges mentioned above, the Mk.122 has an improved morse key and a variable antenna tap on the tank coil of the RF output stage.
 
Mk.122 with top lid Mk.122 spy radio set Mk.122 spy radio set Morse key mounted on the front right Antenna matcher Receiver tuning scale Receiver tuning dial Power connector

 
Technical description
The Mk.122 was a very compact transceiver; slightly larger than an A4 sheet of paper. The radio can be divided into three sections: a power supply, with a convential transformer at the center of the unit, a freely adjustable receiver on the left, and a crystal-operated transmitter on the right.

Power Supply
The Mk.122 can be powered directly from any mains AC voltage between 100 and 250V in 10 steps. It consumes 20W in standby and 34W when the receiver is used. When transmitting (with the key down) it consumes 65W. The radio was often supplied with a external Vibrator Pack (No.14), to allow it to be powered by a 6V battery. In that case, the maximum current would be 3A (standby), 5A (receive) or 10A (transmit). The battery was recharged with the Mk.812A Charging Unit. Alternatively, the radio could be powered by the Mk.810A hand/pedal genarator that supplied 110V (45-80W).

Receiver
The receiver is freely adjustable between 2.5 and 20MHz, divided over 3 ranges as shown above. The sensitivity is depending on the frequency and lies between 1 and 5 µV (at 20dB S/N). The Audio output stage produces approx. 20µV, which is enough to feed a high-impedant (50K) crystal earphone. The receiver consists of three valve-based circuits:
 
  1. Mixer/oscillator (CV3888 or ECH42)
  2. IF/detector (CV3883 or EAF42)
  3. Audio/BFO (CV3888 or ECH42, CV1833 or OB2)
Transmitter
The transmitter, that covers the same three frequency ranges as the receiver, is crystal operated and is only suitable for CW (Morse). The crystal should be placed in a socket along the front of the transceiver, to the left of the morse key. It produces an output power of 10-13W and consists of only two circuits:
 
  1. Crystal Oscillator/Doubler (CV3990 or 2E26)
  2. RF Power Amplifier (CV3889 or EL41, NE48)
Crystals can be used in three modes:
 
  1. Fundamental frequency
  2. Double frequency (using the doubler)
  3. Third harmonic
Accessories
  • Main lead
  • Battery lead
  • Mains voltage tester (with neon light)
  • Crystal earphones
  • Reel antenna
  • Wire antenna (36m, with insulators)
  • Crystal adapter
  • Tools (screwdriver, soldering iron, knife, paper, pencil, etc.)
  • Spares (vibrator, fuses, lamp and valves)
  • Mk.810A Hand/pedal generator (optional)
  • Mk.812A Charging Unit (optional)
References
  1. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN 0952063-36-0, September 2004.

Further information

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