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Mk. 122
Clandestine transceiver

Mk.122 was a compact valve-based transceiver, developed in the early 1950s by HMGCC (now CGHQ) at Hanslope Park (UK). The device was the successor to the Mk.121, and was used by Special Forces (SF), agents, stay-behind organisations and the Diplomatic Wireless Service (DWS).

The device is housed in a metal wrinkle-finished enclosure that measures 33 x 23 x 8.5 cm and weights 5.6 kg. It has a removable protective lid at the top, and can be powered directly from the AC mains (100-250V in 10V steps), or by an external 6V vehicle battery with a vibrator pack.

The device has several improvements over its predecessor, the Mk.121, including an improved internal morse key – mounted at the front right. Furthermore, it combines the full 2.5 to 20 MHz frequency range in a single device, whereas five Mk.121 variants were needed to do the same.
  
Mk.122 spy radio set

In addition, the Mk.122 as a a variable antenna tap on the tank coil of the RF output stage. The transmitter is crystal-operated, whereas the receiver can be tuned freely by means of its Variable Frequency Oscillator (VFO). For reception of CW signals (morse), the built-in Beat Frequency Oscillator (BFO) can be enabled separately. The device was succeeded in 1956 by the Mk.123.

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
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Mk.122 with top lid
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Mk.122 spy radio set
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Mk.122 spy radio set
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Morse key mounted on the front right
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Antenna matcher
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Receiver tuning scale
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Receiver tuning dial
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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)
Specifications
  • Organisation
    HMGCC (now: GCHQ)
  • Users
    Special forces (SAS, SBS), agents, stay-behind, DWS
  • Power
    Mains 100 - 250V in 10V steps, 40 - 400 Hz AC
  • Battery
    6V accumulator and external vibrator pack No. 14, 6V
  • Consumption
    Mains: 20W (standby), 34W (RX), 65W (TX)
    Battery: 3A (standby), 5A (RX), 10A (TX)
  • Dimensions
    330 × 230 × 85 mm
  • Weight
    5.6 kg
Receiver
  • Circuits
    Mixer/oscillator, IF/Detector, AF/BFO
  • Modulation
    AM R/T, CW
  • Bands
    3 (see below)
  • Sensitivity
    1 - 5 µV (at 20dB S/N)
  • Selectivity
    5.5 kHz at -6dB, 13 kHz at -20dB
  • IF
    470 kHz
  • Output
    20 µW into 50 kΩ earphones
  • Valves
    2 × CV3888 (ECH42), CV3883 (EAF42), CV1833 (OB2)
Transmitter
  • Circuits
    Crystal oscillator/doubler, RF power amplifier
  • Modulation
    CW
  • Bands
    3 (see below)
  • Output
    10 - 13 W
  • Valves
    CV3889 (EL41), CV3990 (2E26), NE48
Bands
  • 2.5 - 5 MHz
  • 5 - 10 MHz
  • 10 - 20 MHz
References
  1. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN 0952063-36-0, September 2004.
Further information
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Crypto Museum. Created: Wednesday 05 August 2015. Last changed: Saturday, 17 October 2020 - 08:21 CET.
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