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Mason A3B
Portable intercept receiver

The A3B was a portable intercept receiver, designed by Mason Engineering Inc. (USA) as the successor to the A2. Design of the A3 started in 1968, but it was not released until 1971. Although the A3 was an improved version of the A2, the A2 remained in production until 1990, and was sold alongside the A3 and later models, such as the MPR-1.
 
Design of the A3 is similar to the A2, but with a number of significant improvements. The rather (vulnerable) antennas are no longer mounted directly to the tuning units, and the oscilloscope is much more compact and user-friendly. Furthermore, the cableing is organized better.

The oscilloscope is mounted vertically inside the lid of the case, with the display facing upwards. It is held in place by four velcro dots. A small hinged mirror is present at the top, to allow the display to be read easily by the user (see image).

The separate tuning sections are no longer supplied as separate plug-in units, but are mounted together in a single case. Short patch cables are used to connect a tuning unit to the receiver. The receiver itself is housed in a separate metal case, seated behind the tuning units. The oscilloscope is a separate unit.
  
The complete A3B unit with the mirror up

All units are packed together in a nice compact leather case, allowing the A3 to be carried around without attracting attention. A separate 10 GHz (MWT-1) module was available as an option. When present, it would be mounted with four velcro dots in the bottom right corner of the lid of the leather case. This option is not present with the A3B shown here, but the outlines are clearly visible as cut-outs in the green foam. The additional unit covered all frequencies between 2 and 10 GHz in two ranges (2-5 GHz and 5-10 GHz).

The A3B was very popular with the US intelligence agencies and also with the agencies of various European countries. The unit shown here has been used for many years by the United States Information Agency (USIA). A similar unit, with the optional 10 GHz extension present, was revealed as a public exhibit by the US Department of State, Bureau of Diplomatic Security's, Countermeasures Directorate at an exhibition in 2008 [1] (see the rightmost image below).
 
The closed suitcase containing the A3B The complete A3B unit with the mirror up The complete A3B unit with the mirror down All three units packed away in the foam of the suitcase The 4 velcro dots in the top lid marking the position of the oscilloscope A selection of (probe) antennas used for various frequencies Close-up of the tuning units. Note the transparent disc for easier control. Close-up of the oscilloscope display and the mirror An A3B unit with the optional 10GHz unit (mounted inside the lid)

 
References
  1. Scientific American, Spying on the Spies
    State Department shows off Cold War-era electronic evesdropping gadgets.
    Larry Greenemeier, 22 July 2008.

  2. Mason Engineering Inc., Radio Receiver with Spectrum Display Model A-3B
    User manual and technical description of the Mason A3B receiver. 1971.
    12MB PDF file.

Further information

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