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Racal MA-4224
Voice Encryption Unit

The MA-4224 is a fairly sophisticated voice encryption unit, designed by Racal Datacom Ltd. in the UK from 1978 onwards. It is a stand-alone unit that can be integrated with standard military radio sets (Racal and others) by inserting it in the audio line.

The image on the right shows a typical MA-4224 unit. It can accomodate up to 3 Traffic Encryption Keys (TEK) that are selected with a rotary switch at the front panel. Keys are entered by means of a fill gun or directly on the keyboard. The latter can be cumbersome as the unit doesn't have a display to verify the input.

The MA-4224 can also be used as a phone encryption unit. For that purpose it would be fitted in a standard briefcase, together with an acoustic coupler and a power supply unit (PSU). A hinge allowed the front panel to be tilted.
  
Front panel of the MA-4224

As becomes clear from the description below, the MA-4224 is built from conventional (discrete) components, spread over 3 rather large PCBs. The MA-4224 was eventually replaced by the much smaller MA-4225, which is effectively a miniaturized version of the MA-4224.

MA-4224 unit Front panel of the MA-4224 Front panel of the MA-4224 Close-up of the audio connectors Close-up of the connector for the transceiver (TCVR) and the key selector. Selecting a key Entering data on the keyboard Rear of the MA-4224 unit

Interior
Looking inside the MA-4224 reveals a rather nice piece of conventional electronics. From the manufacturing codes on the chips it appears that the unit was designed in the late 1980s and manufactured in 1990. The interior consists of three PCBs that are nicely stacked together.

The three board are connected by means of a series of blue-ribbon flatcables. The image on the right shows the digital board that is at the bottom of the PCB stack. Surprisingly, it doesn't contain a microprocessor, which means it is completely driven by discrete CMOS logic.

At the top right of the board is a molded backup battery, that is used to retain the cryptographic keys that are stored in the static memory chip on one of the other boards. At the other side is the analog board that contains the RAM chip and in the middle is the logic board.
  
Digital board of the MA-4224

At the left side of the digital board is a row of small sockets with tiny little plugs that can be used to change the configuration of the unit. At present the effect of the plugs in unknown.

The MA-4224 is very service-friendly. By removing the right screws from the PCBs, the complete unit can be unfolded. The PCB are held together by two clever hinges, as can be seen in the image on the right. The rightmost board in the picture is the logic board that is normally folded in between the other two boards.

When unfolded like this, the front panel can be re-attached and the unit can be operated normally. All components are on the upper side of the boards. After all these years one terminal of the battery (on the digital board) is corroded.
  
The 3 PCBs unfolded

Digital board of the MA-4224 Analog board of the MA-4224 Logic board Configuration plugs Changing the configuration The 3 PCBs unfolded Close-up of the corroded battery terminal The static memory chip that is used to store the cryptographic keys

References
  1. Racal, PRM-4031 Workshop Manual
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Crypto Museum. Last changed: Tuesday, 07 October 2014 - 12:59 CET.
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