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Racal MA-4224
Mobile voice scrambler

MA-4224 was a mobile frequency- and time-domain (F/T) voice scrambler, also known as a two-dimensional secure speech device, introduced in 1978 by Racal COMSEC in Salisbury (Wiltshire, UK). The device is interoperable with the portable MA-4225 and the MA-4400 scrambler phone.

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
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MA-4224 unit
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Front panel of the MA-4224
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Front panel of the MA-4224
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Close-up of the audio connectors
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Close-up of the connector for the transceiver (TCVR) and the key selector.
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Selecting a key
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Entering data on the keyboard
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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
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Digital board of the MA-4224
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Analog board of the MA-4224
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Logic board
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Configuration plugs
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Changing the configuration
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The 3 PCBs unfolded
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Close-up of the corroded battery terminal
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The static memory chip that is used to store the cryptographic keys

Documentation
  1. Audio Encryption Unit MA 4224, Technical Manual
    Racal-Comsec Ltd. WOH-8333, Issue 9, July 1986.
Further information
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Crypto Museum. Created: Thursday 26 August 2010. Last changed: Monday, 02 March 2020 - 21:16 CET.
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