I found two surplus ACS 2 voice scramblers, with one handset damaged. After repairing the connection I opened both units to see how the audio, controls and synchronization were arranged.
Construction
Each handset contains an audio board, a larger logic board, controls, a microphone, an earpiece and the wiring between them. The boards use through hole parts and socketed integrated circuits. This makes the unit practical to inspect and repair.
The two handsets exchange an audible startup sequence before the scrambled audio path is ready. The video records that exchange. It is useful for timing and signal study, but the sound alone does not reveal the scrambling method or its security.
Tracing the signal path
- Record the model, board markings and connector wiring.
- Check supply rails and audio paths with the unit disconnected from any radio.
- Feed a known audio tone into one handset.
- Record the signal before and after each analog stage.
- Compare the two units to separate normal design features from faults.
With the covers off I could start tracing the microphone input, analog processing, synchronization and earpiece output. A tone source, oscilloscope and audio recorder should be enough to follow the signal through the boards, although I would not trust an old scrambler like this for secure communication.
Testing should use a cable, audio load or permitted laboratory radio link. Do not connect to a service or frequency without authorization.
Having two units open side by side is especially useful. Repeated sections and matching wire paths help separate the microphone and earpiece audio from the control and synchronization circuits.
Photographs
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