That is 3 pairs of speaker outputs, CD,tuner and tapes ok. Left unused for a few years. Will check for amp noise on the speaker outlets in the next hour. At the moment I'm thinking muting via corroded erroneous phones bypass switch , impossible to see /probe without taking all apart. Any other ideas to look at. Looks like 2 stereo SIP hybrid amp packages, unlikely both failed, no approprieate fuses seen yet.
Now found a pair of relays that had no sound of click-over, but monitorable for signal and control, assuming a minimum number of board interconnects is enough to test out. Managed to get to the phones socket and the isolated C/O sw is fine (or I disturbed it), but at least I now know which lines from there go to the relay board and the output lines from the 2x STK4142II .
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