Sony MHC 901av phones output but no speakers
 

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.

Sony MHC 901av phones output but no speakers
 

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 .