Tuner works but nothing on any input function speakers or phones. Cold
testing what is accessible of the amp seems ok.
I suspect uPC1237 protect area, probably drift causing false protect,
but not removed the top board to probe that area yet.
Good SM is out there.
Any known drift component in those amps over 30 years?
Plenty of yellow peril, now dark brown, milk-glue but tuner working ,
where marginal conductivity would show up.