Cleaning the contacts did seem to work, I wouldn't trust them much but for the time being it works.
Problem now is the sound amplifier IC STK402-100S, the one I found locally was the STK402-100 (without the S) and for any one searching in the future, those two models are not the same and not compatible, the unit did not produce sound with the model without the S, at least the power protect message went away.
Now since I'm ordering some ICs from China, might as well through in some relays. That relay worked for no less than 15-18 years before failing, I consider that good enough.
It's a Panasonic ALA2F24, I also found an Omron G5PA-28 which seems to be compatible with it.
I'm now sure that intermittent problem was the relay like cheeseit suggested, when the unit disconnected one of the speakers and I used some percussive maintenance it sometimes came back to work, which suggested either a cold solder joint or something mechanical that is the problem. And since there is no cold solder joints as far as I can see, it must be this relay, also there was a good amount of black sot underneath the unsoldered relay which is not normal dust.