Yamaha R-700 STK3056

[argonrepublic]

I have this old receiver. It plays sometimes then doesn't. It was playing for a bit then it crackled a bit. I wacked the internals a couple of times with a rubber screwdriver handle. It seemed like I knocked out the sound. I have shifted everything around and it doesn't seem to make a difference.
When the protection realy engages by itself, it sounds good. If I close it manually, the right side is crackly unless I turn in up loud. It seems to clear up. With protection relay manually engaged, at low volume I hear faint hum. Would that be capacitors or STK3056?
Thanks

[Mooly]

That's typical symptoms of a tarnished relay contact when the proplem clears when the volume is turned up. I'd clean those up first before looking for other issues.

[argonrepublic]

I will clean those. Thanks.
This is really strange. I resoldered all of the pins for the STK3056 just in case but the issue is still related to movement of the STK3056. The heatsink does not appear to be grounded or electrically connected anywhere yet when I apply pressure to the heatsink toward the STK3056, the hum switches off and within seconds the relay engages itself. Hot spots of the backside? STK3056 physically separated? Appears to be plenty of compound. I was thinking of cleaning it, applying more compound and reinforcing it with a strap to clamp it against the heatsink.

[Mooly]

I suppose its possible that the STK has an internal fault thats affected by movement but it would be a real one off. It won't be hot spots.

Sometime the best approach can be to resolder all the joints on the PCB... everything... it doesn't take long.

The real answer is to use a scope and see whats going on checking rails, grounds inputs and outputs etc.