Yamaha TSR-7810 receiver power drift

[ponteley]

Apparently same as RX-V681/RX-V781/RX-A760/RX-A860

Everything working fine when plugged in and turned on.
Output voltages on all speaker terminals are few mV.
As time goes by (10-20 minutes), FR chanel goes to 10mv>100mv>200mv>300mv and up to 3V and then PROTECTION kicks in.

Very strange issue. Can't find anything blown/overheating/shorted/open.
All diodes and transistors checking out OK. Resistors too.

Can't find service manual or schematic....

Any help is greatly appreciated.

 

[Harald Kapp]

Can't find service manual or schematic....

I can.

 

[ponteley]

I can.

Well.... Yeah....
You know what I meant.....
manualslib, elektrotanya, ...etc

 

[PETERDECO]

If it takes that long for the voltage to start rising and you don't feel anything getting hot, get a can of "freeze" spray and spray the semiconductors one by one. You found the problem IC or transistor when the problem immediately goes away. Wait a moment between freezing each part.

 

[Ylli]

Start with the diff pair on the bad channel.

 

[PETERDECO]

Yes. Back in the 80's, that was a common problem with (I forget the brand) a receiver I used to repair. The part was always the same dual transistor at the input. Being direct coupled all the way to the output, the outputs would drift and activate the protection circuit.