Look for the "big" ones called electrolytic capacitors
. You should find them spread across the board, but probably a few very big ones at the point where the power supply enters the board.
The comic situation is that I am pretty much familiar with electronics but I have never seen something like this. So consider my question as a dilemma posted by a very competent person.
If it is the power it would affect the amplifier section too but it doesn't so I don't think it is in the power supply. This receiver has the beauty of separate pre-amp & amp sections which allows you to hook something (equalizer) in between. I thoroughly checked the schematics and the signal can bypass all the other boards with proper selection of the switches which leaves only the tone amp circuit as a suspect (and eventually the main potentiometers which would be horrible if true). The amplifier part when separated works perfect.
Unfortunately I do not have nor I can find oscilloscope, and as I mentioned all the components look fine (and believe me I have disassembled the unit and checked everything visually). Unless somebody had similar situation I have to go part by part which means a lot of money and time.
It might be a potentiometer as somebody suggested, burned transistor (which I suspect) or dead capacitor (none of the electrolytes looks suspicious
I will try to change the transistors (if I find them) then I will see ...