Sony stereo amp repair?

[crafty717]

I have a Sony home theatre receiver model STR-DE575 ( 5x100w) that is about 8 years old. the front two channels are hooked up to a pair of Bose speakers and when the tv is watched we are in two channel mode. Over the years the the sound has been slowly degrading and now it is to the point where if we turn the volume up to max it is loud but nowhere near what it used to be. We used to have the volume level at 25 on the display and now for normal use we are at about 40. If I switch to surround the center channel will blow you away. I hooked each of the speakers separately to the center channel and what a difference. I put back the way it was. So it looks like there must be an amplifier stage or two blown somewhere for the two channels. Was wondering what the easiest way to check this is. I am an Electrical Engineer ( Not in electronics though) so ripping into it a bit shouldn't be a problem. Just need to know where to start looking. Could be as easy as the output stage transistors or something else? Any thoughts, I'll start at the speaker terminals and work my way back. One other note also about the same time the volume started decreasing the fm radio started to not work. When you put the radio on there is a constant buzzing sound through the speakers but no radio. Maybe this is related.

 

[Sceadwian]

Sounds like degrading electrolyte from bypass and DC isolation capacitors.
Basically, if there's a capacitor in there in the power stage bigger than your thumb, replace it, at least on the channels you use, might as well do em all while you're at it.