Bryston 4b loud buzz

I bought this amp on eBay about 2 months ago partly because it had significant service which included replacing 10 transistors. Everything was fine and we were just watching TV when out of nowhere a pronounced loud buzz came out of the right channel. At the time the volume was pretty low and because of my tinnitus I can't turn it up much anyway. I did the input and speaker wire swap to confirm that it was the right channel and the fuses are good. It had operated in this configuration with the other components for some time. I disconnected it and now just run everything through my HT unit and all is well. So what's wrong with this?

Thanx