Thank you all so much for the excellent information.

Last night I had the unit on my bench, multimeter in hand. The first
thing I did was to check the grounds. One on the main board showed
signficantly higher resistance than the others so I re-tightened all of
them and cleaned all the ones on th main PCB. I re-checked resistance
to ground from all those points again and got a consistent 0.3 to 0.4
ohms (not accounting for test cable loss).

I plugged the unit back in and voila, no more "PROTECT" fault. My jaw
was agape! I was thinking I needed to go out and buy a Powerball
ticket. This NEVER happens. I was convinced that one of the output
transistors had failed but lo, twas no more than a poor ground.
Amazing.

I'll go back and put a heat sink onthe upc2581. This receiver runs
fairly warm generally and that is good counsel.

re5513