In your specific case I'm not sure what the problem may be, ( since there are a few different causes that will give you the same symptom ) but about 5~6 months ago I repaired an SA-HT720 (almost the same design as ht-650's) that had a couple of leaky transistors that I think are on the DC detection circuit, (I don't remember exactly) of the amplifier section. I don't remember location numbers but I think Both were 2sC3199 There are a few of them on that board So, with your voltmeter you should be able to find them fast. Disconnect the suspicious ones from the circuit if necessary to make sure you found them. E- mail me if you have any question good luck!