I have repaired a different model Sony doing the same thing. The thing I love about alot of Sony units is, they use individual transistors for the "finals", or last stage of ampllification (as opposed to steak-packs, one big hybrid chip). I havent seen inside your model yet, as I havent opened my 740P yet, but here is one thing I would check for if you havent already. With a ohmmeter or digital multimeter with diode check, look for shorts across any of the high-power transistors (these are usually mounted on the aluminum heat sink in pairs, (1) positive channel and (1) negative channel). The ones in mine had a number like SK brand MN2488 ( the negative) and MP1620 (the positive) which were both shorted. (try combinations of all 3 pins) There were several pairs of these, and unfortunately required me removing an upper set to get access to the lower set, soldered to the main board. I cant remember the exact resistance values for the "good ones", but the "bad ones" were @0 ohms, regardless of the probe polarity. I desoldered them and retested them out-of-circuit to verify the short. I found replacements through MCM (www.mcminone.com)for less than $20 total. This included new fuses as well, AFTER the transistors were replaced. Hope that helps!!!!!! Elliott