blown fuses, sony STR 635

I bought a Sony STR 635 at a garage sale, where it was playing just fine.

I hooked it up to my speakers at home, and noticed that the left side didn't sound right. Took the cover off the speaker and found the woofer was dead. (Seemed to happen overnight!)
Anyway, the receiver shut down and flashed "protection".
I unplugged everything and when I tried it again with different speakers, there was no sound at all, from anywhere.
I opened it up and found the F901 and F903 8A fuses blown. Replaced both fuses, and they both blow immediately upon power up. No speakers or anything else connected.
Did I toast this thing?
-Zippy

 

Possibly. If the blown woofer has a shorted voice coil it would present too high a load to the amp, same as if you shorted the speaker wires together. The amp went into protection, but the output transistor(s) may have already been damaged -- which means they're shorted to ground and popping fuses.

Pull the output transistors and power it up. If it still blows fuses the fault also took out something in the power supply.

There's a reason why this happened in the first place, though. If you're like me you got it home, connected the speakers and immediately cranked it all the way up to see what it could do.

Were the speakers rated too low (impedance) for the amp? Were the speakers' power ratings less than half of the amp's output power? Either or both of those conditions are common causes of failure of amps & speakers.