Audio Systems: Onkyo A-SV420 gives PROTECTION message, bose acoustimass, output transistors

Question
Hi, I purchased this Onkyo A-SV420 Audio Video Control Amplifier along with Bose Acoustimass Series II (two satellite and one sub woofer passive) back in 1998 July. Now it's giving PROTECTION error message and no out put. One thing I noted was that right after switching on the amplifier at times a light hum comes out and at times a rustling sound. Another thing noted was that a relay normal made sound after switching which is quiet now.
As I don't have any Onkyo service Center in my country (Pakistan), I did contacted the Onkyo support on e-mail. Who plainly refused everything no service,no schematic diagrams. Even I agreed to send it over but denied. Found this page and now writing it to the experts to solve the problem. Should I replace the out put relays I saw two behind the speakers connector and any other part which need to be changed. Waiting for your reply.

Answer
First, unplug the amp power cord.  Disconnect the speakers at the back of the Onkyo.  Then power it up again with out the speakers connected.  See if the brings it back to life.   If so, it means there could be a short in the speaker wiring or in one of the speakers.

If the set still goes into protection mode with no speakers connected it means the problem is, indeed, internal to the amplifier.  Most likely the audio output transistors have become overheated. This could be due to overpowering of the amp (playing too loud for long periods of time) OR the heat transfer grease on the output transistors have dried out allowing the transistors to get too warm and burning them out.

Replacing the output transistors, in this case, would solve the problem.

I don't think it is the relays; they are acting on a shorted load.  Therefore, I am predicting the problem is a shorted voice coil or speaker problem; something like that.  

Also, the Bose speakers have a rather low impedance, as low as 4 ohms at certain frequencies, which may be placing too low impedance on the amp which, over time, could be part of the cause.

Hope this helps. Let me know if there is more.

C