Rotel power amp posessed?

[pburke]

A friend of mine asked me to help him with a power amp that has been "acting up" lately. Symptoms are rather odd: it plays fine most of the time, but once every few days it will emit a very loud sound that he describes as something similar to the noise you get when you short out wires and listen to the electrical sparkle fireworks. The sound comes up when the unit is stone cold and has just been powered up, or after running fine for a full day, just random, out of the blue. He thinks it's coming from both channels, but can't confirm 100%. It will do the sound without a source connected, cold or warm, but again, random, not in any way predictable.

I had the unit open yesterday and can't find any obvious problems. All caps look fine, no aparent discoloration on any part, nothing looks fried.

I think the model is RB971 - it's about 5 years old.

Any ideas as to what may be going on in that unit? I'm not quite the amp expert, but can find my way through schematics and swap just about any part, except I don't even know where to start looking in this case.

Peter

[anatech]

Hi Peter,
I think the first thing to do is reproduce the fault on your bench. Try tapping around the board with a non-conductive item (not a rubber mallet! ). Then you will know for sure whether it's one channel or both. The most likely thing is a cracked solder joint, but it could turn out to be something more fun to service.

-Chris

Edit: Hit the wrong key - again!