Hi, and welcome.

I don't know the lay of the land where you are, but don't buy anything yet.

Call Onic's Electronics
8402 Barnwood Ln
Riverside, CA 92508
(951) 789-7115‎

If they can't fix it, they will likely refer you to someone who can. I bet this is a relatively simple fix.

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These guys, in Orange County, know speaker repair very well and may be able to direct you better than I:

http://www.speakerrepair.com/

Yamahas have a very good protection curcuit. Does it power down with NO speakers connected? How old is the Yamaha? I believe they have a 2 year warranty.

mrlowry may be right. The amp may shut down due to a short somewhere, possibly at the output (speaker wires shorted?). Some other amps will turn on but won't fully open up if the input signal is too high at turn-on. It depends. Get it checked at the repair shops it it's not a simple short that you can find.