there are two likely answers, first this amplifier uses discrete transistor as drivers for the amplification stage and though I haven't worked on this model I bet that there is an aluminum heat sink that the power transistors are mounted against. Check to see if this is getting hot. one or more of the transistors are over heating and causing the protection circuit to turn off the amplifier (this is a guess as I don't have the amp in front of me). Check to see if the amp is dissipating a lot of heat. It shouldn't be warm at all after a few seconds unless one of the components is shorted, which creates heat. unplug all the inputs and outputs from the amp to make sure the problem is with the amplifier, not a bad speaker or shorted input.
Secondly there is a sleep feature that might be misbehaving, though this is exceedingly unlikely.
lastly use you sense of smell to see if you can detect the odor of burnt plastic/insulation.
If you have no experience in electronics this is the type of problem that is best left to an expert, but you can at least help narrow down the cause, speak to a repair person knowledgeably and possibly save yourself from being swindled by a bad repairman...
I repair equipment and work at a sound and lighting company (20+ years)
here's a listing for one on ebay for $99 that'll be cheaper than paying someone else to fix yours...
BEFORE ATTEMPTING TO SERVICE THE RECEIVER DISCONNECT THE POWER!!!
okay, you've disconnected everything and the problem presists. That means the system has gone into protection mode. Before it turns off does it produce sound at all? If so the problem isn't too bad, and it's worth a shot at fixing it.
If it doesn't make it that far (no sound) then there are several possibilities. There is a circuit that provides a "soft on" which consists of a timer circuit, made of capacitors and resistors to give a couple of seconds delay before engaging a relay which brings the amp output on line. This prevents a loud pop by allowing the transistors to come up to power before switching in the speakers. Sometimes the caps fail and that usually can be verified visually by taking the cover off the amp and looking for capacitors with bulges or a sticky coating on the circuit board around the capacitors http://en.wikipedia.org/wiki/File:Condensador_electrolitico_150_microF_400V.jpg
If you find a dead cap it can be verified with an voltage ohm meter by placing the leads of the ohm meter across the leads of the capacitor. The meter will either slowly count down to nothing then start counting up, or it'll just count up to a very high resistance. Reversing the leads will start the cap charging or discharging and you should get similar readings. If it just reads 0, your cap is shorted and bad... If the cap has an extremely large resistance and the meter never counts up or down, even after reversing the leads, the cap is bad. But in either case look first for the sticky coating that looks like the cap sprayed out its insides...
Caps are cheap and can be bought at a local radio shack.
If you don't see anything like that look for discolored or burnt components, smell is very helpful as overheated components can be very lightly discolored but the smell of a burnt/overheated component is unmistakable.
Read the numbers off the discolored part and type them in to yahoo search including the term datasheet, this'll give you a small explaination of what the component does who makes it and some sample applications.
Finally, the 99 dollar direct replacement on ebay is probably going to be your best bet it you have no idea about what the above means or if your nervous about trying to repair it yourself. Todays electronics have become throw away, most people buy new electronics rather than trying to have them repaired and there are few good repair shops left that don't charge an arm and a leg. I always tell people that I don't charge to look at their equipment. Places that require an up front payment for looking at the gear are to be avoided, they have no investment in actually doing the repair. They'll take your money and unless the repair is easy, will tell you "it isn't worth fixing."
Finally, the yamaha service link is included below, and if you can't solder consider replacing the unit, it might be cheaper than paying to ship it back and forth and the cost of repairs.
You can email me if you have any questions and I'll do my best to help you from here.
I was working on another amp today and it reminded me that some amps us a protection circuit that is triggered by dc voltage in the power amp (output) section of the receiver. This happens when one or some of the output transistors leaks internaly or shorts. Call the yamaha service center and find out their rates. Any of the above problems are cheap and easy to fix with a basic knowledge of electronics.