Tripping Breakers

I have a musical fidelity KW500 that trips a 20 amp (dedicated curcuit/12 gauge wire/hospital receptacle) the instant the power switch is pushed. I have checked everything I can think of that could cause it (hookup is fine). Everything else about the system and circuit is unchanged and has worked for years. When it did turn on before it sounded great and played without tripping the breaker. Anybody else have this problem? If so what was it? Your help is appreciated. Chris.
Sounds like the KW500 has a short and fairly large fuses?
Sometimes you can see or smell what is fried by popping the cover off (after ac power disconnected and caps are discharged!) and giving everything a good look see. Other thing, if it is tripping a GFCI breaker you might just be looking for a slight short to ground, sometimes you can get resistance to ground back up to snuff by just blowing clean with an instrument air bottle (radio shack) and wiping away obvious dust.

Does the problem occur repeatedly after you reset the breaker? Does the fuse in the amp blow? Often, any device which uses a torroidal transformer in the power supply will have a current inrush that momentarily exceeds the rating of the breaker (I'm assuming the KW500 uses one, though I can't confirm this). Usually, most circuit breakers can handle the momentary current inrush, but as circuit breakers age, they become more finnicky (I wish I knew why). I recently had to replace a 20A breaker in my kitchen circuit, since every time the heating element on the espresso machine cycled, it would blow the breaker. I'd say if the fuse isn't blowing on the amp, or if after resetting the circuit breaker the amp works, then the culprit is most likely the circuit breaker itself. If the fuse is blowing, I'd say there's an issue with the amp, and it must be sent for service. Good luck.

Breakers do wear out. FWIW whenever we ran the coffee maker, dish washer and the clothes iron simultaneously it would trip the breaker. Replacing the breaker with one of the same value was the solution.
I had a similar problem some years ago and it was, in fact, the breaker that had failed. It's cheap and easy to replace the breaker so that's certainly something you can isolate as a potential cause of problem.
Try plugging the amp into another circuit. If the same thing happens it may be the amp or power cord. If it powers up successfully and your breaker doesn't trip then the breaker on the circuit you usually use should probably be replaced. This is not unusual as breakers age, as most people above have pointed out.
Are you sure that you did not add a lamp or some other electrical consumer to the circuit? I was having a similar problem until I put in dedicated lines. In another room every time my wife turned on a few lights, I threw the breaker because I was pulling to many amps.

Here is to dedicated lines and great music.
The breaker is a good choice. Also if you are using the "thin" style breakers, (these are the ones that fit two to a socket) they will work poorly. If it is a GFI type breaker they work poorly and die frequently. You may have to try more than one replacement to verify the breaker problem. When installing our spa the first two from the local Home center ran hot, emitted a stench and were down right scary. I had to go with one from an electrical distributor and it has worked properly. Beware of cheap junk.
Good luck, John
I agree with all of the comments on aging circuit breakers. I have both been there and done that. But I also have to add some potentially worse news. I have noticed occasions where a buss bar became burned and continued to be a problem after the breaker was replaced.

Definitely replace the breaker with the best one you can find but also take a good look at the buss bar. I am not sure what brands of breakers are best. For my old house I am just thankful when I can find one that is the right style. Good luck.
I am still investigating the issue. I really appreciate the advice everybody has given and time to post a response. Thanks, Chris.
What make is the electrical panel? Aprox age of the branch circuit breaker that feeds your Amp? At first when the problem started, aprox, how many times did you reset the breaker before it would not hold anymore?
First and foremost you should treat a tripping breaker as a sign of potential trouble. Don't assume a worn out breaker replacement will solve the problem. Hire an electrician to change the breaker and to check all the wiring. Old wires fray or shed insulation causing faults; wires get exposed to moisture or water; there could be loose screw connections at the outlets, or the circuit may be overloaded with other appliances you never knew were connected. Play it safe - hire an expert to give everything a look.
Thinkl Gmmueller and Bryanhood have fist line test and then as everybody say's swap the breaker.I'd do all soon since I have waited on some fixes and dallying has caused real catastrophe.