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.