I own an old Spectra 210 amp. I hadn't used it in a while, about twenty years. The other day, I tried it, just for the hell of it, and as soon as I turned it on (dry, no instruments, effects, etc), It had this very loud low-pitched hum; sounds sort of like a motor with a locked rotor. When I pulled it apart, I had noticed one of the 10-watt resistors was corroded at the terminals. I isolated it, and checked it for resistance; no deflection. I am looking at three possibilities: there are two big electrolytics , four big transistors, and two power regulators I see as possibilities. I checked the transformer for shorts and found none. To me, this is a run-of-the-mill solid-state amp, nothing special about it's circuitry that jumps out at me; but this has me puzzled. Any takers?