I'm going to outline my initial steps in assessing the state of amplifiers in an unknown but assumed to be malfunctioning condition. I'll attempt to be thorough as I dig through a small heap of amps, but it's very possible I'll overlook one of those obvious to me steps that has become a thing of second nature. Long story short, this is a document of my (largely self taught) approach, and shouldn't be treated as an all inclusive instructional document.
First up is a Peavey 260 series amplified mixer, visual inspection reveals that someone evidently did not like the Peavey logo and blotted it out with an indelible marker or polishing compound of some sort. The logo remains intact, so it should prove easy to return to its original beauty with something like a silver paint pen. Visual inspection also confirms that the power cable appears intact and stress free, so let's plug it in.
Amplifier powers on and stays on, so far so good.
Before putting a load on it of any sort I'll probe the speaker output for abnormal voltage.
Speaker output is delivering over 39 volts DC to whatever is plugged into it.
This unit is a speaker killer, and illustrates why unknown amplifiers shouldn't be plugged into speakers prior to a brief preliminary inspection.
My hunch is shorted output devices shunting rail voltage to speaker jack. This one is heading for the bench.