Hi Roly,
Thank you for your time.

The amp is probably circa 1980-ish. It's old, but in good shape. No corrosion and i see no evidence that anyone's been under the hood before. i will verify the grounding of the various components, and i will test voltages that you suggested. I'll be back to you with that. Hopefully, I'll have time to do all that today!

Thank you so much for responding.

Pete

Hi anyone reading this. i really appreciate you trying to help me. Not only am i a complete NOOB, but i know almost nothing about electronics. that said, i have actually managed to make some simple repairs, and save a few junk-heap destined amps.

so i have this pa head. everything works EXCEPT, if i turn the volume, the tone, or the bass pot for any particular channel to full output, the "protection" light comes on and the amp makes a loud buzz. mind you, the volume might only  be at 50%. but turn the tone or bass up full, and the buzzing starts. the main output volume might only be half way up (or less) also.

Any suggestions what to look for? i do have a small multi-tester, but that's about it for test equipment. i'm willing to replace parts as an experiment. any ideas? attached is a pdf with a schematic for the individual channels, and one for the power amp. nothing there for the EQ section (in the pre-amp?)
Thanks in advance,
Pete