BGW 250 problem

[sgelectric]

I was recently given a 1972 BGW 250 stereo power amp and told it has issues.
After powering it up slowly, a cursory examination revealed an amp that seems to be functional.
Upon powering down, in addition to the expected slight thump, the amp emitted a fairly loud screeching sound from the left channel only.
What is causing this?
TIA

[Enzo]

Well, really, who knows?

AMps make noises when you turn them off because as their power supplies drop down, they reach a point where they become unstable. This can result in the output swinging over to one or the other power rails with the remaining voltage trying to appear on the output, however briefly. The classic thump. SOmetimes the output swings over one way, only to be swung back the other. That would be the Ka-thump, rather than the plain thump. In other cases, the amp is unstable to the point where the output swings over one way only to be swung the other by the differenctial circuits, and it happens back and forth at a frequency heard as a screech sound. It really is pretty much just the thump a few octaves higher.

What to do about it? First we need to decide if anything really needs to be done about it. It only happens at the end of the night, after we are done playing. Probably doesn't hurt it.

But one could engineer in speaker relays, I guess. One could see if the power supply filter caps in this old beast need fresh replacements. Might cure it, might not, but it coldn;t hurt to have 30 year old caps replaced.