2/
I opened up the one with the issues to have a look, and happily, the input section is on a separate board! Well then, hopefully I can swap it out for the input section from the newer unit, to A, confirm that the input section was the problem, and B, get the better amp fully working.
I removed the suspect input board and immediately my eye was drawn to these semitransparent yellow polymer caps. I can’t say exactly why, but I don’t trust them.
…
3/
I then went and opened up the other unit, and pulled out the input board. Under all the dust, a few things became apparent.
First, they revised the design to remove the stupid metal tab that you had to bend to be able to remove the input board without taking every other board out first. Nice.
Second, the only noticeable revision to the input board was replacing those exact yellow capacitors with a different kind of film/poly cap. Aha! Vindication perhaps?
Anyway, I was able to swap the newer input board into the older amp (after removing all the dust).
And then test, and yay! It worked great!
Nice quiet fans achieving plenty of airflow, chassis doesn’t get hot, no input popping. This 20-year old amp is ready for another decade or two of service, I hope.
And maybe I’ll get some filter foam over the front intake…
