Sounds alot like a dirty switch or control.
How many did you disassemble to clean ?
How are you turning off one channel or the other ?
Don't use deox-it to clean the boards...Its for electrical contacts. Not cleaning and defluxing circuit boards.
Your" power on" switch may be internally arcing.
I know what relay clicks are. I need to know where you made the measurements.
Before the relay clicks.:idea: Are you measuring this at the amplifier board or the speaker binding posts ?
Omron basic snap action switch
When it acts up. exercise the with the speaker select switch as well as the rest of the switches and controls. It sounds like a dirty switch. But...
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