I have this DENON PMA-630 DC coupled amplifier from 1978 when bought new. Recently I had an intermittent problem with no output on both channels. Thinking it was a transistor in the power supply that I changed in 1985(!), I went back inside and found all the voltages from the PS are correct and the unit is in fact functioning. I did find that the speaker offset voltage is about 35-45 mv on both speakers which seems a bit high. The bias adjustment in the service manual is explicit about setting the speaker offset to 0 +/- 10 mv, but goes on to have you set idling current (at a test point} when warmed up without giving a final value for the speaker offset voltage. The procedure has you turn the gain all the way down when making the adjustments and gives no guidance of where the offset voltage should track when the volume is ramped up. Measuring the speaker voltage per the manual is done with a high impedance voltmeter, and in doing so, I see the voltage go to over
2.5 volts without a speaker load. Both channels are similar and I have had no history of speaker popping or any anomaly.There is no spec or procedure calling for checking the bias at other than zero volume. Is this voltage rise normal?
Since this is probably a good time to go through the bias adjustments, I would like to do so. However I am concerned that the pots for the bias adjustments might be dirty and scratchy from old age.
I don't want to spray them with cleaner and find them way out of whack and blow a transistor. Can anyone suggest a good method for maintaining the nominal bias while cleaning those pots?
Finally, what is a good pot and switch cleaner that will not damage the parts?