Resoldered those points on the left board. Now I'm monitoring bias to see if the intermittent shows itself again.
In the meantime. . . when I flip the muting switch from Normal to -20dB, I hear a thump through the right speaker. Checking for DC at the pre-out RCA for the right channel, I measure approximately 380mV with the Normal position and 40mV when switched to -20dB. I suppose that 380mV could be coming more or less from anywhere in the preamp section prior to that point, so I'll start checking the outputs of the various boards. That said, shouldn't the coupling cap C601 at the amp board input be blocking this?
In the meantime. . . when I flip the muting switch from Normal to -20dB, I hear a thump through the right speaker. Checking for DC at the pre-out RCA for the right channel, I measure approximately 380mV with the Normal position and 40mV when switched to -20dB. I suppose that 380mV could be coming more or less from anywhere in the preamp section prior to that point, so I'll start checking the outputs of the various boards. That said, shouldn't the coupling cap C601 at the amp board input be blocking this?
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C532 should stop any DC from the circuitry to the left. It is possible something off to the right that we can not see is at fault and back feeding into the switch.
Good thought about something from the amp board backfeeding. With the coupler switch off and the pre and amp sections separated, I checked for DC at the Main-In jacks. It was a few millivolts. But the Pre-Out shows the 380mV. I also checked for DC at the output of the Loudness circuit, which immediately precedes the filter board where the muting switch is. Again, just a few millivolts. So it appears the issue is on the filter board. Maybe a leaky cap?
