Making some progress. I was getting inconsistent readings to ground on one of the input jacks. Sometimes one channel looked shorted, and sometimes it didn't. (IIRC it was the right but I'm not sure).
I had already ordered-in all of the small electrolytics needed to replace the 20+ year old 'lytics on the amp board as part of routine PM, so I swapped them all out. I also replaced some metallized film caps that were in the burned spots on the PCB. The short to ground problem on the inputs went away, so it was likely a bad cap somewhere. As we had discussed before, there were a lot of areas on the board that had visible heat damage. With the new lytic caps, a lot less bias current is required to meet the 6 mV spec, so some of them must have been leaky.
(As an aside, how easy is it to blow up an opamp? After doing the recapping, the signal generator mistakenly had maybe 5mV of +DC offset because of a knob that wasn't properly zeroed. When the amp turned on, the signal trace at the input looked OK for a short while, and then became very ugly and spikey -- kind of like a tangent function if you remember them from your geometry days. I pulled the IC and swapped in another.)
Now the Left channel produces a clean sine wave output at the speaker terminals into no load.
The right channel is causing problems though -- the amp will idle fine with no Right channel input, but the protection circuit keeps triggering when an AC test signal is fed to the R channel input and I scope the speaker terminals. The relay energizes after the 3 second turn-on delay, and on the scope I see a blip of what looks like a tall clipped waveform on the screen for a fleeting instant, then the protection relay triggers again, flattening out the scope trace. The relay continues to cycle in periodic fashion, and I intermittently see a fleeting blip of what sometimes look like either: a) a clipped AC waveform that's transposed upwards with a DC offset, or b) a tall square wave with rounded corners that looks like its all DC. After the waveform appears on the outputs for only an instant, the relay cycles into protection mode. Then the cycle repeats over and over again.
Now here's what's interesting -- the odd behavior I've mentioned above only appears at the speaker terminals. At the center-taps of the R channel ballast resistors, the sine wave outputs look completely normal throughout the amp's power range. I'm at a loss to explain why the DC appears on the outputs but not on the ballast resistors. The only thing between those two points is ... the relay. But if the DC is coming from the relay and appearing at the speaker terminals, I would think it should appear on the ballast resistors as well. Maybe its coming from somewhere else.
I had already ordered-in all of the small electrolytics needed to replace the 20+ year old 'lytics on the amp board as part of routine PM, so I swapped them all out. I also replaced some metallized film caps that were in the burned spots on the PCB. The short to ground problem on the inputs went away, so it was likely a bad cap somewhere. As we had discussed before, there were a lot of areas on the board that had visible heat damage. With the new lytic caps, a lot less bias current is required to meet the 6 mV spec, so some of them must have been leaky.
(As an aside, how easy is it to blow up an opamp? After doing the recapping, the signal generator mistakenly had maybe 5mV of +DC offset because of a knob that wasn't properly zeroed. When the amp turned on, the signal trace at the input looked OK for a short while, and then became very ugly and spikey -- kind of like a tangent function if you remember them from your geometry days. I pulled the IC and swapped in another.)
Now the Left channel produces a clean sine wave output at the speaker terminals into no load.
The right channel is causing problems though -- the amp will idle fine with no Right channel input, but the protection circuit keeps triggering when an AC test signal is fed to the R channel input and I scope the speaker terminals. The relay energizes after the 3 second turn-on delay, and on the scope I see a blip of what looks like a tall clipped waveform on the screen for a fleeting instant, then the protection relay triggers again, flattening out the scope trace. The relay continues to cycle in periodic fashion, and I intermittently see a fleeting blip of what sometimes look like either: a) a clipped AC waveform that's transposed upwards with a DC offset, or b) a tall square wave with rounded corners that looks like its all DC. After the waveform appears on the outputs for only an instant, the relay cycles into protection mode. Then the cycle repeats over and over again.
Now here's what's interesting -- the odd behavior I've mentioned above only appears at the speaker terminals. At the center-taps of the R channel ballast resistors, the sine wave outputs look completely normal throughout the amp's power range. I'm at a loss to explain why the DC appears on the outputs but not on the ballast resistors. The only thing between those two points is ... the relay. But if the DC is coming from the relay and appearing at the speaker terminals, I would think it should appear on the ballast resistors as well. Maybe its coming from somewhere else.
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