Finishing up a rebuild, wanted to reset unit bias. Was following the directions given in this thread: https://audiokarma.org/forums/index.php?threads/mcs-3275-idle-current-mystery.859915/
The problem is I don't understand where to put the probes. If I put them across the indicated emitter pins I can get bias, but it is probe directional, and switches depending on which amp I'm on. For example, if I am measuring positive to red, negative to brown on right amp, I get a reading in positive and the trimmer has to go counter-clockwise to raise to 14mV. I do the same thing on the other channel and I get a negative reading. Only now the pot is working backwards- clockwise to lower, counter to raise- and it won't go past 7mV all the way "down".
What's going on here? This isn't how emitter resistors nor trimmers are supposed to act?
Edit: figured it out, mostly.... ended up going directly across the emitter for each side with mini clamps. Right side dialed right in, left had issues... I cleaned trimmer but didn't help. Ended up reflowing the drivers and that seems to have done. So now both channels are 13-14mV.
Only issue I have now is a quirk upstream of the volume knob I think. When unit first starts up, even with no speakers selected and no volume applied, the right meter will twitch a bit. When put on a source it will only play sound if on mono. After a few minutes, stereo just comes back. I already recapped the control amp circuit for the volume control and cleaned all switches/knobs.... should I try to clean them again? Or am I looking at another issue?
The problem is I don't understand where to put the probes. If I put them across the indicated emitter pins I can get bias, but it is probe directional, and switches depending on which amp I'm on. For example, if I am measuring positive to red, negative to brown on right amp, I get a reading in positive and the trimmer has to go counter-clockwise to raise to 14mV. I do the same thing on the other channel and I get a negative reading. Only now the pot is working backwards- clockwise to lower, counter to raise- and it won't go past 7mV all the way "down".
What's going on here? This isn't how emitter resistors nor trimmers are supposed to act?
Edit: figured it out, mostly.... ended up going directly across the emitter for each side with mini clamps. Right side dialed right in, left had issues... I cleaned trimmer but didn't help. Ended up reflowing the drivers and that seems to have done. So now both channels are 13-14mV.
Only issue I have now is a quirk upstream of the volume knob I think. When unit first starts up, even with no speakers selected and no volume applied, the right meter will twitch a bit. When put on a source it will only play sound if on mono. After a few minutes, stereo just comes back. I already recapped the control amp circuit for the volume control and cleaned all switches/knobs.... should I try to clean them again? Or am I looking at another issue?
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