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Old 7th April 2011, 03:15 AM   #1
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Default Bias or loss of power in one channel problem

I am using a Harmon Kardon Citation II amp, but with a different circuit than the original. It is a differential push pull setup. This is kind of a generic, what do I check question.

The problem began when one channel kept cutting way down in volume. For a while tapping the amp brought it back, but now it won't play at normal volume on that channel at all. The bias is very hard to settle down. If I turn the adjuster on a tube up enough, it will draw 70 ma. Turning it down slightly, has little effect, then turning it down more suddenly drops it almost to nothing. Turning it back up follows the same pattern in reverse. I notice that the tubes seemed almost connected in that adjusting one often adjust the others, and their output is similar.

The other anomaly occurs with the .33 capacitors the bias pot wipers to ground. They read .55 in the circuit. This is not the case with the other channel, which reads .33. (The other channel is fine.)

The bias resistors on the pots and the coupling caps on the driver tubes seem fine. I swapped out tubes but that had no effect.

This seems like a short somewhere obviously. Any ideas where to start looking?

Thanks.
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Old 7th April 2011, 04:01 AM   #2
kevinkr is offline kevinkr  United States
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Roger, have you grabbed your trusty voltmeter and checked to see what the voltage is on the wipers for that channel?

I would not be too surprised if the bias was high enough to cut off the output tubes. Common resistor on low side of pots may be open or cold solder..

Yeah, I'm the guy who designed the circuit Roger is using in his amp.. (20yrs ago) And no the design is not available..
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