CS-800C strange problem

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Miniman82
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Joined: Thu Jun 15, 2017 11:48 am

CS-800C strange problem

Post by Miniman82 » Thu Jun 15, 2017 12:00 pm

Hello all,

I'm primarily a vacuum tube guy, so I admit these transistor amps usually stump me. This one really has me going though, I can't figure it out. When I got it both output stages had shorted T-03's, so I pulled them all out and tossed the bad ones. There were enough good ones left to populate a single channel, so I did and it works fine.

Recapped both driver cards just to be on the safe side, and the OPAMP's got replaced as well.

The other channel got new MJ15024G's, but here's the strange issue: it will work fine on a variac up till the rails hit 74v or so, then it's like a light switch and the speaker terminal slams to -75v. Clipping lamp comes on when this happens. If you back the variac down under 65 rail volts, the clipping lamp will go out and things return to normal. Also when the clipping lamp is on, the +15v zener supply drops to around 5v. I've never seen anything like this.

I can swap the driver card from the other channel to this one and it doesn't happen, so the issue is definitely in the driver card. The card that does it got some additional work, but even after replacing the driver transistors, limit transistors and the voltage amp Q2 it still has an issue.

Any ideas?

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