JohnRoberts

Re: Sunn Beta Bias problem
« Reply #20 on: October 17, 2020, 09:43:06 AM »
Ah, o.k. Thanks. I don’t have a lot of experience with solid state.
This is how to get some..
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It looks like I need to remove the active components to measure.
That can cause incidental damage, first measure in place. Only after you identify a component as possibly faulty remove it and measure it out of the circuit to confirm.
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I thought Q5 might be bad since there seemed to be no voltage drop. I replaced that, but it didn’t change. I then removed Q7 and there was still 44v at the collector of Q5. Then I replaced Q13, Q14, and Q12 one at a time with no change. At that point I suspected C13 could be short, so I replaced that. Still problems. I then replaced Q7 and looked at signal on a scope. That is when I decided to start this discussion. I took another look at Q7 last night and it was bad, so I put another new device there. Now I get signal through the amp, but the bias is still off. I see -8v at the base of Q7, and -12v at the base of Q16
The base of Q7 should be roughly 3 diode drops above the output voltage, or nominally +1.5V DC with no signal. Likewise Q16 should be a symmetrical minus 3 diode drops. I suspect the output is -10V, and class A bias current is strong.

I am not a fan of the class A bias trim configuration. Those trim pots tend to fail open circuit which would be maximum bias current (not good). For debugging you can set the class A current to minimum. You can measure the class current by the voltage drop across the 0.33ohm emitter degeneration resistors (R18, R21, R48, R50). Generally you can measure only one, not all of them.

 
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and the signal looks like a cowboy hat at those points. I’ll keep at this until I get it 100%. Then I can move on to the next one. The next one sounds good, but has several broken knobs, pot shafts. Where can I get pots and knobs for these?

If the output stage is working and output voltage is -10V when it should be 0V, inspect the negative feedback circuitry (R7 and R31) and input long tail pair (differential pair).

Another possible suspect is the DC servo IC1, again I am not a fan of using a polar electrolytic with indeterminate polarity, and it looks like I may have too much control range.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.


 

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