Electronics > Beginners
Question about the function of a capacitor in a circuit
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xavier60:

--- Quote from: fedaalis84 on March 08, 2019, 09:40:02 pm ---IC700 is out, but everything else is back in, except instead of transistors, I have 1K Ohm resistors from the base to emitter.

I'm getting about 0.5V DC on the left channel, but up in the area of 3-4V DC on the right.

--- End quote ---
I would expect to see some negative voltage coming from the protection  circuit. It should be the same on both channels. Did the Emitter resistors get damaged on the right channel?
Do you mount the power transistors well?
Thermal paste must be used. The transistors have to sit tight and flat against the mounting surface. The leads must be soldered last of all.
xavier60:
On second thoughts, I think it's stray leakage. Are you using extra soldering flux?
fedaalis84:
When I did have the right transistors in, I did Mount them with thermal paste and also use the little mica squares. And Soldered them in as the last step.

The emitter resistors on both sides still check out good. The protect circuit absolutely does put out some negative voltage, it brings the protect voltage (should be 3.3V) down to -50 or so millivolts.

I think the problem on the right channel is the biasing transistor Q751.

I had pulled Q752 out of circuit and it tested fine. But it did have some thermal paste between the collector and emitter. I cleaned that off, put some fresh thermal paste on the back and remounted it.
fedaalis84:

--- Quote from: xavier60 on March 08, 2019, 10:02:25 pm ---On second thoughts, I think it's stray leakage. Are you using extra soldering flux?

--- End quote ---

Yes, and I haven't cleaned it up in a while. The first few times I made some changes oh, I would brush it off with acetone. But as I started going to the back of the board more often, I started leaving the flux instead of cleaning it.
xavier60:
When you get the parts, as a precaution, check the bias voltage before fitting the output transistors as before.
If I have doubts, I temporarily fit 10 to 47 ohm resistors in place of the emitter resistors when the output transistors are fitted. They act as fuses and hopefully will protect the output transistors if there is a bias problem.
Unless IC700 was faulty the whole time, it would be difficult for it to be damaged by other faults. The VOUTs source/sink a limited amount of current which makes them inherently immune to overload.
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