| Electronics > Beginners |
| Question about the function of a capacitor in a circuit |
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| fedaalis84:
You must be right and there's a problem with the biasing circuitry on the right channel. But everything had checked out good on my basic tests.. At least, the transistors were functioning as expected (diodes in the correct direction, etc.), resistors were the correct values... I think I identified a starting point for an investigation: R778 is reading ~19kohm resistance (EDIT: resistance to ground) on the emitter resistor side, but anywhere from 200kohm-3mega ohm on the other side. On the left channel, R728 is reading ~47kohm on both sides, reliably similar on either side. These are "4.7 ohm" resistors, but with an inductor on the other side of the board it looks like. It's almost as if the inductor is connected on only one side of R778.. Do you think that could jazz things up? |
| fedaalis84:
So I made a few more observations. Firstly - I think the weird resistance measurements on R778 were just because the positive rail capacitor had some charge in it. So I don't think that's an issue. I tossed a potentiometer in between the base and emitter of Q754, and have it sitting at around 1.8kohms. Both VOUTs are reading OK voltages. HOWEVER... I removed a toasted replacement of R759, so that part of the circuit is open... And the emitter on Q753 is reading +15-20VDC... Its only connection anywhere in circuit is to the collector via C768. So now I'm thinking C768 is leaking DC from the collector...? |
| fedaalis84:
Well I'm done for a while... I let some magic smoke out of IC700 today. Once all parts come in - I'll replace everything as it should be, and hope for the best. I think R759 is why the right channel continued to short out - I had put a ~380ish ohm TINY SMD replacement, probably below the 1/4W rating of the original 100ohm there. And that was fried after If it wasn't that, then it was IC700, which I'll be replacing anyway. I did my due diligence on the biasing circuitry prior to the output transistors, and after double checking, triple checking, that all looks OK. If it still doesn't work I think I just need to let it go, maybe keep it on a shelf until I've learned some more (and am willing to spend more money trying to fix it). |
| fedaalis84:
Looking like Q751 is not functioning properly actually. The diode readings from base to emitter are reading about twice as high as the left channel equivalent. Will try replacing & see what happens. |
| fedaalis84:
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. |
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