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Help troubleshoot a previously working board

beta22

Help troubleshoot a previously working board

Postby strogg » June 12th, 2019, 7:58 pm

This was exciting. I was just about to finish up the 4-channel project by putting a new DAC that just came in the mail. I got it installed into the chassis and turned everything on. Something weird happened. One of the boards stopped working based on my right ear being half the volume of the left. OK. I took it apart and narrowed it down to one board. When I unplug the audio source while everything is running, the volume doesn't change. I do the same for the other channel, and I get very quiet audio.

I took the board out and started doing measurements with the multimeter. R9 was at 6.8V. R34 was at 200mV. DC Offset was 21mV. And of course, the heatsinks were piping hot. I turned all the pots down a few revolutions so R9 is at 4.5, R34 at 50mV, and DC Offset near 0. I started doing other measurements from there, and everything was just way off. R10 and R12 are at 3.5V. I started doing standard test points from the troubleshooting section, and everything was just not quite right. It'd be 10% off at half the places. The worst is C2/R20 where it measured 950mV where it should be 3.06 for a +/-24V PSU, followed by the aforementioned R10 and R12.

Because everything looks off, and both sides of the voltage rail are equally effected, I can't begin to imagine where to begin troubleshooting. Do note that this board was properly functional for over two weeks before this happened. Any help would be appreciated. Thanks in advance.
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Re: Help troubleshoot a previously working board

Postby amb » June 12th, 2019, 10:46 pm

I suggest doing a BJT sanity check on all the BC550C and BC560C transistors on that board to see if any of them might have blown out.
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Re: Help troubleshoot a previously working board

Postby strogg » June 13th, 2019, 7:48 pm

Well, that was odd. They all passed the sanity check. But wait. While I was down there, I decided to re-tap the solder joints at critical junctions just in case. I did work on wire management while installing the DAC, after all. Maybe something got loose from a poorly tended joint. I think that was it, because the board all of a sudden worked again.

Word to the wise: When working several hours straight doing solder work, be sure to take breaks when your eyes gloss over. I'm guessing that's what happened. I like to think I do a bang up job with clean joints and connections, but whoops. *shrugs*
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Re: Help troubleshoot a previously working board

Postby amb » June 13th, 2019, 9:13 pm

Well, I'm glad it was simple. :)
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