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Thread: Circuit repair: capacitor questions

  1. #1
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    Default Circuit repair: capacitor questions

    While cleaning a recent acquisition, a QumeTrak 143 DSDD 5.25 drive, I realized that the drive had blown a capacitor. Looking closer I saw where, but not what, had blown; all that was left was a smudge and a couple of leads.

    I found the schematics and located the area of the problem, but before I attempt a repair I want to ask for advice from the EEs among us. Referring to the photos attached:

    It's not clear from the screen whether C12 or C15 has blown. I think it's C12, and probably a small tantalum like others on the board. Schematics show C15 as a 400V capacitor, 0.01 ufd, which I think is the blue one just above in the closeup picture. That capacitor has black smudge all over it but I can read 103 / J5A / AVX. I don't know what all the markings mean but 103 should mean 0.01 ufd.

    This still leaves me with several questions. There is no voltage marked on the schematic for C12. I think the value is 0.1 ufd but it's hard to tell for sure. And there is no polarity indicated either; if I were to insert another tantalum I'd have to figure out the path to ground for polarity, unless I'm missing a convention here - entirely possible. Lastly, if C12 blew would you replace C15 as a matter of course?

    Thank you in advance for your suggestions.

    -CH-

    Overall.JPG Closeup.JPG Schematic.JPG

  2. #2
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    Looking at my own drive, C12 is the blown one--I don't believe it's a tantalum unit. At any rate, it's across the +12 and ground power rails, so it isn't too critical. A modern 16+ volt MLCC would be the perfect replacement. C15 is a 400V film cap and is unlikely to go bad. More to the point, it's a timing-critical component, so leave it alone.

  3. #3
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    Thanks, Chuck. I have a replacement for that.

    -CH-

  4. #4

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    Does the schematic have any notes like "All unlabelled caps are x value"?

    Otherwise just go with a 100nF to 1F ceramic cap. 16V or higher...

  5. #5
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    All unlabeled caps are .1 ufd but no spec on voltage that I saw.

    Stuck a 0.1 ufd 25V ceramic in and we are responding to commands. Rotational speed a tad slow at 296.5 or so but a lesser worry now.

    Thanks to all for their suggestions.

    -CH-

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