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21059Re: Need 3B2/310 Help!

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  • epsilonprocess
    May 2, 2011
      Thanks for the help Ian.

      The cover was on when it started smoking, so the smoke I saw came out of the floppy drive opening but I can't be sure it was the floppy that was the issue. What I might do is try to fire up the floppy drive using a standard ATX supply to see if it still works.

      I completely disassembled the machine, removing all power supply connectors. I did indeed measure +12 and +5 on one of the 4-pin drive Molex connectors. So it seems the output is ok. The wires seem to come out of single bundles from the power supply of Red (+5v), Yellow (+12v), Blue (RTN/ground) and Orange (-12v).

      See photo: http://home.prog.fm/3b2-ps2.jpg

      However, I did find where there was OBVIOUS damage on the system board power supply connector. While I can't be 100% certain this wasn't there before, I would go with the fact that it happened as a result of whatever caused the smoke. Unfortunately, I can't find any sign of a short...yet, anyway.

      See photo: http://home.prog.fm/3b2-ps1.jpg

      What I found odd was that measuring the resistance between the red wire bundle and the blue (or ground) there is only 20ohms of resistance, while measuring the yellow bundle to blue (or ground) is almost an open. Is that normal even though I got a clean +12v and +5v? Oh and, I measured while the system board power was still connected BEFORE I noticed the damaged connector.

      I also didn't see any obvious caps blown on the power supply. Where is the square cap located approximately?

      Thanks,
      Anthony

      --- In midatlanticretro@yahoogroups.com, Mr Ian Primus <ian_primus@...> wrote:
      >
      > --- On Mon, 5/2/11, epsilonprocess <als@...> wrote:
      >
      > > Rather than go through the measurement details for
      > > everyone, if anyone with 3B2 experience can offer some help,
      > > I'd like to hear from you.
      > >
      > > Luckily part of the package he sent me included a backup
      > > main system board. But before I even try it, I would like
      > > some help from anyone with experience servicing 3B2's.
      >
      > I've spent a fair amount of time inside my /310, so...
      >
      > Where did the smoke come from? Did it come from the power supply? Are the output voltages at the floppy power connector still OK? I would put my money on some faulty mylar caps in the power supply - they go across the AC line and when they fail, you get lots of smoke. The same sort of thing happens to Apple II's and the like...
      >
      > Take the power supply out, and take the cover off. You're looking for a little yellow block capacitor with a hole blown in it.
      >
      > -Ian
      >
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