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Re: HELP! Altair 8800B blew smoke!

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  • stevenaleach
    ... and ... next ... Oh, forgot to update my email address on Yahoo.. it is now stevenaleach@mac.com...
    Message 1 of 4 , May 25, 2003
      --- In altaircomputerclub@yahoogroups.com, "stevenaleach" <leachm003@h...>
      wrote:
      > Hello, I just retrieved an Altair 8800B from storage, blew away the dusts, checked
      > connections, etc. finally I powered it up, lights flickered in a promising manner,
      and
      > then smoke came rolling out of the front panel.
      >
      > Capacitor C4 on a voltage regulator VR1 is fried... crispy.. charcoal.. I have no idea
      > what the value of this cap was in order to replace it.
      >
      > Also, assuming I can replace this, any suggestions of things to check before the
      next
      > attempt? (I will leave any nonessential cards, ie serial parallel, disk cont. etc. out)

      Oh, forgot to update my email address on Yahoo.. it is now stevenaleach@......
    • alltare
      C4 is 22UF/35Volts. It s used at the input of VR1, an LM320, as the -9V (VDD) supply. To protect as much as possible, I suggest that you pull out ALL boards
      Message 2 of 4 , May 26, 2003
        C4 is 22UF/35Volts. It's used at the input of VR1, an LM320, as the
        -9V (VDD) supply.

        To protect as much as possible, I suggest that you pull out ALL
        boards from the buss after repairing this, and check the 3 buss
        voltages to be sure they're within spec (should be about +8, +18,
        and -18 volts). Sometimes, a blown regulator circuit can take out
        the raw supply(s). Also, in case that regulator is bad, you might
        lift the output pin of VR1 from the PCB so you can check the -9V.
        Doing this, if the regulator is bad you won't be delivering -18V to
        the circuitry.

        Good luck.

        steve
        =============================

        --- In altaircomputerclub@yahoogroups.com, "stevenaleach"
        <leachm003@h...> wrote:
        > Hello, I just retrieved an Altair 8800B from storage, blew away the
        dusts, checked
        > connections, etc. finally I powered it up, lights flickered in a
        promising manner, and
        > then smoke came rolling out of the front panel.
        >
        > Capacitor C4 on a voltage regulator VR1 is fried... crispy..
        charcoal.. I have no idea
        > what the value of this cap was in order to replace it.
        >
        > Also, assuming I can replace this, any suggestions of things to
        check before the next
        > attempt? (I will leave any nonessential cards, ie serial parallel,
        disk cont. etc. out)
      • horacehall
        ... check before the next ... It is common for old electolytic capacitors to fail if hit with rated voltage after long storage. The electrolytic barrier needs
        Message 3 of 4 , Jul 22, 2003
          --- In altaircomputerclub@yahoogroups.com, "stevenaleach"
          <leachm003@h...> wrote:

          > Capacitor C4 on a voltage regulator VR1 is fried... crispy.. charcoal..
          > Also, assuming I can replace this, any suggestions of things to
          check before the next
          > attempt?

          It is common for old electolytic capacitors to fail if hit with rated
          voltage after long storage. The electrolytic barrier needs to be
          reformed before it is put into service. This is a big problem with old
          radios, TVs, etc., and the Altair should be no exception. If possible,
          just replace the cap. If you want to do it the hard way, to keep
          original parts, you need to 'reform' the cap.

          To reform the cap, connect it to a source of variable voltage, and
          slowly raise the voltage from zero (or close to it) to the working
          voltage. Preferably, if you have a meter, monitor the current through
          the cap to see that it doesn't get too high. Alternatively, see that
          the cap doesn't start warming up. If watching the current, you should
          see it rise with increased voltage, then over a span of time it will
          drop off. The idea is to get up to working voltage while keeping the
          current low. At working voltage, the current should eventually go to
          nearly zero after a while. If not, replace the cap.
          --Wayne
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