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Re: HELP! Altair 8800B blew smoke!

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  • 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 1 of 4 , May 26, 2003
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      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 2 of 4 , Jul 22, 2003
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        --- 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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