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29104Re: IMSAI 8080

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  • s100doctor
    Jan 29, 2013
      --- In midatlanticretro@yahoogroups.com, "DanielB" wrote:
      > I just managed to get my hands on an IMSAI 8080 that probably hasn't been power up in decades.

      Simple advice. Look around the Web at descriptions of what OTHER people have done, to start up and inspect and repair THEIR IMSAI or MITS (Altair) or other S-100 computers. They probably have descriptions, photos, and discussions of WHY they did what they did. Consider their experiences and see what is reasonable for you to do.

      Additional advice, since it's in discussion here. Spend money and buy a variac. Use it often with your IMSAI, and consider running it for quite awhile at say 100 volts AC or so. Why? Again, see what other people have said. But IMSAI power supplies (many S-100 supplies) were built to supply TENS of amps. If you only run a few cards, you aren't supplying that much amperage. The power supply voltage will be HIGHER than necessary. That strains the caps and voltage regulators. Google "linear power supply" for more information.

      You only need a few volt 'drop" across the S-100 card regulators for them to work OK. So use the variac to manage the input DC. And check those regulators for good DC output, and add heat sinks, and thermal paste. There's reasons why the PC boards are cooked under many S-100 voltage regulators.

      People's advice about "forming caps" is entirely reasonable, a Variac can help with that too. Bring up the IMSAI from zero volts, slowly, again look for other people's Web sites.

      You "could" use a series incandescent light bulb in series with the AC line. Or you CAN spend $20-$50 and invest in a Variac. To protect a $1000 to $1500 computer. To use with any other old piece of equipment. It's good to have one around.

      I suppose I can add "ohm all your DC voltage lines to ground" on all S-100 cards, both sides of all the regulators, LONG BEFORE applying power. Did you know that an ohmmeter has "polarity" and the red lead is usually "positive"? If you read under a few hundred ohms, or even 10 ohms, look for shorted tantalum caps. Note that all the TTL and chips collectively measure "several" hundreds of ohms "resistance" on most DVM's - learn what your "meter" does on your S-100 boards, look for the "odd duck" ohms measurement on your DC lines.

      You don't HAVE to do that - but I think "power it up and look for smoke" is a VERY BAD idea. A shorted cap will draw AMPS of current and FRY a DC circuit trace, and with poor luck it will be on your MOTHERBOARD. Tantalum cap smoke is kind of noxious, that's burning plastic. And that's just the start.

      Turning off the AC doesn't stop the damage. 20,000 microfarads of capacitor holding 8, 12, 14, 16 volts of DC can probably supply AMPERES for SECONDS through several ohms, until the PC board trace burns open, or the tantalum cap's now melted blob of tantalum metal burns through. Oh, by the way, many S-100 power supplies don't have fuses on the DC side. Of course the AC fuse won't help you, 10 amps on the DC side is under an amp on the AC side. Anyway, you turned off the AC.

      It's a death race - which will burn out first? Copper or tantalum? I've seen bright red, or purple smoke (vaporized plastic that is). It's a great show. Make sure you keep your digital camera on during "the smoke test" so you can show it on YouTube. I think I show some fried traces on my Web site, don't know if I caught any smoke.

      As I implied, some of this is on my Web site, where I suggested you find it with Google. It happens to me too, as I think Will Donzelli posted, caps fail over time as well. I power my IMSAI though the Variac anyway, just to keep the regulators "cool".

      Funny thing, this is all just "advice" to get to POWER ON, nothing to do with running!

      Herb Johnson
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