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Re: [midatlanticretro] Re: IMSAI 8080

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  • Systems Glitch
    ... Bought a bare variac (knockouts for wire or clamps) from Will for this exact purpose. In the winter it doesn t matter, but in the summer with no air
    Message 1 of 61 , Jan 29, 2013
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      > 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.

      Bought a bare variac (knockouts for wire or clamps) from Will for this exact purpose. In the winter it doesn't matter, but in the summer with no air conditioning the regulators will get HOT. Also keep in mind that with the cover off, you're getting very little airflow to the end of the card cage opposite the power supply...which is usually where the regulators are.

      > You only need a few volt 'drop" across the S-100 card regulators for them to work OK.

      I run my Morrow Wunderbuss on my benchtop with 7V DC on the +8 rail. Haven't found a vintage card with original regulators yet that won't handle it. They run much cooler, especially the heavily loaded boards like 2102 based RAM boards.

      > You don't HAVE to do that - but I think "power it up and look for smoke" is a VERY BAD idea.

      I tend to prefer this method with linear power supplies (all boards out). Usually the caps are either bad or they're serviceable. Then load it down with a big power resistor dummy load and check the AC ripple (either an oscilloscope or a true AC RMS meter will work). Run it like that for a couple of hours. If the caps look especially crusty I don't bother powering the system on with them, but I've got a decent supply of huge old can caps.

      As to testing boards, you can get board extenders that have fuses inline with the power traces. These fuses are much smaller than the main supply fuses and will usually blow before a board trace or tantalum pops. If you have an extender that doesn't have fuses, you can cut them in yourself.

      Keep in mind that with a hefty supply like the IMSAI's, the old hermetically sealed tantalums in metal tubes become little bombs. My Cromemco chassis popped one on a memory board...fortunately down in the card cage. It's a very loud bang and glowing bits of capacitor went flying.

      Thanks,
      Jonathan
    • DanielB
      Yep, front panel is working good now. One odd thing I have run into is that from the Zapple monitor I can write to the SMB s onboard RAM, but I cannot write to
      Message 61 of 61 , May 14, 2013
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        Yep, front panel is working good now. One odd thing I have run into is that from the Zapple monitor I can write to the SMB's onboard RAM, but I cannot write to the RAM on the memory board but I can write to the memory board from the front panel.

        Dan

        --- In midatlanticretro@yahoogroups.com, joshbensadon <no_reply@...> wrote:
        >
        >
        >
        > --- In midatlanticretro@yahoogroups.com, "DanielB" <danlb_2000@> wrote:
        > >
        > > After bad chips I finally got the front panel working, and after a couple bad tantalum caps on the SMB I also got that working. Very exciting when I got the Zapple monitor prompt!
        > >
        > > https://lh6.googleusercontent.com/-uQShOzInDeE/UZFoWnYYpsI/AAAAAAAACII/80vupyb6Psc/w883-h662-no/IMG_3933.JPG
        > >
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        >
        > Daniel,
        >
        > That's looking great! Very well done :)
        >
        > Can you now Examine/Deposit via the front panel?
        >
        > :)J
        >
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