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Re: IMSAI 8080

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  • joshbensadon
    ... Hi Daniel, Congrats on the IMSAI, very nice computer! Step 1, check out the power supply. Pull out all the cards, remove the front panel (CP-A) electronics
    Message 1 of 61 , 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. I am very interested in getting this thing up and running again. Any recommendations on what I should check before attempting a power up? Any advice would be much appreciated.
      >

      Hi Daniel,

      Congrats on the IMSAI, very nice computer!

      Step 1, check out the power supply.
      Pull out all the cards, remove the front panel (CP-A) electronics (UNPLUG the computer, there is 120VAC to the front switch at the upper right corner of the CP-A board, it's an Orange & White wire on my system).
      To Remove the CP-A board:
      There are several plastic spacers that might fall out of place and get lost on the floor. So I do the following:
      -remove the front panel switch escutcheon (the lower metal part) by removing 4 screws from the bottom.
      -unscrew the 8 allen screws but do not pull them out. Just unscrew until you feel the last thread skipping.
      -unplug the ribbon cable to the CPU board and the Power Connector, let these cables fall over the front of the panel, even secure them on the front by pressing them between a couple of the switches.
      -Carefully, wiggle the plexiglass & board straight up, mind any screws that might still be threaded in.
      -Keep the whole assembly vertical until you can safely lay it down on your bench.

      Now, the system is strip down, Either jumper the plug for the 120V switch or install an Amp meter here. Plug the computer into a variac and bring the voltage up slowly, meter your caps for some voltage. Here the choice is yours, either apply a low voltage (say ~6VAC in) for a few hours to allow the capacitors to reform** or skip to next step. Next, bring the voltage slowly up to full, watch your current. Then start testing the capacitors, put a load on the +8V, +16V and -16V (one at a time) and check your ripple voltage. On my system, I had 8.2V while drawing 5Amps with 620mV ripple. I did not record the other voltages, but I would say they are good if they have less than 1V ripple at 1Amp. The +16 and -16 aren't used very heavily.

      Let me know when you get this far, I can offer more advice if you found this helpful.

      Cheers,
      Josh

      PS. There's a lot of information online about Capacitor Reforming. I recommend reading about it. But, I don't think the IMSAI is old enough to warrant this step. I will stand down to others more experienced on this forum.
    • 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
        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
        > >
        >
        >
        > Daniel,
        >
        > That's looking great! Very well done :)
        >
        > Can you now Examine/Deposit via the front panel?
        >
        > :)J
        >
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