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IMSAI 8080 Deposit/Deposit Next Examine/Examine Next not working

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    IMSAI 8080 Deposit/Deposit Next Examine/Examine Next not working

    Hi, I am the excited new owner of my dream machine. Brought up with a variac, and everything works as expected, except for the examine/next and deposit/next switches (when halted) They do work electrically. Single step does work. It looks like the mod has been performed to allow it to work properly with non-IMSAI RAM boards. Simple troubleshooting steps have already been performed, such as reseating loose ICís. Before I start hunting with the logic tester, any likely suspects that could prevent both from working? Looking at the schematic, I was hoping that u14 or u13 could be the problem. They appear to be functioning properly.

    Includes the following boards:

    TDL ZPU z80 board
    CCS Disk Controller
    Solid State Music Rom board
    Processor Technology 3P+S I/O serial & parallel interface card
    COEX 64K Static RAM board.

    The problem still exists when only the front panel and z80 board are in place.

    Just curious - was the variac for capacitor forming? I'm also a somewhat recent new owner of the same, and was debating on replacing the two large caps inside mine.
    IBM Computers: 5150 (16-64K, 64-256K), 5160, 5161, 5162, 5170 (regular and Tempest), Option 370 16 bit, PC Convertible
    PS/2 Machines: 30, 40SX, 50Z, 55SX, 60, 70, 77 (Lacuna & Bermuda), 80, 85, 90 XP, 95, "E"
    Vintage ThinkPads: 300, 360C/CE, 510CS, 560/E, 600E/X, 701C/CS, 700, 720C, 730TE (tablet), 750C/P (color and tablet models), 755C/CE/CD/CDV/CX, 760ED, 765L, 770
    Vintage UNIX workstations: 2/120, 3/75, 3/60, SS 2/5/20, U2 Ent, HP 715/100, several RS/6000s


      Once it is at full power and with a full load of boards, look at the ripple on a scope. That will tell you if you need to replace the capacitors. These are hard to get now days and if they are not needed to be replaced, don't replace them, until they no longer work.
      I use the variac method of reforming but don't really recommend it. I do it because I'm lazy. The best way is to use a high value of resistor as well as the variac. While forming, the capacitor it designed to vent a small amount of gas, without rupturing the seal. Most of the big capacitors can easily handle 5 ma or so. Start with a lower amount of current at first because you don't want it to use all the leakage current on a single spot. Increase the current over time. This will help you in determining the desired series resistor.

      As for the front panel, make sure the connector cable from the front panel is the right way around. It will work funny if the bits are swapped.
      Usually a failure of the front panel to work right is caused by one of the 7407 open collectors used to jam the data or instruction onto the bus. This can be diagnosed by a scope triggered by the enable signal for the various parts of the deposit and load switches. Look at the schematic to figure what you are looking for.
      You have said that they don't work but often they do something. What they do is a valuable symptom as to the actual cause of the failure.
      Also, note that the RAM needs to be static RAM of the front panel may not work right.
      Last edited by Dwight Elvey; March 15, 2021, 07:48 PM.


        Thanks Dwight! I will hook the IMSAI up to my oscilloscope over the weekend. That will be a production, as it's an 80's era scope and weighs almost as much as the IMSAI.

        I was able to make some progress by creeping down the schematic with a logic tester. On the examine/next side, everything tested fine until I got to U23 (74123). U23 gave no reading on output pin 12. I swapped U17 (known good) with U23. Deposit/Next and Examine/Next came to life! But because there was now a bad chip in U23, I'm getting new problems. So I have halted testing until the replacement 74123s arrive over the weekend. I have also ordered some 7400s, 7402s, 7405s, 7410s, 7427s, 74108s, and 7430s in anticipation of what I will find next.

        Pkhoury, no I wasn't using the variac to reform the capacitors. The previous owner assured me that it had been receiving regular use. Rather I brought it up on the Variac just to have a controlled power-on after a cross country shipment. It might not have made a difference vs just flipping the switch, but it sure made me feel better.


          Shall we play a game? It lives! The chips arrived early.

          It turns out that I had to replace u20 and u23 (both 74123s).