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Grid 1520 Keyboard Problem - Input no longer accepted

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    Grid 1520 Keyboard Problem - Input no longer accepted

    Hello all.
    I was wondering if any of you can get me looking in the right direction.
    I have a 1520. I power it up and it works great - for about 5 minutes. After that, if you press a key, nothing happens (on the screen, at least). After 16 presses, there is an error beep with every key press and nothing appears on the screen.
    It stays this way until it is rebooted. When rebooted, it does the same thing: works for about 5 minutes, nothing on screen, then error beeps when keys are pressed.
    In my troubleshooting, I wanted to remove as many variables as possible. I removed the modem, all but the minimum 1 MB of RAM, and the hard drive.
    But, it still has the same problem.
    I have swapped 286's (microprocessors rarely fail in this way. Since it's socketed, I thought I'd try it).
    I've swapped (and now socketed) the keyboard controller PROM.
    I replaced 2, 100 uF electrolytic caps (1 near the keyboard PROM, and 1 near the 286)
    I poked around the controller PROM with an oscilloscope, but I don't know what's a good signal, or what's a bad signal.
    Are there any buffer latches that I should check?
    I'm open to any ideas.
    Thanks,
    -Shawn
    Last edited by shawnerz; March 27, 2021, 11:13 PM.

    #2
    First rule of all electronic troubleshooting, did you check the power supply? not the one that goes in the pod but the big black section behind the drives? I would confirm that the 5 VDC and 12 VDC bus are good and stable before anything. You can run the system with the cover removed, the display to the side and see if its heat related but i have seen many of the now forty year old power supplies suffer from capacitor failure.

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      #3
      If it starts beeping after each keystroke after 16 presses, it sounds like the keyboard is probably ok. Bios keyboard buffer gets filled up because the OS keyboard handler isn't servicing the interrupt. My off-hand guess would be ram, after 5 minutes one of the chips heats up and starts glitching and the OS locks up. Is that one with SIPP ram? Might try swapping it around, or running a 286 compatible memory test.

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        #4
        The problem was a broken and cracked capacitor on one of the SIPP RAM modules.

        Qbus: Thanks for the suggestion. At first, I was like, "Of course the voltages are stable. It's booting, right?"
        I wasn't getting anywhere in my troubleshooting. I had checked the 5V, but what about the others? Rather than probe around to find the various voltages, it was easier to swap out the power supply. I had the same problem with another power supply. So it was a good and valid suggestion, but it was not the problem.

        Turns out, it was, in fact, a bad SIPP. I did not see eswan's response until just now. But thank you. The "Ah ha!" moment was earlier tonight when I powered up the laptop without a disk in the drive. I left it powered up while I went to locate a boot disk. When I finally got back to the laptop, I put the disk in, hit return, and it booted.
        By this time, the computer should have stopped accepting keyboard inputs.
        I assume that because the OS hasn't booted, that RAM stick wasn't being used. Once the OS loaded, DOS was attempting to use the bad RAM chip.
        I swapped out the 1 MB of RAM with other SIPP's and the laptop worked for longer than 5 minutes. I inspected the SIPPs and found a cracked cap on one of them.
        Thanks for the responses,
        -Shawn

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