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Thread: Request For Help on 5150 Pc Motherboard

  1. #1
    Join Date
    Feb 2019
    Location
    South Carolina, USA
    Posts
    19

    Exclamation Request For Help on 5150 Pc Motherboard

    Hi everyone!

    I recently got a seriously unkempt 5150, and I've made it my mission to restore and repair it.

    So far, I've cleaned the beast, and tore it down. Thought the motherboard appears dead.

    I took out the 3 socketed banks of ram, deep cleaned it, replaced the ROMs, and checked for shorts on the power rails and all seems fine.
    But still the board doesn't post at all, nor does it beep. Only a small blip when the board is powered.

    Heres some pics, any help would be amazing..

    IMG_20190610_203514.jpg

  2. #2
    Join Date
    May 2006
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    Melbourne, Australia
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    Default

    I figure that this is the same 64KB-256KB motherboard that you wrote of at [here].

    Quote Originally Posted by Xarpi View Post
    I took out the 3 socketed banks of ram ...
    You can also remove the BASIC ROM's (U29 through U32) if you like.

    Quote Originally Posted by Xarpi View Post
    But still the board doesn't post at all ...
    The POST could be starting, but failing at one of the early checks (where, on a detected failure, the POST simply halts the CPU, giving no visual nor audible indication). In the IBM 5150 POST sequence at [here], those are checks appearing among steps 1 to 17.

    In an earlier thread, you wrote that you have an oscilloscope. Note step 9 of the POST. The linked image in step 9 shows what to expect on pin 13 of the timer chip.

    If you observe those timer pulses on your motherboard, then obviously, your BIOS chip is probably good, and the POST is getting as far as step 9 at the least. Based on history, faulty RAM in bank 0 then becomes a likely suspect (causing failure of POST step 13), and use of the Supersoft/Landmark diagnostic ROM (with adapter), or Ruud's diagnostic ROM (with adapter), should reveal if bad bank 0 RAM is the case or not.

    If you do not observe those timer pulses on your motherboard, then you need to go back to basics. E.g. Swap out socketed chips (CPU, 8284A, BIOS ROM) with known good chips. At [here] is the pre-POST motherboard startup sequence, and included there are some oscilloscope waveforms.

  3. #3
    Join Date
    Feb 2019
    Location
    South Carolina, USA
    Posts
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    Quote Originally Posted by modem7 View Post
    I figure that this is the same 64KB-256KB motherboard that you wrote of at [here].


    You can also remove the BASIC ROM's (U29 through U32) if you like.


    The POST could be starting, but failing at one of the early checks (where, on a detected failure, the POST simply halts the CPU, giving no visual nor audible indication). In the IBM 5150 POST sequence at [here], those are checks appearing among steps 1 to 17.

    In an earlier thread, you wrote that you have an oscilloscope. Note step 9 of the POST. The linked image in step 9 shows what to expect on pin 13 of the timer chip.

    If you observe those timer pulses on your motherboard, then obviously, your BIOS chip is probably good, and the POST is getting as far as step 9 at the least. Based on history, faulty RAM in bank 0 then becomes a likely suspect (causing failure of POST step 13), and use of the Supersoft/Landmark diagnostic ROM (with adapter), or Ruud's diagnostic ROM (with adapter), should reveal if bad bank 0 RAM is the case or not.

    If you do not observe those timer pulses on your motherboard, then you need to go back to basics. E.g. Swap out socketed chips (CPU, 8284A, BIOS ROM) with known good chips. At [here] is the pre-POST motherboard startup sequence, and included there are some oscilloscope waveforms.
    Thanks for the insight. Yes, its the same board. I reposted here, cause I thought this would be a better thread to put it in, and the original post was kind-of all over the place.

    Ill check the signals you mentioned in a just a bit. I also DO have a Supersoft/landmark Diagnostic ROM, but no adapter. I just ordered an adapter as we speak though.

    I'll get back to you once I can check these things.
    Thanks

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