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Thread: Dead 486 Motherboards -- Help!

  1. #1

    Unhappy Dead 486 Motherboards -- Help!

    (Good thing I don't have much hair left to pull out...)

    I have three 486 motherboards that POSTed occasionally in the past but now not at all.

    I have tried everything within my level of expertise: verify the jumpers, swap out the processor and cache, reseat all the DIP chips, try different sticks of RAM, use different power supplies, use different video cards and move them around to different slots. Also tried one of those POST code cards but only get codes with no information for American Megatrends BIOSes.

    After completely recapping one board and partially recapping the other two, the fully recapped board worked fine for a few power cycles before dying again.

    In one last desperate attempt, I tried putting the 'good' board in the oven. I figured my fidgeting might have broken a solder joint. No luck.

    Is there something obvious I'm overlooking here!?

    I would really appreciate some advice at this point because I'm all out of ideas.
    Collecting PS/2s and boxed software from ca. 1987 to 1994

  2. #2

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    were these boards at any point exposed to battery leakage? could be it wasn't fully cleaned off and continued to erode the traces/vias over time

  3. #3

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    One of them had a Varta battery which had leaked. I removed it, cleaned and repaired the traces. The other two came with 2032 holders, including the 'good' board. No corrosion on them.
    Collecting PS/2s and boxed software from ca. 1987 to 1994

  4. #4

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    If you have a way to dump the BIOS then you could investigate the POST code by looking inside the BIOS disassembly to see where that code is written out.

  5. #5

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    Quote Originally Posted by bakemono View Post
    If you have a way to dump the BIOS then you could investigate the POST code by looking inside the BIOS disassembly to see where that code is written out.
    How do I dump the BIOS? I see programs that can do it, but I obviously it can't run on a non-functioning MB. Are there special hardware tools needed?

    (Sorry for the facile questions. I'm in over my head with troubleshooting these boards.)
    Collecting PS/2s and boxed software from ca. 1987 to 1994

  6. #6

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    Quote Originally Posted by allCAPPS View Post
    Are there special hardware tools needed?
    Yes, generally known as a ROM or device programmer. But I guess this means you don't have one...

  7. #7

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    I have build a debugger. You'll find a description and schematics for the 6502 version here. The idea is that you halt the CPU immediately after a reset and start stepping the CPU instruction by instruction. In case of an 80x86 machine the address shown on the display should be FFFFF0 and the data (in most cases) 5BEA (part of JMP FAR F000:E05B where JMP FAR = EA). Is that not the case, then you have a big problem. If it is the case, then you must find out where it goes wrong during the start-up. And that is another challenge.
    Regarding leaking batteries: most (IMHO every) 486 board is multi-layer. So you you are not able to repair damage to internal traces.
    I know some people don't like to hear this but I scrap a board that doesn't start up at its own for parts. The exception: boards with non-VLSI/SMD chips like the original IBM 8088 and 80286 boards (and clones).
    Adding to the problems: replacement parts for failing VLSI/SMD parts, how can I find out which of the many VLSI/SMD ICs is broken in the first place, desoldering them, missing schematics, etc., etc. If 496 boards were rare...., maybe.
    With kind regards / met vriendelijke groet, Ruud Baltissen

    www.baltissen.org

  8. #8

    Default

    Quote Originally Posted by Ruud View Post
    Regarding leaking batteries: most (IMHO every) 486 board is multi-layer. So you you are not able to repair damage to internal traces.
    Most people don't like to hear this either no matter how many times it's repeated.
    PM me if you're looking for 3" or 5" floppy disks. EMail For everything else, Take Another Step

  9. #9

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    I can live with the leaking battery board being dead. I just can't believe all three are dead. Just about every conceivable troubleshooting measure has been taken, and what's frustrating is that one will occasionally decide to work ONE TIME when I inevitably get optimistic again and pull them out of storage.

    I'll look into doing the BIOS dump. Thanks for the suggestion. I just keep thinking there must be something obviously wrong, but maybe it's going to be more involved than basic troubleshooting.

    It's honestly a tradeoff between time and money, and as much as I want these to work again, it's starting to look like buying a working board will cheaper than continuing to pour countless hours into these.

    I built a 486 back in the day and wish I saved it now!

    Please keep the suggestions coming!
    Collecting PS/2s and boxed software from ca. 1987 to 1994

  10. #10

    Default

    Quote Originally Posted by allCAPPS View Post
    After completely recapping one board and partially recapping the other two, the fully recapped board worked fine for a few power cycles before dying again.

    In one last desperate attempt, I tried putting the 'good' board in the oven. I figured my fidgeting might have broken a solder joint. No luck.
    What type of solder did you try to reflow and what oven temperature did you use? Some solders require rather high heat.

    Alternatively, there may be something that is killing the caps, even the new ones.
    PM me if you're looking for 3" or 5" floppy disks. EMail For everything else, Take Another Step

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