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Thread: Help tracking down a very intermittant memory failure on my IBM EGA

  1. #1

    Default Help tracking down a very intermittant memory failure on my IBM EGA

    Note: this is not the infamous false positive checkit test! My EGA has the daughterboard and full 256kB fitted. That said, I'm fairly sure the error is in one of the chips soldered to the card. I could simply desolder them all and replace, but 8 DIP-18s would be a bit tedious.

    Errors will occasionally manifest in visible text mode corruption, usually from a cold start and will go away very quickly. Very occasionally I can get the checkit video memory test to fail, but 99% of the time it passes. I have yet to be able to catch a failure with the IBM diags disk. Attached are two examples of the failing bits reported by checkit. If anyone can help me narrow this down to an IC, or even bank of ICs on the EGA itself, I'd be quite grateful.

    20200519_174229.jpg
    20200519_174329.jpg

  2. #2

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    Occasional cold start errors that go away after system heats up a bit make me think about cracked soldering joints on some memory chip(s). This can easily be an issue on old PCBs. Have you had a closer look on memory ICs pins, if those look well soldered? Sometimes it may be visible, sometimes it may not, so before complete resoldering I'd first just apply some flux and reheat each pin to melt a lead to make sure that each connection is good, and then try to run test again.

  3. #3

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    If the problem is temperature dependent, just try cooling the suspect ICs one after another with canned air, cold spray or even an ice cube in a plastic bag. That way you should be able to identify the faulty IC.

  4. #4

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    All the solder joints look good. No visible cracking, all nice and shiny and smooth.

  5. #5
    Join Date
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    Melbourne, Australia
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    I have done some experimentation, and am creating a web page of the results. See [here].

  6. #6

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    Quote Originally Posted by modem7 View Post
    I have done some experimentation, and am creating a web page of the results. See [here].
    Thanks! So looks like I'm going to have to catch an error with the diags disk.

  7. #7
    Join Date
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    Unless you can find and read the schematics to find out where bit 6 and 7 go to on the board, or there is an existing service manual which has this information, you're going to be stuck with the shotgun approach to repairing it.

    That means remove all the chips and socket them and go through one by one.

  8. #8

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    Socketing them is out of the question. There is not enough vertical space for the daughterboard chips/sockets and chips + sockets on the main PCB.

    I went ahead and purchased a DRAM tester after wanting one for over 2 decades now. Got a Chroma unit on ebay for $50. Supports DIP, 30pin SIMM, SIPP, and 72pin SIMM. So if I do pull them all at least I can get a definitive answer

  9. #9

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    So the plot may have thickened. I'm hesitant to declare this as fact due to the highly intermittent nature of the problem, but the problems appear to have gone away when removing the daughterboard. I have not been able to duplicate the issue for two days now. What I don't fully understand is how the daughterboard memory would affect visible text in the main page? Unless one of the data lines on the expansion memory chips was stuck in a driven state and not going high-z respecting the chip select?

    Looks like my memory tester will be getting a workout when it arrives.

  10. #10
    Join Date
    Jan 2019
    Location
    New Hampshire
    Posts
    241

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    I've seen the same effect from a bad ram chip on the motherboard causing display corruption. I think it is exactly as you describe like a ram output is stuck on. Do any of your previous memory tests indicate which bit contains the error?

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