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Thread: MDA blown SRAM chip

  1. #1

    Default MDA blown SRAM chip

    I got this IBM MDA card to test with my converter but it has a blown SRAM chip :/

    The lower 4-bits for the lower half of the screen are stuck on LOW. It seems a single SRAM 1Kx4 is bad. Would any of you with experience in repairing these boards could spot it ? I could try to find with a scope but the chips are difficult to reach.

    I will have to buy SRAM chips now... If I had to replace many I will end up paying more than I paid for this board...

    mda_blown.jpg

  2. #2
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    Is it a real IBM card or a clone?
    The later clone cards that are switchable between CGA and Herc mode use just 2 RAM chips. Given that you have the four low order bits failing I'm guessing its a single faulty 41464 chip (64kx4)
    Sometimes the RAM chips are surface mount and sometimes through hole, the card I have in my hand now is fitted with surface mount and has unpopulated provision for through hole chips.

  3. #3

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    Its a long green IBM 1501985 0 Black and White Parallel Card. It has 8 NEC uPD2114LC Static RAM chips 1Kx4.

  4. #4
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    Not an expert, just thinking out loud:

    Errors start at around 800H, or 2048, so that's going to be the third bank (2K-3K) ?

    But looking at the logic diagrams in the tech ref, I'm not entirely sure what chip is responsible. My guess would be U12, but it's an assumption based on the chips being numbered based on their position in memory space (e.g, 1K LO, 1K HI, 2K LO, 2K HI, 3K LO, 3K HI, 4K LO, 4K HI). Hopefully someone smarter can provide more useful information, just thinking out loud.

    Sometimes I just enjoy the puzzle.

    (Edit: but I doubt you'll use it for more than testing anyway would you? All of my IBM MDA cards are in static bags with Hercules cards in each machine)
    Last edited by SpidersWeb; October 11th, 2017 at 03:22 PM.
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  5. #5

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    Quote Originally Posted by SpidersWeb View Post
    Not an expert, just thinking out loud:

    Errors start at around 800H, or 2048, so that's going to be the third bank (2K-3K) ?

    But looking at the logic diagrams in the tech ref, I'm not entirely sure what chip is responsible. My guess would be U12, but it's an assumption based on the chips being numbered based on their position in memory space (e.g, 1K LO, 1K HI, 2K LO, 2K HI, 3K LO, 3K HI, 4K LO, 4K HI). Hopefully someone smarter can provide more useful information, just thinking out loud.

    Sometimes I just enjoy the puzzle.

    (Edit: but I doubt you'll use it for more than testing anyway would you? All of my IBM MDA cards are in static bags with Hercules cards in each machine)
    Sure, Hercules are way more fun, especially with RAMFONTS. But I'd like to play a little with assembly, see what can I get from the 6845

  6. #6
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    Can you find the schematics for the board? I don't remember if the MDA board was fully documented.

    2114 SRAMs shouldn't cost all that much. I could sell you some tested/working 2114s, but I suspect it'd be more to ship them to Canada than they're worth.

  7. #7

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    Quote Originally Posted by glitch View Post
    Can you find the schematics for the board? I don't remember if the MDA board was fully documented.

    2114 SRAMs shouldn't cost all that much. I could sell you some tested/working 2114s, but I suspect it'd be more to ship them to Canada than they're worth.
    I paid 25 bucks for the board :P

    I got some 2114 from Bulgaria, 15 dollars for 10 of them, let's see If they ever come. If I find the blown chip I can try adapting a 6116 I have here meanwhile...

    The MDA manual on minuszerodegrees has the schematics but it's too confuse. There are many, many TTLs working together...

  8. #8
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    Living in Canada, like I do, I could mail you a 2114 for about a buck.....
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  9. #9
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    It shouldn't be too hard to work out which one is the problem. I don't have a schematic for it or I'd work it out from that for you.

  10. #10
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    Quote Originally Posted by David_M View Post
    I don't have a schematic for it or I'd work it out from that for you.
    The 'IBM Monochrome Display and Printer Adapter' link at [here].

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