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Thread: IBM 5155 gone to "dollar sign hell" - help!

  1. #11
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    Quote Originally Posted by TheMaritimeMan View Post
    I just noticed something really interesting. When I type commands in IBM PC BASIC, and the computer obviously injects erroneous characters in them, the commands won't execute - it throws a syntax error. So it seems like the characters are not just passively appearing - the computer actually sees them and tries to execute them.

    For example, if I type "run", it changes to "ruj" and it won't execute. If I type "cls", it changes to "cls $", and the cls command actually executes, and then it throws a syntax error, as if it's due to the dollar sign being there afterward.

    Does this still seem like something that just a video card problem would cause? It's not the keyboard because if I boot with the keyboard disconnected it still outputs bad characters during the memory count.
    Entirely possible that BASIC reads back the screen contents. Wouldn't surprise me one bit.

  2. #12

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    From looking at the schematic the ram paths are labeled D0-D7 and D2 is U55. Looking at my card, I see there is a tantalum cap right below it. Look for crud, scratches etc
    Thanks for the info! I looked at all the RAM chips, and the solder joints looked good as far as my vision could see. But I suppose there's no harm in reflowing U55 for the heck of it.

    Entirely possible that BASIC reads back the screen contents. Wouldn't surprise me one bit.
    Looks like you're right - I booted DOS, and everything I typed and executed worked as it should.

    I keep forgetting that when I first powered this on last night, it worked perfect for about 5 minutes before it got sick, so it really does seem like the chip just gave up and died. Looks like I can get replacements cheap enough on eBay.

  3. #13

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    Quote Originally Posted by Chuck(G) View Post
    Entirely possible that BASIC reads back the screen contents. Wouldn't surprise me one bit.
    IIRC they went through the trouble of doing that in graphics mode too, as bonkers as that sounds. To read the input, BASIC checks those 8x8-pixel blocks from video RAM, compares them with the 8x8 characters in the PC BIOS, and tries to find a match for each so that it can tell what you just typed. (I believe that this faux-'OCR' routine is part of the PC BIOS too.)
    This also led to the baffling situation of using a non-IBM EGA/VGA card, where the 8x8 graphics mode font comes from the card's BIOS ROM, and may be somewhat different from the one in the PC BIOS... in which case, INPUT statements would promptly stop working, etc.
    Last edited by VileR; July 17th, 2016 at 10:32 AM.
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  4. #14
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    Why on earth would they bother doing that? Was it to save a few K of RAM on a 16-64k 5150? It seems like a huge length to go to, for not much result.

    Bobby.

  5. #15
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    There's a function in the BIOS to read the screen, whether or not it's in graphics mode, so not so much in BASIC per se. You can see how this works in BASIC by repositioning the cursor over text lines that have been displayed and hitting "enter".

  6. #16

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    Thought I should post an update - I re-flowed U55; no dice.

    Here's something really interesting - my friend with the card he may be able to send me recently tested the card, and his too spontaneously started displaying incorrect characters in text mode (Link). Shortly afterwards, one of the tantalum caps exploded! He replaced the cap, and now his card is working fine again!

    So I'm wondering if it's actually one of the caps on my card causing the problem? Other evidence of this is that if I let the computer sit for a long time (ex. I powered it up today for the first time in a month), the video card will actually function normally for a few minutes, before the problem surfaces again. To me this sounds like a cap issue, as I wouldn't think semiconductor failures would fade in and out like that.

    Does anyone concur? All my caps visually look fine, but I wouldn't mind pulling some to see if that changes anything. Problem is there's 30 of the things and I have no idea where to start (well, I guess U55...)

  7. #17
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    Quote Originally Posted by TheMaritimeMan View Post
    Does anyone concur? All my caps visually look fine, but I wouldn't mind pulling some to see if that changes anything. Problem is there's 30 of the things and I have no idea where to start (well, I guess U55...)
    Are you sure all of them are tantalum? Probably most of them are ceramic...

    And, yes, tantalum caps use to explode...

  8. #18
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    Quote Originally Posted by TheMaritimeMan View Post
    the video card will actually function normally for a few minutes, before the problem surfaces again. To me this sounds like a cap issue, as I wouldn't think semiconductor failures would fade in and out like that.
    They can. Especially RAM chips. They can operate normally under some circumstances, such as lower temperature, and then fail under others. Which can be annoying when trying to trace a specific issue.

    Tantalum capacitors are usually just for filtering/stabilizing power, so it is very strange that that other card started to work.

  9. #19

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    Are you sure all of them are tantalum? Probably most of them are ceramic...
    Here's a pic showing all the different types of caps on the board.

    They can. Especially RAM chips.
    Well, it was worth a thought anyhow.

    The RAM chips are Motorola MCM4517P12 - does anyone know if equivalent chips exist? The cheapest I can get any in Canada is $18 for five of them on eBay, which is a bit crazy.

  10. #20

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    Quote Originally Posted by TheMaritimeMan View Post
    Here's a pic showing all the different types of caps on the board.



    Well, it was worth a thought anyhow.

    The RAM chips are Motorola MCM4517P12 - does anyone know if equivalent chips exist? The cheapest I can get any in Canada is $18 for five of them on eBay, which is a bit crazy.
    Hmm, looks like a single-supply version of the 4116. You could replace those with 4164s by connecting pin 9 on each chip to VCC or Ground.

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