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

  1. #1

    Default IBM 5155 gone to "dollar sign hell" - help!

    I have an IBM 5155 Portable PC that did work perfect. Today I fired it up for the first time in about 3 months, and it worked fine as usual, and then after a few minutes dollar signs ($) started popping up all over the display. Now when you turn it on, it gives the POST beep code for a video card error. It still boots and shows video, albeit littered with dollar signs.

    I tried reseating all the expansion cards, and I reseated the ROM chip on the video card. No help. I would try reseating the RAM, but I haven't yet because it takes a lot of time and energy to get to the motherboard in this thing.

    Anyone ever experience this before and/or might have an idea of a solution? Someone mentioned to me that it might be failing tantalum caps - I've never heard of them doing anything other than exploding without so much as crashing the system, but does anyone else concur?

    It kinda stinks that this worked perfect after sitting for years before I got it, and it worked perfect when I stowed it away 3 months ago, and now on its first run since then it's totally crapped out.

  2. #2
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    If the system boots, then it would not be the motherboard RAM.

    It is almost certainly a bad RAM chip on the CGA card. I would guess it is probably not any of the intermediate logic chips since you are seeing video at all.

    I assume the "$" is where spaces should be. A space is hex "20" and a "$" is hex "24", so one bit is getting stuck on. Somebody else here could probably tell you exactly which chip that is, but they are soldered on to the board.

  3. #3

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    Bad RAM on the video card? Aw, rats!

    You're right - looking at a picture I took (linked below), the dollar signs do seem to appear only in blank space. Specific letters are also being replaced by other letters - in the picture, many E's are replaced with A's - again a change in one bit, and actually the same bit as the space/$ replacement. I also see '-' getting replaced with '>' - again a change in the same bit!

    Crap, you're totally right! I wish the system RAM was still a possibility because at least that stuff is socketed.

    http://i.imgur.com/1caAZwm.jpg

    Still odd that this happens after just three months in storage.

  4. Default

    There is a possibility it may not be a bad chip since the solder is likely old as well. One thing you could try is warming and relflowing the solder to the chip. At a minimum its an easy thing to do and you dont have to wait for a replacement in the mail or you could do this while waiting for a replacement to arrive.
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  5. #5

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    Quote Originally Posted by mlaferriere View Post
    There is a possibility it may not be a bad chip since the solder is likely old as well. One thing you could try is warming and relflowing the solder to the chip. At a minimum its an easy thing to do and you dont have to wait for a replacement in the mail or you could do this while waiting for a replacement to arrive.
    You say 'chip' like there's only one. There's at least eight RAM chips on that card.
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  6. #6
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    I suspect that it's the chip--over time, the plastic encapsulation can fail and there's basically no other cure than replacement.

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    Quote Originally Posted by Stone View Post
    You say 'chip' like there's only one. There's at least eight RAM chips on that card.
    There are eight. One for each bit. In this case bit 2 is stuck "on", so it is only the chip that stores that bit. Someone who could be arsed to read the schematics could tell you exactly which one. Probably the third from one end or the other if they are in order.

    If it were me, I would test the machine with another video card before breaking out the soldering iron. It wouldn't even have to be IBM CGA if you can hook up an external monitor.

  8. #8

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    Yeah, I'm willing to take a crack at reflowing the solder to the ailing chip, if I can identify which chip it is, which is where I would need help. I suppose I can visually inspect all of them and see. This thing did undergo a 90 minute ride in the trunk of the car before this point, so I wouldn't rule out a cold solder joint.

    While I don't have another CGA card I can test, I do have another XT machine I can test this card in, but unfortunately that machine is inaccessible right now.
    Last edited by TheMaritimeMan; July 16th, 2016 at 09:47 AM.

  9. #9

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    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.
    Last edited by TheMaritimeMan; July 16th, 2016 at 09:54 AM.

  10. #10

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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

    Edit: just saw your last post. Doesn't seem like video ram if it's affecting keyboard syntax too

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