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

T

themaritimegirl

Experienced Member
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Jun 15, 2011
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NB, Canada
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.
 
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.
 
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.
 
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.
 
mlaferriere said:
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.
Click to expand...
You say 'chip' like there's only one. There's at least eight RAM chips on that card. :)
 
Stone said:
You say 'chip' like there's only one. There's at least eight RAM chips on that card. :)
Click to expand...
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.
 
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.
 
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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.
 
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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
 
TheMaritimeMan said:
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.
Click to expand...

Entirely possible that BASIC reads back the screen contents. Wouldn't surprise me one bit.
 
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
Click to expand...

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.
Click to expand...

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.
 
Chuck(G) said:
Entirely possible that BASIC reads back the screen contents. Wouldn't surprise me one bit.
Click to expand...
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.
 
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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.
 
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".
 
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...)
 
TheMaritimeMan said:
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...)
Click to expand...

Are you sure all of them are tantalum? Probably most of them are ceramic...

And, yes, tantalum caps use to explode...
 
TheMaritimeMan said:
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.
Click to expand...
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.
 
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