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Troubleshooting IBM 256K Memory Option Card

Yoder

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Joined
Sep 8, 2011
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12
Location
Tampa, FL
Hi guys,

Last week I picked up a nice clean IBM 5150 with keyboard from the original owner. It's nice and clean and has a HDD,a full 256K of RAM on board, a Paradise CGA/mono card, an asynch serial card, and an IBM 256K RAM expansion card (the type with 4 16-pin memory modules) described here: http://www.minuszerodegrees.net/oa/OA - IBM 256KB Memory Expansion Option.pdf . It worked great for about two hours when it just shut down out of the blue. When I tried to turn it on, the fan whirred for 1-2 sec and then it shut down again. I pulled all the cards and pin pointed it as the RAM expansion card. After setting the DIP switches on the MOBO to only look for on-board RAM, the machine works as usual, so the board seems to be ok. I even tried the RAM card in a different ISA slot, but that didn't help.

I'd like to try to repair the 256K card, if possible and wanted to see if you could give me some ideas. All the tantalum caps look ok, and even after removing all four memory modules, the computer will still not power up with the card in. I've checked the 1st and 3rd legs of the LM317T voltage regulator and I'm only getting 0.6V on each for a second or so...so this is a problem. FYI, I didn't check the center leg b/c I'm accustomed to those being a ground...turns out the center pin is output, but the 0.6V readings on the other legs are a problem. With the card out, I checked the diode near the voltage reg at CR1 and I'm getting a 520 mV voltage drop using the diode test on my DMM. I'm not sure if this is a little low or if it's ok.

So now I'm out of ideas. I'd really appreciate any input you guys can give on how to test this further. It's an original IBM card and I'd love to get it working.

BTW, I attached an image of the card.


Thanks!
 

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RJBJR

Experienced Member
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Feb 17, 2011
Messages
453
All the tantalum caps look ok, and even after removing all four memory modules, the computer will still not power up with the card in.

Considering the age of the board I'd bet that one of the caps are shorted, can't go by looks alone. You can sometimes smell burnt caps.
If it was my board I would be looking for a shorted cap with a multimeter. Maybe someone here has a good procedure for checking caps.
 

Anonymous Coward

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Shandong, China
Is this system using the original power supply? 5150s usually didn't have hard drives because the 63.5W power supply wasn't strong enough to drive them reliably.
 

modem7

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The "the fan whirred for 1-2 sec and then it shut down again. I pulled all the cards and pin pointed it as the RAM expansion card" symptom suggests that your card has a short in it, resulting in the power supply being overloaded and shutting down. Therefore I see no point in making voltage measurements on the card (there is no longer any -5V / +12V / +5V coming into the card).

Most people reading your post will say to themselves, "It is going to be a shorted tantalum capacitor for sure." The probability of a shorted tantalum cap is very high in vintage computers. Quite often, a shorted tantalum cap shows no visible sign of failure.

If your card had say two or three tantalum caps, then I would suggest that you simply cut all of them off the card, and then see if the symptom disappears. But the circuit diagram indicates 7 tantalum caps, 3 on the -5V line, 1 on the +12V line (C20), and 3 on the +5V line.

What you could do is first verify that there is a direct short on one of the -5V / +12V / +5V lines. With the card out of the computer, use your multimeter in resistance mode to in turn measure the resistance of the -5V / +12V / +5V lines on the card. I'm expecting that you'll find that one of the measurements reveals a very low resistance, in the order of a few ohms, on one of the lines - the short.

If you identify a shorted line, remove the associated tantalum caps on that line, and then see if the symptom disappears.
 

Yoder

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Sep 8, 2011
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12
Location
Tampa, FL
Is this system using the original power supply? 5150s usually didn't have hard drives because the 63.5W power supply wasn't strong enough to drive them reliably.

Yes, it does and I know that this is not ideal. I should look at a replacement 140W soon...
 

Yoder

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Messages
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Location
Tampa, FL
Russ and modem7, I agree about the caps. And modem7, thank your for the detailed instructions on how to troubleshoot those lines. I appreciate you taking the time to do that. I'll get the schematic out and start checking the lines with my DMM. I'll report back on what I find!
 

pearce_jj

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The hard-drive may not be an issue; bear in mind that HDD's in 1981 like the ST-412 had significant spin-up draw of up to 4.5A on the 12V line. That reduced in time quite quickly so check the specs of the drive fitted.
 

mikey99

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Jun 15, 2006
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Any luck getting the memory card working ? I have that same card
..... I think these type of memory modules are very
uncommon on PC cards. I recall seeing these memory modules on
IBM mainframe circuit boards back in the 70's.
 

Yoder

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Location
Tampa, FL
Thanks for asking, but unfortunately I have not had time to go through it. It's on my kitchen table waiting for me, but have had some long work hours.

I agree. I've certainly never seen those types of modules before...I didn't even think they were at first. They're kind of cool and pretty easy to pop in and out. I hope I get it working. I'd like to put an EGA card in this thing :) And I need one of those 6 in 1 cards (or whatever they're called).
 

SpidersWeb

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AST SixPakPlus - it's like the holy grail of accessories - with 128KB RAM installed on one of those, you'd be at 640KB (which is enough for anybody)
 
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