NOTICE FOR IBM XT OWNERS

Discussion about old PC hardware.

NOTICE FOR IBM XT OWNERS

Postby dkarguth » 2019-2-10 @ 03:21

I got an unpleasant reminder today that old IBM hardware is very prone to catastrophic failure due to tantalum capacitor failure. The capacitors used in a lot of IBM hardware from the era (e.g. CGA card, MFM HDD card) tend to short out over time, and then fail with a catastrophic "Bang" and a shower of sparks. Just a reminder to check your cards, and replace the capacitors BEFORE they explode.

MFM card
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Closeup of damage
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Exploded capacitor vs his unexploded friend
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Collateral damage
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Re: NOTICE FOR IBM XT OWNERS

Postby novasilisko » 2019-2-10 @ 08:21

[eyes slowly drift to tantalum caps on my own VGA card]

Hmm.

Well, this card is from around 93 I think, so hopefully it hasn't worn down too much. The Trouble with Tantalums (the famous star trek episode), as I understand it, is that they're often run closer to spec than they should be, which slowly wears them down over time, but unlike other capacitor types, they have a propensity toward just spontaneously combusting, instead of any sort of graceful failure mode.
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Re: NOTICE FOR IBM XT OWNERS

Postby DonutKing » 2019-2-10 @ 08:39

I've had a few tantalum caps explode, and they certainly make big racket, but those scorch marks are something else. Looks like it was actually on fire?

Have you replaced the cap and does the card still work?
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Re: NOTICE FOR IBM XT OWNERS

Postby SpectriaForce » 2019-2-10 @ 10:10

An exploding tantalum capacitor can indeed cause fire. Here’s a top tip: if you test an IBM PC, then do it with the enclosure on it, otherwise the exploded burning tantalum can hit you.
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Re: NOTICE FOR IBM XT OWNERS

Postby root42 » 2019-2-10 @ 13:24

Replace them with good brand tantalums of the same rating, or slightly higher voltage rating if you want to be safe. New tantalums have less catastrophic failure modes and should last a long time.
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Re: NOTICE FOR IBM XT OWNERS

Postby digger » 2019-2-10 @ 13:41

The last time I tried to start up the Olivetti M24 I grew up with, it would refuse to boot. I measured the voltages of the PSU and they were extremely low, so probably a few capacitors in the PSU had given out. I never heard anything pop, though. But at this point, the machine is well over 30 years old, so perhaps I should thoroughly check all of the capacitors (not just the ones in the PSU) before starting it up again.

I'm also thinking about poly-modding the motherboard of the M24. I know, polymer capacitors are not "era-appropriate" as many purists will point out. But I want vintage hardware like this to last a long time. These are pieces of history and will become antiques one day. Also, if I'm going to go through the trouble of replacing capacitors in such a vintage piece of hardware, I might as well put the most durable and high-quality caps in there, right? Also, since capacitors go bad and need to be replaced over time anyway, having those not be identical to the caps that the machine originally shipped with isn't that much of an issue, I think.

However, from what research I have done on this so far, poly-modding may not be possible or practical for the PSU, since those typically use much higher capacity capacitors. So for those, I'd just have to find the most high quality (Japanese?) capacitors I can get with the same specs as the original ones.

I really hope the system hasn't yet sustained any damage from leaking capacitors.

Any thoughts on this? Is it crazy, sacrilegious or otherwise pointless to poly-mod a vintage computer?
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Re: NOTICE FOR IBM XT OWNERS

Postby SpectriaForce » 2019-2-10 @ 14:17

digger wrote:The last time I tried to start up the Olivetti M24 I grew up with, it would refuse to boot. I measured the voltages of the PSU and they were extremely low, so probably a few capacitors in the PSU had given out. I never heard anything pop, though. But at this point, the machine is well over 30 years old, so perhaps I should thoroughly check all of the capacitors (not just the ones in the PSU) before starting it up again.

I'm also thinking about poly-modding the motherboard of the M24. I know, polymer capacitors are not "era-appropriate" as many purists will point out. But I want vintage hardware like this to last a long time. These are pieces of history and will become antiques one day. Also, if I'm going to go through the trouble of replacing capacitors in such a vintage piece of hardware, I might as well put the most durable and high-quality caps in there, right? Also, since capacitors go bad and need to be replaced over time anyway, having those not be identical to the caps that the machine originally shipped with isn't that much of an issue, I think.

However, from what research I have done on this so far, poly-modding may not be possible or practical for the PSU, since those typically use much higher capacity capacitors. So for those, I'd just have to find the most high quality (Japanese?) capacitors I can get with the same specs as the original ones.

I really hope the system hasn't yet sustained any damage from leaking capacitors.

Any thoughts on this? Is it crazy, sacrilegious or otherwise pointless to poly-mod a vintage computer?


You'll need the data sheets of the capacitors in your power supply to figure out what you need. Voltage and capacity are just two characteristics :wink:
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Re: NOTICE FOR IBM XT OWNERS

Postby Errius » 2019-2-10 @ 14:42

Damn it, you guys are scaring me.

And I was just reading about electron migration last night. That was bad enough.
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Re: NOTICE FOR IBM XT OWNERS

Postby novasilisko » 2019-2-10 @ 18:34

root42 wrote:Replace them with good brand tantalums of the same rating, or slightly higher voltage rating if you want to be safe. New tantalums have less catastrophic failure modes and should last a long time.


I have a question. What electrical characteristics do tantalum capacitors have that can't be matched by normal electrolytics? What negative impacts might be had by replacing them with equivalent typical tin-can electrolytics, or the solid polymer ones, etc?
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Re: NOTICE FOR IBM XT OWNERS

Postby root42 » 2019-2-10 @ 19:07

novasilisko wrote:
root42 wrote:Replace them with good brand tantalums of the same rating, or slightly higher voltage rating if you want to be safe. New tantalums have less catastrophic failure modes and should last a long time.


I have a question. What electrical characteristics do tantalum capacitors have that can't be matched by normal electrolytics? What negative impacts might be had by replacing them with equivalent typical tin-can electrolytics, or the solid polymer ones, etc?


I guess you can use those, as long as ESR, voltage, ripple current etc all roughly match. I just figure that a similarly rated tantalum cap will most probably have the correct specs to be a good fit. I think Dave from eevblog had a good video on this some while ago.
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Re: NOTICE FOR IBM XT OWNERS

Postby quicknick » 2019-2-10 @ 21:58

Here's a good starting point, but there are quite a few other resources on the 'net regarding polymodding. I'm all for it, and i fully agree with digger about the importance of keeping our pieces of history in good shape :)
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Re: NOTICE FOR IBM XT OWNERS

Postby carlostex » 2019-2-10 @ 22:42

I've been poly modding my cards and boards with success. I've been replacing the standard 10uf 16V tantalums with 10uf 25V KEMET branded solid capacitors
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Re: NOTICE FOR IBM XT OWNERS

Postby Errius » 2019-2-11 @ 01:13

I have a card here (AST Rampage 286) that has 73 of these things on it. I have a box full of similar cards next door. This will be a huge ass operation.
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Re: NOTICE FOR IBM XT OWNERS

Postby carlostex » 2019-2-11 @ 01:42

A couple of days ago i did this to my U386 DX board. It had 50+ tantalums on it. With a desoldering gun it becomes an easy job. Its an essential tool to do this kind of job
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Re: NOTICE FOR IBM XT OWNERS

Postby Ozzuneoj » 2019-2-11 @ 01:47

Just wondering, but was this the first time this XT had been turned on for a while? From what I have read about this, tantalums are generally extremely reliable and do not degrade with time the way that standard aluminum electrolytics do. If they fail it is usually from long term storage (20+ years) or poor storage conditions, then being powered up once they have shorted internally. The really obvious and scary way they fail has made them notorious, not necessarily that they have a really high failure rate.

I have only experienced two tantalum failures in all my years of computing. One was a Trident TVGA8900B that popped and smoked about two years ago when I first powered it up, the other was a brand new in box Everex EGA+Parallel card that I got about 3 years ago... that one simply had a shorted tantalum (no explosion or smoke) that I replaced with a random old aluminum electrolytic with the same rating (it was all I had at the time and that card has worked perfectly since the repair). In both situations the cards hadn't been used in decades (the everex card was over 30 years old and had never been powered up) the Trident was a slow ISA card from ~1990 so it's doubtful it was used in the last 25 years.

One day last summer I photographed every single card I had with SMD or through-hole tantalums and cataloged the ratings of their caps in case any ever blew up (they are usually unrecognizable when they go, making it hard to know what to replace them with). I then took ALL of these cards, ISA, PCI, AGP and VLB, outside with some motherboards and power supplies. I covered the setup with a plastic tote to shield myself from the explosions. I took several hours testing cards... probably between 50 and 75 cards from the mid 80s 8bit PC\XT era to late 90's AGP cards with SMD tantalums. I'd swap a card, cover it up, power on the system and wait a few minutes with it running. The result? Not a single failure.

Since then I've tested many more cards, some very odd, very old and very unlikely to have been used in the last 20 years, and I haven't had another explosion or even a shorted cap.

Replacing every tantalum on vintage boards is excessive and unnecessary in my opinion. There are tons of tiny tantalum capacitors in cell phones and other modern electronics. If they were actually THAT bad, they wouldn't still be in use. That said, tantalums are expensive so unless you're a stickler for keeping things looking original, replacing them with a low ESR electrolytic or some other modern capacitor that didn't exist 30 years ago is probably fine. I am not an engineer or all that knowledgeable about the details of these things, I can only tell about my (limited) experience, for what it's worth.
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Re: NOTICE FOR IBM XT OWNERS

Postby dkarguth » 2019-2-11 @ 02:44

I use the XT occasionally, normally for typing up small documents on Word 5.0 and for testing programs for 8088 compatibility.
This particular incident occurred during a file transfer between my 286 machine across parallel, it had been running for about 10 minutes. All of a sudden, I heard crackling, then looked over soon enough to hear a loud "BANG". The screen went crazy and then collapsed vertically, the HDD light started flickering, and I had it all shut down in about a 1 second of hearing the bang. The obvious smell of toasty electronics then followed, with some wisps of smoke. I have since clipped the capacitor off the board (It's just a power filter cap) and tested everything, and it still works fine, as far as I can tell.
I have taken this as an excuse to completely disassemble the machine for a full restoration. I will be recapping the power supply and all of the boards with brand new tantalums, and I will also attempt to repaint the case.
Speaking of, does anyone know how to reproduce the texture that IBM used on the case lid? Mine was stored in a garage before I had it, so it has some large surface rust spots and I would like to repaint it. I am going to take it to the PPG store to have them scan the inside of the case with their color gun, but I just don't know how to reproduce the texture.
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Re: NOTICE FOR IBM XT OWNERS

Postby dkarguth » 2019-2-11 @ 02:46

I have actually had 2 tantalum failures in this particular machine, one was shorted when I got it, but it didn't explode. The machine just refused to power up. This one is the first one that I've had go bang on me.
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Re: NOTICE FOR IBM XT OWNERS

Postby Merovign » 2019-2-11 @ 06:09

I've had one on a 386SX motherboard (I will eventually recap it because it's a SIPP memory board, I actually have 2 similar ones and they're both "dead"), and one in a USR external modem (KAPOW it went).

I'm trying to figure out if it's possible to test them because of my Kaypro, but I may just replace all the power-related ones.

My first choice would be new tantalums, if you're going to use electrolytics I'd just double-check size first because they can be quite a bit larger. Often they're not.

Polymer caps is probably the best bet if you don't mind them looking non-stock.
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Re: NOTICE FOR IBM XT OWNERS

Postby sf78 » 2019-2-11 @ 10:09

I had around a half a dozen PC/XT's a year ago and all but 2 had shorted tantalums on the mainboard. One actually blew up during testing. Can't really say what effect age and not being in use had on these, but it's pretty obvious that these kind of machines haven´t been actively used in the past 20 years. So according to my experience all of these old boards have quite a high failure rate and require some attention.
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Re: NOTICE FOR IBM XT OWNERS

Postby eisapc » 2019-2-11 @ 14:06

I had this issue twice on my boards. Once with only the bang, once with flames on the board afterwords. The worst is not the bang but the catastrophic smell.
Replacement with good brand electrolytic caps should be sufficient, but I did not replace the missing two ones so far.
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