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286 Motherboard repair - Please assist

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First post, by Paul92

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Hi all.

I was given an old 286 desktop which looked like it had been sitting gathering dust on a shelf for the past 20+ years.

When I plugged it in & powered it on, I heard an audible bang, and funny smell.
I assumed it was the power supply, and opened it, but apart from a loose +5v wire, which I rejoined again the power supply seemed to function fine. It only doesn't function when it's connected to the motherboard.

But upon closer inspection of the motherboard, I found something had blown which looks like an old capacitor. Faulty-286.png
I have attached a photo.

Please let me know if it's fixable, or if there's more likely more damage. My knowledge with resoldering motherboards is not that great, but if given the proper guidance, I should be able to repair.

Thanks

Reply 1 of 27, by kalohimal

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You could try cutting out the blown cap to see if the board works. These tantalum caps love to blow up when not used a long time (they fail short). The blown one is mostly just a bypass caps (since it is next to the power connectors) so the board would still work after it is taken out. But of course for long term it needs to be replaced.

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Reply 2 of 27, by computerguy08

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A CR2032 battery holder on a 286 board, that is something you don't see everyday!

Could you take a photo of the entire board ?

And yes, the board should work just fine after you replace that capacitor. Tantalums are known to blow up when they age.

Reply 3 of 27, by Deksor

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Well it may work fine when you replace that cap ... But it's likely that there are others that are shorted somewhere (or that will be shorted a bit later). You should replace all of them just to be sure none will fail anytime soon.

Use a multimeter to check for any other short.

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Reply 4 of 27, by Jo22

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I highly recommend using a POST card with extra LEDs for the voltages.
Even if it doesn't show any POST codes, the LEDs might give a hint or two, so you can narrow down the issue.
Usually, the LEDs that do not light up or are dimmed are suspicious.
If so, check the voltage line that the Led belongs to.
Certain devices use certain voltages.

Good luck! 🙂🤞

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Reply 5 of 27, by canthearu

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This board will probably work fine once the shorted tantalum capacitors are removed.

Remove the burnt capacitor, check all rails for more shorts, and as long as there isn't any more shorts, give it a go with your PSU.

I wouldn't bother replacing any tantalum capacitors that are not shorted. More likely to do more damage to the board unless you are really good with a soldering iron.

Reply 6 of 27, by Deksor

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Well I've had some good tantalum caps that failed on me afterwards so not changing them is a risk of having to deal with the same problem with the every other caps (law of series).

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Reply 7 of 27, by Intel486dx33

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It’s probably this one. If you dont want to fix it then sell it on eBay. I have a 286 motherboard right now with these same type of capacitors blown I am trying to fix.

Reply 8 of 27, by kalohimal

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Agree that the tantalum caps that had yet to fail shouldn't be replaced. They will actually self recover with use, meaning the more you use the better they become.

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Reply 9 of 27, by TheMobRules

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kalohimal wrote on 2020-07-29, 18:07:

Agree that the tantalum caps that had yet to fail shouldn't be replaced. They will actually self recover with use, meaning the more you use the better they become.

Hmm this quote is interesting:

Wikipedia wrote:

Impurities, tiny mechanical damages, or imperfections in the dielectric can affect the structure, changing it from amorphous to crystalline structure and thus lowering the dielectric strength. The purity of the tantalum powder is one of the most important parameters for defining its risk of crystallization. Since the mid-1980s, manufactured tantalum powders have exhibited an increase in purity.

This may explain why tantalums from early IBMs are so prone to short and/or explode compared to those in boards from the 90s. In my experience tantalums that fail on more modern boards have usually been subjected to voltage spikes or poor storage conditions, while those on IBM PCs from the 80s are almost guaranteed to blow up at some point.

Reply 10 of 27, by Intel486dx33

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Yes, capacitors that have been damaged by leaking batteries tend to fail. Interesting that this one is near the battery.
Was it originally a barrel battery that had leaked ?

Reply 11 of 27, by debs3759

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Intel486dx33 wrote on 2020-07-29, 21:47:

Was it originally a barrel battery that had leaked ?

The circle on the board indicates that it was always a C3032 type battery

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Reply 12 of 27, by Horun

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debs3759 wrote on 2020-07-29, 22:05:
Intel486dx33 wrote on 2020-07-29, 21:47:

Was it originally a barrel battery that had leaked ?

The circle on the board indicates that it was always a C3032 type battery

Disagree. I have a odd old board where the barrel battery was actually mounted vertical and had an odd built in "strap"that ran down the side to one of the solder points. Also the coin cell holder does not fit into the screen print circle and is crowding other components, that is not typical, neither is a coin cell holder on a 286 board with chips dated 1987 (the 74c245 dated 8714). It could be very possibly that the reason that particular Tant cap failed is because someone changed the batt and actually caused some damage to that Tant. I have boards as late 1994 that still used barrel batteries, the earliest with original coin cell holder is 1993. My most recent 286 aquistion is from 1990 with 4 x 30 pin simm slots and it has a barrel battery. Not saying it was not possible but 1987 is way too early for a coin cell holder....

Paul92 wrote on 2020-07-28, 07:26:

Please let me know if it's fixable, or if there's more likely more damage. My knowledge with resoldering motherboards is not that great, but if given the proper guidance, I should be able to repair.
Thanks

Please post a picture of the entire board !

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Reply 13 of 27, by kalohimal

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Actually I have a 286 that has a coin battery holder onboard too. It was bought new in 1988 and had never been modified, except one tantalum had exploded and replaced (the one near the "pass" sticker). I think during the late 80's some manufacturers used them, but when 386s came along they switched to the dreaded barrel battery.

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Reply 14 of 27, by maxtherabbit

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+1
coin cell RTC batteries were certainly not unheard of in the late 80s.

I have an AST SixPackPlus for the IBM PC/XT that uses one on the card, as do many other RTC add-on cards of the era

Reply 16 of 27, by kalohimal

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Sorry I got mixed up. The manual above with blue words is for my XT board 🤣. The 286 manual should be this one.

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Reply 18 of 27, by kalohimal

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Ok sure, please give me some time, I'll scan the 286 manual first. (I knew that was coming 🤣)

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Reply 19 of 27, by kalohimal

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Here you go. They are too big to attach here so I put them in my google drive.
Baby286MB pdf manual
XT Turbo 10MHz pdf manual

Are they for the Ultimate Hardware 2019 project? If so, let me know if you need these too.

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Slow down your CPU with CPUSPD for DOS retro gaming.