I call those Tantalum caps time bombs.

There are some burn marks on the motherboard near the blue cap close to the one that exploded. It could of course just be remains of the blown cap but better clean to board and take better pictures.

Check some old image to see if its a blue cap or orange cap that exploded, the value will likely be the same as the similar cap near it.

I will let some other more knowledgeable member answer if you can replace it with another sort of cap.

Skyscraper wrote:

Check some old image to see if its a blue cap or orange cap that exploded, the value will likely be the same as the similar cap near it.

I removed the orange one near it already to find out. There are pairs of orange and blue in the same configuration elsewhere on the board so I assumed it was orange but you're right I should check to make sure. I have photos luckily.

This is where having a current limit is useful, either using a light bulb or something more exotic.

Thanks guys. When you say having a current limit, gdjacobs, how would that work specifically?

I found this site, which suggests I'll get away with substituting the old 25v caps with newer 50v caps. I know anything but a direct replacement is a risk given it doesn't match the original spec of the board so I'm thinking that ordering rather than salvaging might be better in terms of longevity?

brassicGamer wrote:

Thanks guys. When you say having a current limit, gdjacobs, how would that work specifically?

I found this site, which suggests I'll get away with substituting the old 25v caps with newer 50v caps. I know anything but a direct replacement is a risk given it doesn't match the original spec of the board so I'm thinking that ordering rather than salvaging might be better in terms of longevity?

Regarding current limiting with a light bulb:
http://www.repairfaq.org/samnew/tvfaq/tvtslbt.htm

As the board predates onboard VRMs, you should be good with using a low ESR replacement capacitor of equal capacitance, equal or higher voltage rating, and equal or higher ripple current compared to the original. Panasonic suggests solid polymer caps in their literature, so you can go that direction if you wish.
http://www.digikey.com/us/en/ph/Panasonic/tantalum.html

gdjacobs wrote:

Regarding current limiting with a light bulb:
http://www.repairfaq.org/samnew/tvfaq/tvtslbt.htm

Good stuff mate, thank you. Will certainly help with testing!

Just remove the remains, the board will work without this cap. In case your soul can't rest without place a 10µF/25V tantalum cap.

I have some boards making heavy use of Tantalum capacitors... This kind of thing scares me, but at the same time, I don't want to butcher the boards. I just hope that if this happens to me the rest of the machine survives.

You can replace them with different types of capacitor of the same (or slightly higher) capacitance. You can also use capacitors rated for a higher voltage if you want to shell out for them, they will probably last longer running at the lower voltage.

@gdjacobs; Ah, the old light bulb ballast trick. I've used that method and old hotplates (for more current) to limit current to things before, never computers though. For some reason I think this was implemented professionally, and by that, I mean I'm sure Land Rover used this technique for something. For some reason I seem to think that some model or other limited current to the glow plugs by using the bulb in the dashboard as a ballast... Rather unusual if that was the case. Might have been something else though.

h-a-l-9000 wrote:

Just remove the remains, the board will work without this cap. In case your soul can't rest without place a 10µF/25V tantalum cap.

As long as it was old cap death and not reverse biased fun cap death (which appears to be the case here).

I'll just reiterate here as elsewhere, I don't recommend running a board without a full set of caps for more than a very limited period of time, as doing so will compromise ripple or transient suppression on at least part of the circuit, potentially causing instability and reducing component life.

Reverse polarity sounds a bit far-fetched, that would be a major mistake with the power supply cabling. These caps at the ISA slots are the 'fear caps' that engineers place when they fear instability. Cheap boards get along with far less HF blocking capacitors 😀
The ISA cards have their own caps for this purpose.

With the DC-DC converters for CPUs and such this is of course a different story, there are real ripple currents.

Oh, and if there is something that tantalum caps hate it's high inrush or ripple currents, this is one reason why plugging the board to a running power supply is a bad idea.

This really isn't a big deal at all, and significantly better than a leaking electrolytic as far as damage is concerned.

Just desolder the remains and put an electrolytic of the same value in there. If I was doing this I would replace the whole lot of
them.

A friend of mine has a 286 board where one of these blew up, I put an electrolytic in it and no more problems.

Rather than electrolytic it's preferable to use a polymer. More comparable ESR.

The VGA card is not the ARK1000 based one. I would guess ET4000W32i or p.

gdjacobs wrote:

Rather than electrolytic it's preferable to use a polymer. More comparable ESR.

I might bother with a newer board but, seeing as the only replacements I currently have are SMT types, I'd rather save the money and get an additional / replacement board (thankfully I know where there is one available).

Caps replaced. No dice. Only dashes on the diagnostics board I'm using - it's acting like a board without a CPU at the moment (even though the CPUs are definitely good). So it seems something else is damaged, although it's not visible if that's the case. What's next? I guess it could be the BIOS ROM is hosed, as that would create the same symptoms.

Check voltages, see if you find any missing or out of whack.

Persistence paid off. Ran the board with the ROM removed just to check it behaved consistently and it did. Replaced it and bingo! At first it wouldn't post with the VLB board so I used an ISA one instead. Worked perfectly. Tried the VLB in a different slot and success. It seems I am the proud owner of an ET4000 W32i.

Really nice! I'd be tempted to track down the hardware to join you, if I had the space. Big nostalgia factor. I had a 486 machine growing up, but never had the chance to trick it out much. Still, lots of fun was delivered on that puppy.

I was talking to my brother yesterday and he clarified that our 486DX4 was the machine we had most fun on. I missed out in the VLB era completely at the time though so I'm having fun playing with the equipment now.

I'm lying. It's a pain in the arse. But I never give up.