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Electronics => Beginners => Topic started by: requim on November 30, 2011, 02:05:27 am

Title: Exploding Tantalum Capacitors
Post by: requim on November 30, 2011, 02:05:27 am
Today I received a AMD 386DX-40 cpu and motherboard today.  I anxiously connected a video card, monitor, keyboard, ram, and power supply into the board.  I thought things were good when I turned it on and saw the keyboard lights illuminate on and off like they normally do.  Unfortunately however about a second or two later a tantalum capacitor used to filter the power coming in from the power supply exploded with my face less than a foot away.  Talk about some unexpected excitement.

My question is this -- there are a number of tantalum capacitors on the board (40+) -- Should I go ahead and replace them all now so I don't run into the same excitement again any time soon or should I wait it out?  And can I substitute a different type of capacitor or do I need to replace with them with new tantalums?
Title: Re: Exploding Tantalum Capacitors
Post by: Zad on November 30, 2011, 02:50:53 am
Tantalums are usually pretty reliable unless you reverse the power or over-volt them. Have you checked what voltage your PSU is putting out?

What is the story behind the 386DX-40 anyway, getting an old machine running for nostalgic reasons?
Title: Re: Exploding Tantalum Capacitors
Post by: requim on November 30, 2011, 03:47:05 am
The numbers aren't looking great.  They are as follows:

+5V       +5.35V
-5V        - 4.51V
+12V    +11.42V
-12V      -10.70V

So everything is low except the 5 volt rail.  Could a 7.5% increase over spec have caused the cap to blow?
Title: Re: Exploding Tantalum Capacitors
Post by: requim on November 30, 2011, 05:55:45 am
Oh and the story behind the 386DX-40 is mostly nostalgia.  I've been building retro pc's and loading them up with the best components I can find trying to make them the penultimate example for each platform.  The 386DX-40 was the first PC I ever owned so it has a soft spot in my heart.  I've primarily focused on building DOS and Windows 98 SE machines however I'm still working through the kinks.  The primary reason for my efforts was to play my old Sierra games on the original platforms.  Emulation is nice and all, but it's not perfect, hence the machines.

Title: Re: Exploding Tantalum Capacitors
Post by: Rufus on November 30, 2011, 08:26:46 am
Tantalums are usually pretty reliable unless you reverse the power or over-volt them. Have you checked what voltage your PSU is putting out?

Tantalums are not very reliable. When sourced from a low impedance supply faults in the dielectric are prone to produce runaway thermal heating and exploding capacitors rather than self-repair. It isn't unusual for tants which have been sitting with no bias for a long time to fail when bias is next applied. I once shipped a product containing a new industrial PC to Saudi and it ended up sitting on the docks there for a couple of years while they had the gulf war. It was fully tested before it shipped but when they powered it on smoke came out, a tant on the backplane failed catastrophically.

For the OP I assume the board has been unused for a while, wouldn't bother replacing the others they are less likely to fail as time (under bias) goes by.
Title: Re: Exploding Tantalum Capacitors
Post by: requim on December 01, 2011, 03:48:16 am
While I don't know the history of the board, I think it's safe to assume it hasn't been used in ages.

How about replacing all the tantalum filter caps for the power?  There are about 8 or so of them. 

Also is it possible for the power supply to have caused the damage?  Should I be worried about damaging other boards (it has seemed to work fine on other motherboards I've used it on) by continuing to use it?  If so I'm willing to rip it open and replace the caps on the power supply.  I'll probably do so anyway since the numbers aren't looking too good. But I'd still like to know what the thresholds are for a computer power supply.  Anyone?
Title: Re: Exploding Tantalum Capacitors
Post by: benemorius on December 01, 2011, 04:32:23 am
I wouldn't worry about the power supply. The tolerance for the 5 and 12 volt rails is +-5%, so yes that one rail is a bit high and just might have been a contributing factor to the explosion, but it really isn't that far out of spec. Assuming you aren't relying on this machine for anything, you may as well just leave it as it is unless you really want to open it up.
Title: Re: Exploding Tantalum Capacitors
Post by: requim on December 01, 2011, 07:01:11 am
I'd just like it to be closer to spec.  I have been using it on a variety of motherboards and would rather not risk damaging any of them if the power supply was indeed the cause of the exploding capacitor.  So if I need to open it up and replace some caps then that's what I'll do.  Thanks for the info.
Title: Re: Exploding Tantalum Capacitors
Post by: Afrotechmods on December 05, 2011, 12:42:56 pm
I would definitely worry about the power supply. Most people check them with a multimeter or the onboard voltage monitoring and if they see the voltages within a few % they assume everything is fine. This is not necessarily the case because those voltage readings represent an average voltage. I have tested many supplies that put out near perfect 5V and 12V readings on the multimeter, but as soon as I viewed the output on an oscilloscope there was anywhere from 500mV to 1V of nasty high frequency spiking. Check the supply on a scope or swap it out with a known good one.