Datron 4808 Fail 9 - shorted tantalum bead capacitor but with 10V margin

[perdrix]

Simple problem - either the +15V or -15V in-guard supply is overloaded.  Time to play hunt the tant!

And of course the part that had gone short-circuit was a 10uF 25V tantalum bead capacitor on the +15V rail.  It had a voltage rating of > 50% over the rail voltage.   I thought this should have been sufficient to protect against the classic problem with these parts (failing short when little or no voltage over-rating).   I replaced it with a 35V part which is >100% headroom.   Should I just put it down to "old age"?

Your thoughts?
David

[fzabkar]

Yes, I've seen hundreds of failures in these caps. I saw one burn like a match head while testing a PCB in the workshop.

[TheDefpom]

I did a big repair series fixing my Datron 4700, I replaced every single 25V tantalum in the unit with a 35V, I suggest you do the same...
Also re-cap the main power supplies whilst you are at it, there could be excessive ripple coming through, don't rely on testing the caps low voltage testers don't tell you everything, measure the supply ripple with a scope.

[bdunham7]

I recently repaired a Fluke calibrator and replaced more than 100 tantalum caps because of these failures and I think that margin is not enough.   I used Kyocera AVX 50 volt caps for almost every instance, the only exceptions being a few caps larger than 10uF in less than 10 volt circuits that were not available higher than 35V.  I replaced a number of the larger tantalums with electrolytics and had no issues.

[TiN]

Just replace them all. Burning down calibrator or even your home not worth the tantalum cost.

[Testtech]

What color is the bad part? Yellow, blue, something else?

[perdrix]

It was yellow (FWIW).

PS TheDefPom suggested that I re-cap the main power supplies.   Unless there's an easy way to get those boards out without complete disassembly I'm very wary of attempting this.    Even removing the rear panel is a pain as there's not enough length in the leads soldered to the connectors to allow you to pull it more than about 1" to 2" away from the rest of the calibrator.

D.

[Testtech]

The reason I asked about the color, I have seen many more of the blue tants fail than the yellow, but they all are suspect.
I guess I am going to have to bite the bullet and recap my 4800A.
Would you have a parts list to share?

Thanks,