[Premium-Rx] Cubic R 2411/R 3030 blows caps

[Geoff Fors]

You would think that a 35V tantalum would have more than enough headroom for
a 16V rail, but...

This is becoming really common these days.  I suspect we are talking about
those mustard or blue colored dipped radial lead capacitors which look like
wooden match-heads.  The leading cause of malfunctions of equipment serviced
by a local military test equipment facility where I used to work was failed
tantalum caps.  It got so bad that we couldn't send a newly serviced and
calibrated instrument out to the customer without worrying that it would
"bounce" because a tantalum would short out on location, including ones we
had already replaced.  Specifically, in Datron and Fluke calibrators and
high-end DMM's. We bought fresh "quality" caps from Digi-Key and Allied and
the failure rate dropped dramatically, BUT, not totally!  We finally started
increasing the voltage spec on the replacement caps at least 10-15V and
sometimes higher and the problems  quit.  These caps are typically used as
decouplers/filters on assorted voltage rails and usually have a resistor of
say, 100 ohms in series.  They take the resistor out when they short.  My
practice these days is to use a cap of 2X the original's rated voltage if it
will physically fit.  Statistically, we noticed that for some reason the
biggest failures were with 10 mfd 16V and 25V caps, and replacing them with
a 35V stopped the problem.  I also wonder if they need some sort of
re-forming process such as with electrolytics, because the failures didn't
seem to occur in day to day use, they occurred when the equipment was taken
out of long term storage and used for the first time.

About 8-9 years ago, more or less, there was some sort of worldwide issue
with a tantalum shortage and such capacitors became far more expensive and
the lead times from the manufacturer grew dramatically.  One of our local
manufacturers resorted to buying reels of tantalums on eBay because his
suppliers couldn't deliver.  I have wondered if the quality of tantalum
capacitors began to slip at that time and that's part of the problem.

Anyway, I certainly concur with Todd.  Replace shorted ones with as high a
voltage replacement as you can get, within reason, and the problem should
stop.

Geoff
WB6NVH