George,
it is not unusual for a 30 plus year old tantalum to fail, and that is
about 20 years past the typical lifetime prediction for electrolytic caps.
In spite of air conditioning, any spaces on a ship or oil platform lab
will have salt air enough to cause conduction problems on circuit boards
of a fine film of salt over time. Trust me, I work for the Navy as a
civilian researcher, and we fight this problem on every at sea research
project.
Now, it so happens that epoxy dipped tantalums in the 80's had a lot of
problems. Your message did not make clear if you are speaking of epoxy
dipped or metal cased tantalum caps. The Kemet metal cased ones were
quite good and met mil specs if installed with correct polarity.
I too, have an instrument from the 1980's with epoxy dipped tantalum
caps. An upcoming project is to shotgun all remaining epoxy tantalum
out of it with new tantalum caps from mil stock or even modern epoxy
ones or molded ones which are better.
That is because it has gotten to having at least one tantalum fail every
time this instrument is powered up.
The tantalum voltage rating should be for DC plus AC peak to peak, which
it sounds like yours were. But that has been a problem for some
instrument circuits.
-Stuart
K5KVH
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