John wrote:
Replacing known good filter capacitors is a zero-sum exercise at
best. New aluminum electrolytics are relatively expensive, and
you'll be replacing parts that are probably near the bottom of their
bathtub-shaped reliability curves with parts that are definitely on
the left side of theirs.
* * * After my first few encounters with high-ESR parts out
of the retail box, I stopped replacing good ones.
I have no actual statistics to offer in support of either side of
the question, but in my own case, it certainly hasn't cost me
anything to leave good electrolytics alone.
I basically concur with John. You can get good, reliable aluminum
electrolytics, but you need the sort of Q/C knowledge that hobbyists
rarely have. HP/Agilent has a fine reputation for sourcing good
parts, and very rarely has been caught out by batches of bad parts
(unlike some other well-thought-of test equipment makers). So the
parts in your 3586 and 5370 are likely about as good as you can
do. I would NOT recommend shotgun replacement, absent evidence that
your particular 3586 or 5370 was a victim of a batch of bad parts,
and one failure -- regardless of how spectacular -- does not
constitute such evidence. For that matter, do you have actual
forensic evidence that a capacitor failure caused the damage, or is
that just a guess based on what you think is the likliest
suspect? Perhaps it was a diode (as in a spectacularly-smoked 5345
of mine) or other component.
It is very rare for a failed electrolytic to cause the devastating
damage that your 3586 suffered, so replacing them on an as-fail basis
is a much better strategy. Don't let the shock value of the
magnitude of the damage blind you to a rational analysis of the
probability and potential severity of future failures.
Finally, I would caution against wholesale replacement of tantalum
caps with aluminum electrolytics. Tantalums have a number of
different properties compared to aluminum electrolytics, and it is
not always obvious which difference led the designer to choose the
tantalum for any particular location. So, even if one decides to
shotgun the electrolytic capacitors in a unit, it is probably best to
replace tantalums with tantalums. Even the best, low-ESR aluminum
electrolytic may not do the job if the designer chose the tantalum
for reasons other than low ESR. Do you consider yourself to be a
designer of sufficient talent and experience to second-guess an HP
design team? (I'm not suggesting that nobody is -- just asking if
you are confident that you are.)
Best regards,
Charles
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