A particular board that I'm currently involved in repairing, makes use of a
couple of 15nf 630v dc caps in the primary side of a smps transformer. They
go open or low value, and are easily found initially by running an ESR meter
over them. Good ones go about 38 ohms, bad ones over 80 ohms or beyond the
range of the meter. Once you've got them out, a value check will confirm
that they are faulty. The originals are those 'boxy' blue metalized film
ones with 15mm pitch on the leads. Some are fitted with the shiny brown
epoxy dipped metalized film types, which interestingly, don't seem to fail.

My problem is that I am having difficulty finding any suitable replacements.
I can find blue box metalised film caps with a 630v dc rating, but 15nf
seems to be a rare value. Also, they are smaller than the originals, and
have only a 10mm lead pitch, and legs that are too short to be able to open
out to 15mm pitch. I can't find any 'dipped' ones at all in that value /
voltage rating.

What I have found is an X2 rated metalized film cap in that value which
quotes an ac rating of 300v, and a dc rating of 630v. Its from BC, and it's
their 2222 338-4 Series. It is the right physical size, and seems to work ok
in the application. Thing is, when you run an ESR meter over these caps,
they go about 80 or 90 ohms. Now I know that you can't trust the reading of
an ESR meter that's designed to evaluate large value electrolytics, on a
small value film capacitor, and the fact that I was using it at all, was
just a matter of convenience in that it quickly gave a useful indication of
the state of the caps, without having to dig them out of the board. So, am I
seeing this much different figure on the ESR meter, because of the way the
meter works, and the fact that these caps are designed to be low inductance,
whereas the originals are just bog standardly constructed ? Can anyone see
any problem in using an X2 capacitor in a pulse application like this, given
that they supposedly have a superior pulse characteristic, and claim to be
low loss in pulse applications ?

Opinions for discussion, appreciated.

Arfa