I have a '62 U67 that I have refurbished with a new k67 capsule (I
kept the old one as-is and bought a new one from Neumann).
The circuit uses negative feedback and the signal passes not only
through the electrolytic coupling cap for the primary of the
transformer, but feedback passes through a few .01 WIMA tropyfol
(anyone know the modern equivalent) capacitors and high frequency
feedback passes through a Styroflex (anyone know if this is
polystyrene??) 80 pF cap.
I am going to replace the 40 year old electrolytic and keep the
original for an eventual sale of the mic (for those who love old
capacitors).
This kind of circuit is very sensitive to phase shifts etc. so I am
hoping that the mic will improve once I re-cap.

My question: do the foil WIMA caps in the mic and the Styroflex cap
change as they age?
I know electrolytics and paper change but have no experience with the
ageing of plastic/foil caps

Can anyone offer help??

maxdm wrote:

My question: do the foil WIMA caps in the mic and the Styroflex cap
change as they age?

No.

I know electrolytics and paper change but have no experience with the
ageing of plastic/foil caps

Paper caps don't degrade slowly, they just fail abruptly. (And they never
did perform well when they were new). Electrolytics are really the only
capacitors that degrade slowly as they age.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Scott Dorsey wrote:


did perform well when they were new). Electrolytics are really the only
capacitors that degrade slowly as they age.
--scott

Is that simply because as the electrolyte gradually dries out they become
less efficient conductors?

Does this mean that all "solid state" capacitors do not periodically need
to be replaced, and should only be replaced after failure, or for an upgrade
in quality?

I have heard some people suggest that we gone overboard on recapping
equipment. Perhaps a lot of business is being generated by recapping
loder units that really don't need it.

Rob R.

Rob Reedijk wrote:
Scott Dorsey wrote:

did perform well when they were new). Electrolytics are really the only
capacitors that degrade slowly as they age.

Is that simply because as the electrolyte gradually dries out they become
less efficient conductors?

Yes. And they also undergo chemical changes as they age.

Does this mean that all "solid state" capacitors do not periodically need
to be replaced, and should only be replaced after failure, or for an upgrade
in quality?

Only electrolytic types need to be replaced on a regular basis.

Paper caps should probably be replaced because old ones are prone to
failure and they never were all that good when they were new.

I have heard some people suggest that we gone overboard on recapping
equipment. Perhaps a lot of business is being generated by recapping
loder units that really don't need it.

If manufacturers used quality caps in the first place, there wouldn't be much
need at all for this sort of thing. The fact is that there are a lot of pieces
of gear that do need new electrolytics after ten years, which is because of
shoddy design work.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."