While stripping out parts from a old valve stereo amplifier noticed these capacitors. The amp has not been used for at least 30 years yet all of these Ducon capacitors in the amp are swollen.

Just put this up as a curiosity, have no intention of using these caps. On the other hand the white UCC polyesters look to be in good order.

I can't say I have seen that myself on capacitors before. Interesting.

How do the swollen caps test?

The demarcation line seems to be consistent, one wonders if they were not made that way the UCC's are likely all paper types.

Only answer is to leakage test & measure capacity (after you have discharged them),

Would be interesting to slice one open longitudinally with a thin sharp blade and photograph that.

Was ratting other parts for a project which is under way and not yet completed. Yes it is curious why these Ducons are so bulged, still busy with the project so it will be a while before I get around to further testing the Ducon caps.

Cutting one open with a sharp knife would be a useful option.

Thanks for the comments so far.

Actually those blue Ducon polyesters are, in my experience, always good. I have NEVER had to change one. Swelling is strange though. Might have happened in production.

Same goes for the dipped Ducon polys (like Philips mustards) in dark red or yellow.

Similar UCC polyesters (e.g. Di-Pol 100) are normally OK too, as are those green or white "Terathene" caps.

BUT

Watch out for white UCC "Hi-Qual" caps. They are paper! And always bad. Replace on sight!
Used in a boost cap position, they will swell up very quickly.

Dug out the Heathkit IT-11 capacitor tester along with the 240/110 stepdown transformer and the instruction manual and did a bit of capacitor testing.

Rarely use this tester and my skill with it is not great.

The blue DUCON's measured capacitance and leakage with no problems. The DUCON'S are labelled as polyesters.

As well the UCC "Terathena" type TFC 4863 or 4563 also measured quite ok both capacitance and leakage.

Think the UCC's are polyester type capacitors as well.

Guess the bulge of the DUCON's must be part of the manufacturing process.

Interesting exercise, the amp must easily be a 1970's manufacture and these old Australian made capacitors are still good.

Circa 1964, actually.
Once the changeover to plastic dielectrics happened, Australian manufacturers made reliable caps.
There is no reason to expect that those caps won't last another 60 years.

However, the only reliable small resistors made by anyone in '64 were Philips carbon film.types, recognisable by the prominent end caps.
IRC / IRH, Morganite and Ducon carbon composition resistors (all seen in the above pics) should always be checked.

In the late 60's IRH changed over to metal film technology, which they had developed at their facility at Kingsgrove. A huge improvement in reliability followed.

Unless the white UCC has Polyester written on it, it is just as likely to be paper.

No, Marc, the only white plastic sleeved UCCs that were paper are labelled "Hi Qual 100".

Have done a search re the UCC Terathena caps but nothing came up. It appears they may be poly's as per Ian Robertson so will hang onto them. Will keep the Ducon's as well, these old caps could be used in low voltage non critical applications.

Snipped out a few more and tested them as compared to current generation polys. Surprised as to how consistent in their capacitance values these old caps are. All pass the leakage test with no difficulties.

Given them a clean, marked as tested and put aside for further usage.

Interesting exercise and thanks to all for the comments.