Film Capacitors - can they be revived?!

[sonata149]

I still have some 4.7uF polyprop caps which I had bought many years ago. They have been in store for a future project which came too late!! When I tested them recently I found them to be in low nF range! As these parts come from a noted manufacturer I can't believe they have such a short life span when there are so many vintage parts still working perfectly.

Is there a way of getting them back to spec like you do with electrolytics. I have built one such circuit which has successfully brought many vintage 'lytics back to life!! Can I use it to revive non-polarized metal-polyprops? If not, is there another way? It's a pity to throw so much money into the bin!

Thanks and regards,
Joe A

[indianajo]

I don't believe plastic film caps age, at least not since they were invented in 1966?
There are a lot of ways to mark a capacitor, and if you don't have the datasheet the markings are sometimes difficult to understand. Some vendors specialize in mismarked stock; rejects are hardly ever thrown away, just surplussed off to the lesser firms. Nowadays we have E-bay for mismarks and counterfeit.
A 5 uf 50 v cap in 1960's technology should be over 2 cm in diameter and over 5 cm long. I bought some 63V 10 uf film caps last year, new stock from a distributor, they were 1 cm diameter and about 5 cm long.
I see in revision that you live in Malta. You might put your caps in a toaster oven and bake them at the lowest temperature (warm) for a couple of hours and see if you had water in therm. If that fixes them, you need to wax dip them before you use them. High quality paper dielectric caps were wax dipped before 1966.

[Minion]

Unused Polly caps will last 4ever if stored properly and even if not stored properly ......

[kevinkr]

Originally Posted by sonata149

I still have some 4.7uF polyprop caps which I had bought many years ago. They have been in store for a future project which came too late!! When I tested them recently I found them to be in low nF range! As these parts come from a noted manufacturer I can't believe they have such a short life span when there are so many vintage parts still working perfectly.

Is there a way of getting them back to spec like you do with electrolytics. I have built one such circuit which has successfully brought many vintage 'lytics back to life!! Can I use it to revive non-polarized metal-polyprops? If not, is there another way? It's a pity to throw so much money into the bin!

Thanks and regards,
Joe A

Possible these are actually 4.7nF caps? 4.7uF films in higher voltage ratings will be quite large - how large are these?

I have very old film caps that measure well within their stated values and tolerances.

I'd not use them if they are wet internally as I expect the foil or metalized film would have corroded quite badly by now.

American Radionics film caps in ceramic tubes were used in HH Scott audio gear as early as 1961 or perhaps even earlier. These are noted to be somewhat unreliable because the terminations become intermittent, a shame because they are otherwise quite good caps.

[davidsrsb]

Moisture would cause the leakage to go up - easy to check with a ohmmeter.
If the foil is corroded away, its gone for good and you can"t repair the caps

[sonata149]

The capacitors measure 4.5 x 1.7 cm dia and are clearly marked 4.7uF +/-5% 150VAC 250VDC. The capacitors are well sealed and there is no apparent damage. I noticed the problem when one of the channels of a preamp I built was not emitting any sound. The output cap was the culprit. Took six of them and checked them for capacitance. They were all out of spec.

From what you say, the only explanation I can think of is that because humidity levels are often high (my country being a small island surrounded by sea) they could have become moist. Checking them with a dvm I get infinite resistance on the 2000K range. So leakage doesn't seem to be the cause.

[Elvee]

Originally Posted by sonata149

As these parts come from a noted manufacturer I can't believe they have such a short life span when there are so many vintage parts still working perfectly.

BTW, what is the manufacturer in question?

[sonata149]

They are SCR PB470s manufactured (?) 12.87.

[benb]

I found this page:
1PB4700 SCR Metalized Polypropylene Cap.
which links to this data sheet:
http://us.100y.com.tw/pdf_file/05-SC...Capacitors.pdf

It looks like a very good sealed cap that (when kept in storage or operated within spec) nothing should go wrong with.

Perhaps high humidity with the caps in storage (no voltage across them) could have an effect, but it seems that would also cause significant leakage current. I would guess this could be a manufacturing problem. Do you recall testing the capacitance, or testing or using any of these caps when you first got them?

Poking Google a little further, there's a dealer for this brand right here on diyaudio:
MTZ Audio - SCR
which lists the manufacturer's site (click English for an English-readable site):
La société des Composants Record, fabricant de composants électroniques condensateurs, condensateur acoustique, condensateur laser, condensateur de puissance, condensateur de haute tension,resistances, resistance bobinee syliconnee, resistance bobine

At this point I'd contact the original dealer you bought them from and/or the manufacturer and see if you can get replacements. The manufacturer might want these back to see what went wrong.

[kevinkr]

That's unfortunate but if they have been stored in a wet environment I am not too surprised to hear they have failed, the SCR types are not guaranteed to be hermetic.

The possibly better news is that there are some caps made by Clarity, the Clarity SA for example that is much more transparent for just a little more money than the SCR types you currently have. They have even better types available although they are more expensive.