Film Capacitors - can they be revived?!

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
 
no age

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.
 
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kevinkr

Administrator
Paid Member
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.
 
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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.
 

benb

Member
2010-04-24 1:52 am
I found this page:
1PB4700 SCR Metalized Polypropylene Cap.
which links to this data sheet:
http://us.100y.com.tw/pdf_file/05-SCR-MKP-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.
 
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.
 
Thanks, friends, for your interest, help and suggestions.

The caps were definitely not stored in wet conditions. They were always stored in a carton box, never in the open air.

Benb, I only measured them recently as I didn't have a capacitance meter. A couple of them were used in speaker crossover modification but were removed because I didn't like the resulting sound. Not sure if it was due to the capacitors. I didn't investigate.

Thanks for your links. I'll decide whether to send them to the manufacturer.
 
I have some large value SCR's that did the same thing. It is because zinc, rather than aluminum or tin was used as the base metal, I am told. This was so unusual, that I sought the answer by asking a cap manufacturer.

That's unwelcome news because I have other large value SCRs (25-50uF) to replace the stock caps in the power supply of Quad II amps. They are still stored...!! I'll check THEM too!!

Thanks anyway for your informed opinions.

JA
 
The caps were definitely not stored in wet conditions. They were always stored in a carton box, never in the open air.
I see on the map, Malta is not big enough to escape the effect of high humidity and salt air.Unless your storage is completely air conditioned. I was given a '59 Ford car in 67 that had been a bakery saleman car in Galveston, TX. Every nut and bolt was stuck and had to be heated, oiled, and in some cases cut off with a crowbar when a chisel wouldn't reach it. A saleman's car never goes to the beach. By contrast, a 30 year old car of the same model I bought here in Indiana in 1990, is relatively easy to work on. An owner on organ forum report that a Farfisa compact organ that might have spent a lot of time in Italy, had the case and legs rotted right off the irreplaceable germanium transistors. Could be the effect of acid flux solder at assembly, could just be the effect of salt air on steel transistor parts. I didn't see any classic car cruises in Maui, Hawaii. The 10 year old cars I did see were real rust buckets.
You should see what the people in the wet parts of Africa and India on pianostreet.com says humidity does to their pianos. Even if they have air - conditioning, the power may not be reliable enough to keep the wood parts straight.Hope you are not out of pocket too much money.
 
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I have some large value SCR's that did the same thing. It is because zinc, rather than aluminum or tin was used as the base metal, I am told. This was so unusual, that I sought the answer by asking a cap manufacturer.

Hi John,

Thanks for the tip.. These are metalized film so is it that the base metal used in the metalization is zinc? I used a bunch of these in the power supplies of some of my tube amps, so far none have failed but now I probably should expect this.

Never liked the way they sounded in my cross-over designs either..

I live a few hundred yards from the Atlantic Ocean, but have had relatively little trouble with corrosion in my gear, however I can often smell the sea here at home..
 
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I see on the map, Malta is not big enough to escape the effect of high humidity and salt air. Unless your storage is completely air conditioned.

Hi indianajo, I accept you have a point there, but OTH why did this happen to this particular range of caps? And all 12 of them? I have other SCRs marked MKP which didn't suffer the same fate. I also have other similar caps from other manufacturers which measure correctly to stated value. All of them were purchased round about the same time (for a cross-over which didn't materialise).

To my mind, all this points to the possibility of something wrong with the construction of these particular items. John Curl's tip could be one of the reasons:

It is because zinc, rather than aluminum or tin was used as the base metal, I am told. This was so unusual, that I sought the answer by asking a cap manufacturer.

Also, thanks Benb for your links. I've written to the manufacturer, more out of curiosity to find out about the problem. Anything else would be a bonus.

Thanks a lot, guys.

Joe A
 
marking

If your caps all read the same value +-20%, then you or your supplier was snookered by the marking, which you thought was 4.7 uf, and actually meant 4.7 nf. The nf value is a much cheaper capacitor. Or they could just be surplussed off mismarked units. Capacitor marking varies by continent, the language they were produced in, decade, and just plain corpe\orate greed. I bought some 0.1 uf capacitors from Newark last year, made by MSR in Korea; they perform fine at that value. They are marked "1k 630H". They are rated at 630V, so I suppose H means volts in Korean.???? 1K pF would be .000001 uf, 1k nF would be 1 uF, so that legend makes no sense at all. The baggie from Newark had the correct marking. I re-marked the caps themselves with a sharpie marker when I installed them.
If your readings are kind of random, maybe these caps are not sealed against humid air, and the others were. The final poly or wax dip is an extra cost, that maybe some manufacturers do, some don't. When I worked in oil exploration in 1973 at the beginning of the plastic cap era, our company was really into "Tex-Caps" which were locally produced for oil exploration. Sprague orange drops were a lot cheaper, but the steel leads would rust. That exploration company used a lot of gold connectors, silver solder, and zip-lock bags to store stuff. Most of their exploration was off-shore.
 
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SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
I have some large value SCR's that did the same thing. It is because zinc, rather than aluminum or tin was used as the base metal, I am told. This was so unusual, that I sought the answer by asking a cap manufacturer.

Zinc isn't used for the film, it's used as a spray on the ends to connect the wound extended conductor. And that's almost universal in film cap manufacture.