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Why do wax capacitors become leaky?
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July 18, 2017
10:09 pm
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PYE625
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Including Philips tar capacitors and Hunts Mouldseal, etc,etc.

Probably a silly question everyone know’s the answer to….. It’s because the paper dielectric breaks down and allows DC to pass…. Right?

But why?  Is it moisture ingress?  Is it trapped moisture rotting the paper?  Even some sealed oil capacitors can become leaky so it can’t be moisture there can it? 

And yet, it is sometimes (probably rarely?) possible to find wax capacitors that are not leaky and are perfectly ok. I found that with a 50’s Ekco radio some time back and was quite surprised.

Your thoughts will be very welcome and interesting wink

Andrew.                                                                                   

                         

July 18, 2017
10:21 pm
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nuvistor
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No scientific knowledge but I thought it was moisture. 

Frank

July 18, 2017
11:24 pm
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acj1980
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I think it could be the isolate material that over time getting bad, another thing could be the air leaks in between craks in the wax/tar, 

the same problem is on yellow wax capacitors from Wicon from the 1950ies

 

Alex 😉 

July 19, 2017
1:05 am
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Marconi_MPT4
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Together with moisture, it is likely when these capacitors were manufactured fifty odd and more years ago, contaminants in the atmosphere and present in the production process have started to degrade.

Some may even support mould growth that permeates through the paper and most likely cause a conductive path. Even if no contaminants were originally present these could be easily be later introduced from the environment when moisture is absorbed.

It has often been found that wax paper capacitors survive better if the equipment is in continuous use.

Rich

July 21, 2017
6:02 pm
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PYE625
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Marconi_MPT4 said

It has often been found that wax paper capacitors survive better if the equipment is in continuous use.

Rich  

Now that is interesting because I have had similar findings with Hunts mouldseal (black 0.1uf) and TCC oil (0.1uf) capacitors in vintage Quad Hifi. 

Quite often this equipment has had frequent use over the years and it is unusual (or used to be, not so long ago) to find any caps that were badly leaky. I certainly remember that it was not standard practise to replace caps unless they were faulty whilst I was at Quad.

Andrew.                                                                                   

                         

July 21, 2017
6:30 pm
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nuvistor
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I suppose keeping it warm keeps the damp out. Certainley in the 60’s paper capacitors were not changed on spec but the ones that could damage the set were tested, I.e audio couplers.

Only exception I can readily think of were the toffee Wima caps in Grundig tape recorders. It usually wasn’t worth looking for electrical faults until those had been replaced. Then a quick mechanical service and the recorder was capable of giving the excellent results they were known for. There were not that many to change and this was done with the consent of the owner.

Frank

August 6, 2017
10:28 pm
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Till Eulenspiegel
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The first attachment shows four capacitors which were taken out of my Pye model 65A battery table radio. The second and third pictures show the state of the chassis before restoration started, it’s a bit rusty to say the least. The leakage of the capacitors was down to less than 20Kohms, thus totally useless.

To restuff the TCC type 343 0.1mfd 350volt capacitors I’ve chosen the RS Components type 185-4347, these are small enough to fit inside the original capacitor tubes.   These old capacitors don’t look too pretty but I think they are still better than having those yellow hi-viz capacitors in vintage receivers.                                                                      Waxies.jpgImage Enlarger

 Pye65A_3.jpgImage Enlarger
Pye65A_7jpg.jpgImage Enlarger

Till Eulenspiegel.

August 7, 2017
8:51 pm
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PYE625
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Till Eulenspiegel said

 These old capacitors don’t look too pretty but I think they are still better than having those yellow hi-viz capacitors in vintage receivers.                                                                       

Till Eulenspiegel.  

 With you all the way on that one Till. thumb_gif

Andrew.                                                                                   

                         

August 8, 2017
10:20 am
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Till Eulenspiegel
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As an alternative to the hi-viz capacitors it might be a good idea to contact the Lancashire firm Suppression Devices Ltd.  The company makes polypropylene capacitors in black cases instead of the usual yellow.  There’s no doubt these will look a lot better in vintage equipment.

The companies’ products are not as cheap as the offerings from the Far East but the general purpose capacitors should not be much more expensive than the capacitors on sale from RS.   A decent quantity order will be required, say +500 pieces.

http://www.suppression-devices…../index.htm

Till Eulenspiegel.

August 9, 2017
12:47 pm
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nuvistor
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An interesting comment from UKVRRR  member SueButcher post 17 on how contact with air degrades paper. If moisture  can get inside wax caps so can air.

http://www.vintage-radio.net/f…..p?t=138092

Frank

August 9, 2017
8:51 pm
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occiput
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It’s possible that the effect referred to has some influence though I would imagine it to be quite small.  Archives, which tend to be full of paper, are frequently controlled for temperature and humidity, but I’m unaware of any that are evacuated of air when not in use.

The main problem with paper as a dielectric is, as I have always understood, that its principal constituent is cellulose, a material with a prodigious affinity for water.  The atmosphere is, of course, full of water almost all the time, especially in this country, so the problem becomes that of keeping the two well apart.  Wax, which tends to be quite good at repelling water, is just a cheap means of doing this.  I don’t imagine for a moment that it even crossed the minds of the engineers concerned to wonder how the properties of the waxes they used might change over the course of 70 years or so.

It’s worth remembering that domestic electronics were built down to a price in a competitive environment, and so therefore were the component parts.  For someone like Hunt’s or TCC, getting an order for, say, 50 million 0.1 microfarads at 350V might mean shaving 1/10d per unit off the line-end price of a component where lots of other people had already had a go at doing the same thing.  Viewed in this light, it’s perhaps not so surprising that the components no longer meet specification after 5, 8 or maybe 10 times their design life.

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