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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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4th Mar 2018, 9:50 am | #1 | |
Dekatron
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Capacitor problems
While reading through some old copies of 'Television' I came across an interesting reply regarding moisture ingress of capacitors.
The set in question dates from late 1954, the article is from 'Your problems solved' December 1972. Quote:
Mark |
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4th Mar 2018, 10:25 am | #2 |
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Re: Capacitor problems
They went leaky in use, never mind being stored, wax paper capacitors of most makes suffered, some more than others, the brown toffee shaped Wima’s were, shall we say, not very good, perhaps worse than Hunts.
Yes a well known problem many years ago.
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4th Mar 2018, 10:33 am | #3 |
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Re: Capacitor problems
Faulty wax capacitors can make equivalent circuits of 4 components:
THe problem with the paper capacitor (and you can totally ignore the wax) is that paper contains salts. When it is dry, it is a pretty good insulator. Add H20 and you get a weak electrolyte. This causes the value of the original capacitor to increase by a factor or 10 approx. however it acts like a higher value capacitor with about a 1 meg or more series resistor, that is placed in parallel with the original value. Then because it has parallel leakage, this adds a parallel resistance. So what you end up with is: The original capacitor value, a series capacitor of about 10 times that value with a large resistor in series with it placed across the original capacitor.. And a pararallel resistor across the capacitor's terminals representing the leakage from the electrical equivalent circuit perspective. So, if you have a paper capacitor that has absorbed H20, it can only be modelled as a new network of 4 components, with varying proportions for three of the values, except for the original value partially masked by the other 3 added R & C values Of course, in some circuits, no issue would be noticed, especially say If in an audio circuit where the grid resistors were 100k or less. But it really shows up in timing circuits, like vertical or horizontal oscillators in TV's where the impedance in the oscillator circuit might be 1meg and as a result the frequency goes way low due to the larger value created, despite its large effective series resistance. Some years ago I recorded the effects of this in wax paper capacitor's displaying two time constants in high impedance circuits, It actually represents an extreme case of dielectric absorption when a paper capacitor absorbs water, which is prevented if it is immersed in oil and becomes a Pio capacitor. This is why PIO types give a lot less trouble than wax paper types. |
4th Mar 2018, 11:43 am | #4 |
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Re: Capacitor problems
Yes the wax doesn’t matter, those Wima caps had a hard brittle covering, some Philips ones had a tar like substance and Hunts used a plastic.
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4th Mar 2018, 11:56 am | #5 |
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Re: Capacitor problems
Just keep out the damp air. Oil filled and hermetically sealed line output transformers give few problems. The actual construction of most wax capacitors allowed for a 10 year life but these are are now over 50 years old!
I well remember the 'Plastiseal' capacitors giving loads of trouble when I was first in the trade. They were only 5 years old and resulted in massive leaks resulting in frame creep when warm. It was very noticeable with Sobell early double sided print receivers [TPS series] but these now perform with 100% stable linearity with good capacitors, 50 years on. John. |
4th Mar 2018, 12:02 pm | #6 | |
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Re: Capacitor problems
Quote:
Although waxed paper caps had been superseded by the plastic encapsulated type, these seem no better. I had a nasty experience with one of those blue & white plastic caps that exploded & narrowly missed my face when working on a set! The only vintage types that seems to hold up well is the Sprague metal clad type & the Philips 'Mustard' caps. Mark |
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4th Mar 2018, 8:04 pm | #7 |
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Re: Capacitor problems
The worst ones I have come across in American TV's are the "Bumble Bee" capacitors which are a paper type sealed in a black plastic case with stripes of colour hence the naming. These ones appear to absorb more water over the years than most. How long paper caps last, if they are not perfectly sealed, might have a lot to do with the paper quality and how it was prepared prior to using it.
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4th Mar 2018, 8:58 pm | #8 |
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Re: Capacitor problems
A couple of WIMA caps were used in the Decca DM35 series. They used to 'snap' short circuit resulting in either a frame collapse or uncontrollable brightness. That was a long time ago. J.
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4th Mar 2018, 11:17 pm | #9 |
Triode
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Re: Capacitor problems
From what I can remember. And it is a long time ago the Wima toffee capacitors gave a lot of trouble in Decca sets DM45 etc. Strangely they were ok in Grundig tape recorders and radios.
Wax capacitors failed randomly the blue and white ones not so good either. Philips mustard coloured were the best I don't think I replaced more than two or three in 40 years servicing ! |
5th Mar 2018, 9:37 am | #10 |
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Re: Capacitor problems
I think the most troublesome caps are those nasty brown or black Hunts caps used widely in most electronics through the 50's.
The plastic casing always cracks and falls off, making it impossible to identify the value without a circuit diagram to hand, they tend to go off like little firecrackers These are definitely replace on sight, most if not all will have turned into resistors, or be short circuit. The photos below are from a couple of my recent restorations, the first photo is from a Pye 'Black Box' amplifier, I replaced these before attempting to power it up, these caps will destroy these amps in seconds! It is not uncommon to find open circuit output & fried mains transformers in these due to these caps. The second photo shows on the left, the amount removed from my Marconiphone VRC74DA, these caps caused many problems in the RF deck back when the sets were quite new! Mark |
5th Mar 2018, 10:46 am | #11 |
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Re: Capacitor problems
That is interesting, my main exposure to the toffee Wima capacitors was in Grundig tape recorders, I had plenty failures in those.
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5th Mar 2018, 10:50 am | #12 | |
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Re: Capacitor problems
Quote:
I never found any paper capacitors that reliable, and that’s in their service life.
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5th Mar 2018, 7:40 pm | #13 | |
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Re: Capacitor problems
Quote:
I guess the degradation is due to the heating & cooling of the set over time, causing the casing to crack, thus exposing the innards to the elements. Sometimes, just out of interest, I will test all the old caps from a set I have just restored. Although it is rare to find any that are not leaky when tested at their working voltage, the Mouldseals are leaking like a sieve at a fraction of their rating, most have morphed into resistors or are shorted. Far worse than most wax caps I have tested. The best type are the Sprague metal clad type used in the 1940's. I recapped a Pye D18T a while ago & removed several of these, I was amazed that most were still up to spec when tested! All of the wax type failed... Mark |
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6th Mar 2018, 12:27 am | #14 | |
Heptode
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Re: Capacitor problems
Quote:
I got a 1960 telly that had been stored in a damp garage for years. The Rifas in that set were cracked like I never have seen before. |
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7th Mar 2018, 4:59 pm | #15 |
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Re: Capacitor problems
These were EX military high quality caps surplus to WW2 requirements. They would have been very expensive compared with paper types but were available cheaply in huge numbers during the immediate post war period. The TCC Visconol tubular range [not EHT] were a very close second but even these are leaky now especially in critical positions. J.
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8th Mar 2018, 10:07 am | #16 | |
Dekatron
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Re: Capacitor problems
Quote:
It amazes me that at 70+ years old that the Sprague metal clad caps show no signs of leakage, they obviously are extremely well sealed, are they the paper in oil type? Mark |
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9th Mar 2018, 10:39 am | #17 |
Dekatron
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Re: Capacitor problems
I would guess so Mark. That was about all that was available back then.
Capillary action is amazing it can allow dampness to travel along leads encapsulated in a solid material such as the 'Lockfit transistors'. I can only think of encapsulation in glass as a perfect seal [valves and CRTs] but the temperature required may be a little high for the component itself. Of course it can take years to do this, long after the intended service life of the component. Generally that is.. John. |
9th Mar 2018, 10:50 am | #18 |
Heptode
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Re: Capacitor problems
Glass to metal seals can cope with extreme pressure over extended periods. I have experience of intact seals after 20 years plus at over 2000psi, in seawater.
Ken |
9th Mar 2018, 12:50 pm | #19 | |
Dekatron
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Re: Capacitor problems
Quote:
The set is still working well (apart from some hum from the smoothing cap)with all original parts, When I get round to restoring it I will check the resistor values. It will be interesting to see if they are still in tolerance. Mark |
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