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Old 4th Mar 2018, 9:50 am   #1
mark pirate
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Default 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:
MASTERADIO TE7T
The height decreased until the picture occupied a
strip about 2in. high across the centre of the screen.
This was some three years ago and the set has not
been in use since. Although it is old it gave a good picture until the fault developed and being a well built chassis I am thinking of trying to put the fault
right. As the set has not been in use I do not expect any deterioration to have taken place.-T. Ryle
(Worthing). We do not agree that the set would not have
deteriorated while out of use for three years. Normally disuse for this length of time would result
in paper capacitors absorbing sufficient dampness to make leakage develop. This usually shows up in
the field timebase, giving the symptoms of lack of height, bottom compression and so on. Which leads us to your fault. The likely cause of this is that
the supply to the field output transformer is much
below the correct 275V. The set is unusual in employing
a 12BH7 double triode as the field oscillator and output valve, with the supply to the output triode taken from the boost line via a 4.71w 1W
resistor decoupled by a 16uF 450V electrolytic. Check both these components and the 50,uF electrolytic
decoupling the cathode (pin 8). Then if neces- sary turn attention to the blocking oscillator section
of the 12BH7 where the 471w resistor in series with
the height control and the 8,uF (200V) field charging
electrolytic should be checked.
I was quite surprised on the comment that the capacitors in question were only out of use for three years, it just goes to show that the problems were well known 46 years ago!

Mark
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Old 4th Mar 2018, 10:25 am   #2
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Default 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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Old 4th Mar 2018, 10:33 am   #3
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Default 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.
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Old 4th Mar 2018, 11:43 am   #4
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Default 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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Old 4th Mar 2018, 11:56 am   #5
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Default 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.
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Old 4th Mar 2018, 12:02 pm   #6
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Default Re: Capacitor problems

Quote:
the brown toffee shaped Wimaís were, shall we say, not very good, perhaps worse than Hunts.
From my experience, I would certainly agree.
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.

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Old 4th Mar 2018, 8:04 pm   #7
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Default 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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Old 4th Mar 2018, 8:58 pm   #8
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Default 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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Old 4th Mar 2018, 11:17 pm   #9
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Default 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 !
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Old 5th Mar 2018, 9:37 am   #10
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Default 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
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Old 5th Mar 2018, 10:46 am   #11
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Default Re: Capacitor problems

Quote:
Originally Posted by johntech View Post
Strangely they were ok in Grundig tape recorders and radios.
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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Old 5th Mar 2018, 10:50 am   #12
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Default Re: Capacitor problems

Quote:
Originally Posted by mark pirate View Post
I think the most troublesome caps are those nasty brown or black Hunts caps used widely in most electronics through the 50's.
Mark
Is that because the sets you restore are of the age when the mouldseal capacitors were the main type used?

I never found any paper capacitors that reliable, and thatís in their service life.
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Old 5th Mar 2018, 7:40 pm   #13
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Default Re: Capacitor problems

Quote:
Is that because the sets you restore are of the age when the mouldseal capacitors were the main type used?
Hi Frank, I restore mainly sets from the mid 30's through to 1970's, and although I agree that any paper cap is a change on sight component, Hunts Mouldseals are the worst type, the plastic casing degrades so badly that many crumble away at the lightest touch. No wonder they absorb moisture!

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...

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Old 6th Mar 2018, 12:27 am   #14
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Default Re: Capacitor problems

Quote:
Originally Posted by Nuvistor View Post
Quote:
Originally Posted by johntech View Post
Strangely they were ok in Grundig tape recorders and radios.
That is interesting, my main exposure to the toffee Wima capacitors was in Grundig tape recorders, I had plenty failures in those.
This might just be a storage environment coincidence?

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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Old 7th Mar 2018, 4:59 pm   #15
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Default Re: Capacitor problems

Quote:
Originally Posted by mark pirate View Post
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
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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Old 8th Mar 2018, 10:07 am   #16
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Default Re: Capacitor problems

Quote:
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.
Considering that Pye played a major role in producing radar and other military equipment during the war, it comes as no surprise that that they must have had huge stocks available at the end of the hostility's.

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?


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Old 9th Mar 2018, 10:39 am   #17
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Default 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.
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Old 9th Mar 2018, 10:50 am   #18
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Default 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
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Old 9th Mar 2018, 12:50 pm   #19
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Default Re: Capacitor problems

Quote:
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.
I have an early 30's Cossor 533A radio that has glass encapsulated resistors, never seen that before!
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.

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