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Old 17th Nov 2023, 9:14 pm   #1
WaveyDipole
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Default Capacitor foil deterioration?

I have recently started wortking on a Grundig TK18 tape recorder. Normally I would not worry too much about film caps, but wanted to ask about this example as it looks a bit rough. The others seems to look consistent with their age (e.g. the one on the right). But what is going on with the one in the centre? Is that some kind of foil deterioration?

It is rated at 1uF/125V and measures 1.4uF in circuit. Not sure what the tolerance rating is but I imagine that 40% is too far of the mark to be in tolerance. I will be replacing it anyway, but just wondered what the cause of the appearance is.
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Old 18th Nov 2023, 1:19 am   #2
Maarten
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Default Re: Capacitor foil deterioration?

There's a name or series indication on the backside. This will indicate whether it's a polyester or paper capacitor. The on right next to it looks possibly cracked.

Measuring 40% high is usual for a deteriorated paper cap, but very unusual for a deteriorated polyester cap. Though it could be due to the in circuit measurement.

You'll have to remove it both to take a second measurement and to look at what's printed on it.
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Old 18th Nov 2023, 10:42 am   #3
Simon Gittins
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Default Re: Capacitor foil deterioration?

The name appears to be "WIMA", a make that often needs replacing in tape recorders of this age.
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Old 18th Nov 2023, 10:49 am   #4
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Default Re: Capacitor foil deterioration?

I suspect that it is too small to be paper. Here is the other side and yes, turning it over confirms the brand is WIMA. There are a number of those in here (0.1, 0.01, 0.022 etc) but this 1uF looks much worse than the others. One or two of the smaller ones do also appear to be showing similar signs at the edges of the foil.

The blue Hunts Electrolytic will, of course, have to be replaced.

I took a measurement out of circuit and got a value of 1.04uF, so it does still appear to be in tolerance.
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Old 18th Nov 2023, 10:51 am   #5
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Default Re: Capacitor foil deterioration?

Is that not just an outer foil to make it look nice? Cap foil is usually silver in colour, that's looks goldish. the only certain way to be sure it's ok is to test it.

Andy.

Edit posts crossed.
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Old 18th Nov 2023, 11:39 am   #6
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Default Re: Capacitor foil deterioration?

In my expereirence the TK 18 and similar TK range (14, 17, 19, 23 & 27) do not normally use the black/brown Wima paper capacitors (said to look like toffee sweets) which potentially can have electrical leakage problems. Hunts paper capacitors also are not normally seen in these models.

All the machines I have seen either have yellow/gold coloured Wima Tropyfol (TFM) Polyester film/foil capacitors (which generally are very reliable) or yellow/brown/orange ERO FOL II capactors which are another Polyester capacitor, generally very reliable again.

I think your examples are TFM.

Of course if there is any doubt or suspicion then just replace.

Earlier Grundig TK models do use a lot of the dreaded Wima toffee capacitors and to a lesser extent some Hunts.

David
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Old 18th Nov 2023, 12:34 pm   #7
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Default Re: Capacitor foil deterioration?

Why don’t you disconnect it and connect it across a power supply near its rated voltage, with a milliammeter in series, to see if it’s leaky?

I'm working on a 1950's West German set at the moment. All of one type were fine when tested thus, all of the others leaky as sieves.
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Old 18th Nov 2023, 1:19 pm   #8
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Default Re: Capacitor foil deterioration?

Back in the day we used those types of Wima capacitors for service replacements in radios, TV's etc at one firm I worked for, they were good quality.

Lawrence.
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Old 18th Nov 2023, 3:02 pm   #9
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Default Re: Capacitor foil deterioration?

Lots of those Wima TF & TFM caps were used in 1970s BR electronics, they sometimes had to replaced due the plastic crazing & falling apart. Also had problems with them changing value near a dropper resistor, in one product we repaired. Apart from that usually quite reliable.

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Old 18th Nov 2023, 3:49 pm   #10
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Default Re: Capacitor foil deterioration?

Quote:
Originally Posted by Diabolical Artificer View Post
Is that not just an outer foil to make it look nice? Cap foil is usually silver in colour, that's looks goldish. the only certain way to be sure it's ok is to test it.
It sounds quite plausible that the outer layer would be for display and the actual capacitor layers would be of a silver colour. It is hard to tell what colour the inner layers are because the ends are darkened.

Quote:
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Lots of those Wima TF & TFM caps were used in 1970s BR electronics, they sometimes had to replaced due the plastic crazing & falling apart.
Its hard to discern from the photo, but this particular capacitor does have lots of what appear to be tiny cracks in the outer plastic layer, which could quite appropriately be described as 'crazing' (like crazy paving). I also noticed in my first photo that the 0.01/400 capacitor on the right has a large crack all the way down the length of the body which I hadn't initially picked up on. I imagine that if the outer layer is cracked then its only a matter of time before the damp/deterioration penetrate to the deeper working layers of the capacitor? On closer inspection, all of the WIMA capacitors have at least some degree of cracking on the outer plastic layer.

The board also has a couple of Ero Foil capacitors on it and both of them seem to be OK.

Its beginning to look like I will need to replace quite a few caps here, both foil and Electrolytic types. I don't know whether these WIMA caps are prone to exploding like RIFA's when the outer plastic layer is compromised and damp gets in, but given the high voltages involved, it seems safer to replace.

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Old 18th Nov 2023, 5:01 pm   #11
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Default Re: Capacitor foil deterioration?

Quote:
Originally Posted by WaveyDipole View Post
The blue Hunts Electrolytic will, of course, have to be replaced.
Of course nothing!

Hunts are renowned for making some of the best electrolytics ever - it's only their paper types that have the bad reputation.

I still have some original old stock from the late 60s of unused Hunts electrolytic capacitors like the one you show and I am still fitting them as replacements for failed other brands with no problems. Don't confuse the brand 'name' with the capacitor 'type'!
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Old 18th Nov 2023, 6:09 pm   #12
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Default Re: Capacitor foil deterioration?

Thank you. Interesting to note. I wasn't aware. Just associated the bad reputation of the paper caps with the brand.
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Old 18th Nov 2023, 6:31 pm   #13
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Default Re: Capacitor foil deterioration?

Quote:
Originally Posted by Techman View Post
Quote:
Originally Posted by WaveyDipole View Post
The blue Hunts Electrolytic will, of course, have to be replaced.
Of course nothing!

Hunts are renowned for making some of the best electrolytics ever - it's only their paper types that have the bad reputation.
I think *all* waxed-paper capacitors have developed a bad reputation ovet the last half-century.

Hunts, as well as making good electrolytics, also made some rather decent ceramic-mica ~Doorknob~ type capacitors; I have some of them here in an antenna-matcher and despite my 'pushing it' a bit as far as their rated RF voltages and currents are concerned, they're showing no sign of problems.
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Old 18th Nov 2023, 6:55 pm   #14
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Default Re: Capacitor foil deterioration?

I recently replaced a failed axial gold coloured WIMA in a 1970's power supply...it was plain metal cased so would never have had the appearance shown here. It was in test eqpt. that originated from a lab, so could well have been powered up for long hours in the past.

Dave
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Old 18th Nov 2023, 7:00 pm   #15
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Default Re: Capacitor foil deterioration?

Quote:
Originally Posted by Maarten View Post
There's a name or series indication on the backside. This will indicate whether it's a polyester or paper capacitor. The on right next to it looks possibly cracked.

Measuring 40% high is usual for a deteriorated paper cap, but very unusual for a deteriorated polyester cap. Though it could be due to the in circuit measurement.

You'll have to remove it both to take a second measurement and to look at what's printed on it.
I have some of those WIMA caps with the red printing in a scope that I'm trying to get working at the moment. I've attached a photo below. The leads have been folded over on the PCB, so it's not easy to remove them and test out of circuit without damaging the PCB (very fragile - it's so much easier when they don't fold the legs! Can do it in seconds with a Hakko FR300).
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Old 18th Nov 2023, 8:39 pm   #16
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Default Re: Capacitor foil deterioration?

I had trouble with one of those!

https://www.vintage-radio.net/forum/...ad.php?t=42853
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Old 18th Nov 2023, 10:37 pm   #17
Maarten
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Default Re: Capacitor foil deterioration?

Quote:
Originally Posted by WaveyDipole View Post
I suspect that it is too small to be paper. Here is the other side and yes, turning it over confirms the brand is WIMA. There are a number of those in here (0.1, 0.01, 0.022 etc) but this 1uF looks much worse than the others. One or two of the smaller ones do also appear to be showing similar signs at the edges of the foil.

The blue Hunts Electrolytic will, of course, have to be replaced.

I took a measurement out of circuit and got a value of 1.04uF, so it does still appear to be in tolerance.
Very likely to be a polyester foil capacitor, so it doesn't need to be replaced as long as it's in tolerance. Unless you don't want to take the risk based on the looks of it. It might be slightly more sensitive to humidty, and then instead of leak it will just gradually reduce in capacitance over the years.
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Old 18th Nov 2023, 10:40 pm   #18
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Default Re: Capacitor foil deterioration?

Quote:
Originally Posted by knobtwiddler View Post
I have some of those WIMA caps with the red printing in a scope that I'm trying to get working at the moment. I've attached a photo below. The leads have been folded over on the PCB, so it's not easy to remove them and test out of circuit without damaging the PCB (very fragile - it's so much easier when they don't fold the legs! Can do it in seconds with a Hakko FR300).
I'm not sure what kind of dielectric the ones in your picture use. 10u 100V is quite high for non-electrolytics but they do look largish so probably polyester?
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Old 18th Nov 2023, 10:42 pm   #19
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Quote:
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I think Wima Durolit are paper capacitors.

A 10u 100V capacitor is likely to be plastic foil or electrolytic, not paper.
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Old 19th Nov 2023, 7:51 pm   #20
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Default Re: Capacitor foil deterioration?

The TF & TFM could be OK, but the ones we replaced at work were in safety critical equipment, that meant replace on sight if visibly damaged, not worth the risk.

David
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