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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 8th Mar 2023, 5:54 pm   #21
paulsherwin
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Default Re: Capacitors "Black Museum"

Thread reopened by request.
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Old 8th Mar 2023, 6:09 pm   #22
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Default Re: Capacitors "Black Museum"

I have just removed two Dubilier Type 700.

Tried some DC on them and was up to 12mA by 250V (and it started to climb), so bottled out. Thought 3W dissipation was enough...!

Given they are toast, applying a rather unfair 500V insulation tester yielded a few tens of kΩ resistance. Straight resistance measurement was about 1MΩ. Both very similar and one cased showed signs of splitting open.
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Old 8th Mar 2023, 6:09 pm   #23
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Default Re: Capacitors "Black Museum"

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Thread reopened by request.
Thanks Paul! Useful thread.
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Old 8th Mar 2023, 6:23 pm   #24
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Default Re: Capacitors "Black Museum"

BEC electrolytic 1964 vintage.
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Old 8th Mar 2023, 6:25 pm   #25
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Default Re: Capacitors "Black Museum"

A gaggle of bubblers.

TCC Type 343 and 543.
DCC "WM" although suspect latter is a date code. Not this is ELV rated.
Hunts A306.
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Old 8th Mar 2023, 9:06 pm   #26
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Default Re: Capacitors "Black Museum"

Is the BEC electrolytic shown really duff?- they're generally one of the better survivors, though of course the odd exception can be excused after all this time.
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Old 8th Mar 2023, 9:19 pm   #27
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Is the BEC electrolytic shown really duff?- they're generally one of the better survivors, though of course the odd exception can be excused after all this time.
One section was modestly leaky: 3mA @ 200V (that's still 600mW internal dissipation), but possibly might have improved as it did seem to be stable/slightly dropping.

The other section was quite exciting: 15mA by 200V and starting to run away.

In each case I did bring them up from 0V progressively - I didn't just bang them on at 200V.
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Old 8th Mar 2023, 10:00 pm   #28
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Default Re: Capacitors "Black Museum"

Black 1Kv labelled 'Plesseal' 0.1mfd caps from 1962. All spurting molten wax but not gone dead short. Appear to be dip coated in a black shiny coating. Found in a Irish built Pilot 23/625 TV from around 1962. Oval in section and approximately 3 inches long.
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Old 9th Mar 2023, 8:56 am   #29
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Default Re: Capacitors "Black Museum"

Your Plesseal caps look very similar to the Wima and ROE types used in German radios of the '50s - Grundig etc. Those types tend all to be leaky too.

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Old 9th Mar 2023, 10:49 am   #30
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Default Re: Capacitors "Black Museum"

Another bad kind are the "micamold" capacitors from the USA - they look like dominoes.

I am surprised by one or two shown above.
Perhaps what we really need is the list of the ones that are known to usually be OK?
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Old 9th Mar 2023, 11:02 am   #31
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Default Re: Capacitors "Black Museum"

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Perhaps what we really need is the list of the ones that are known to usually be OK?
Good idea, with the usual YMMV caveat!

I'll open one to kick it off.
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Old 9th Mar 2023, 3:41 pm   #32
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Default Re: Capacitors "Black Museum"

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Another bad kind are the "micamold" capacitors from the USA - they look like dominoes.
Yes, be aware that there are some of these 'dominos' that - though they have a Mica-sounding name like Micamold or Micasil - are actually paper dielectric.

And they go leaky.

There are two of them in parallel serving as "that capacitoir" in the RCA AR88 receiver, so doubling your chances of something bad happening to the output valve!
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Old 9th Mar 2023, 6:13 pm   #33
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Default Re: Capacitors "Black Museum"

To me, that "Plesseal" looks like a re-badged Wima, and it certainly seems to have behave like one!
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Old 9th Mar 2023, 7:47 pm   #34
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Default Re: Capacitors "Black Museum"

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be aware that there are some of these 'dominos' that - though they have a Mica-sounding name like Micamold or Micasil - are actually paper dielectric.

And they go leaky.

There are two of them in parallel serving as "that capacitoir" in the RCA AR88 receiver, so doubling your chances of something bad happening to the output valve!
Well, probably not actually! The two capacitors aren't independent random failures, they'll be the same batch, exposed to the same environment, so they'll run out of life at the same time.

Two rather than one won't halve the lifetime (luckily!)
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Old 9th Mar 2023, 8:05 pm   #35
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Default Re: Capacitors "Black Museum"

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The two capacitors aren't independent random failures, they'll be the same batch, exposed to the same environment, so they'll run out of life at the same time.
I doubt it, if they do it's a coincidence.

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Old 9th Mar 2023, 8:44 pm   #36
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Default Re: Capacitors "Black Museum"

It would be interesting to check this out. I would agree that paper capacitors from the same batch are likely to have the same life as it looks like the quality of the paper is what leads to the leakage.

There is strong evidence that environment is not the cause because hermetically sealed paper capacitors have been found to go as leaky as any other package. The water that ends up causing the leakage is created chemically by the decomposition of the paper.
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Old 10th Mar 2023, 3:18 pm   #37
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Default Re: Capacitors "Black Museum"

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Quote:
Originally Posted by turretslug View Post
Is the BEC electrolytic shown really duff?- they're generally one of the better survivors, though of course the odd exception can be excused after all this time.
One section was modestly leaky: 3mA @ 200V (that's still 600mW internal dissipation), but possibly might have improved as it did seem to be stable/slightly dropping.

The other section was quite exciting: 15mA by 200V and starting to run away.

In each case I did bring them up from 0V progressively - I didn't just bang them on at 200V.
While the capacitor was probably duff anyway, bringing up the voltage gradually - especially without a current limit - won't reliable reform an elecytrolytic. The current limit is needed to form a new oxide layer instead of just punching holes in the remains of the oxide layer. The voltage can be chosen somewhere near the WV of the capacitor and doesn't necessarily need to be brought up slowly. A rule of thumb for radio repair implemented by Philips in the 1950's is to just put a 50k resistor in between the rectifier and the electrolytic and remove any load of the power supply and let the capacitor reform that way.
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Old 12th Mar 2023, 9:14 pm   #38
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Default Re: Capacitors "Black Museum"

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A rule of thumb for radio repair implemented by Philips in the 1950's is to just put a 50k resistor in between the rectifier and the electrolytic and remove any load of the power supply and let the capacitor reform that way.
This HT via a few tens of kilohms has long been my favoured method of bringing HT electrolytics back into service- it can be left for hours quite safely current-limited like this, even if things go awry internally and leakage shoots up or a short develops, little power will be developed either internally or by the resistor. With modern LEDs giving noticeable light output at less than 1mA, one in series gives a useful indication of how things are going without tying up a meter in the meantime. Even at 400V HT, a shorted capacitor will only draw 8mA through the LED, a nice bright "Achtung".
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