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Old 17th Oct 2018, 1:53 pm   #1
CuriousNerd
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Default Equipment for detecting capacitor's outer foil.

Hello
i am new to this forum,
it's been about 25 years since i last restored a valved radio and they used to be mainly amateur radio receivers i.e collins and hammarlund etc....

I have recently acquired a Kolster-Brandes MR10 for my first project and need to replace some of the capacitors with some polyesters( the yellow axial ones).
While doing some research to refresh my memory on many things long forgotten i remembered that it is advised to find out which side the outer shield is connected to and place that side to either Earth or the lower resistance side in order to maintain shielding as you all know this was marked on the old caps but not on the new ones, i know a scope (which i don't have at the moment) can be used to do this but do you guys use any other ways/equipment of finding out?

many thanks
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Old 17th Oct 2018, 2:01 pm   #2
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Default Re: Equipment for testing capacitor Direction

This is not necessary with modern plastic film capacitors as the method of construction is different.
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Old 17th Oct 2018, 2:32 pm   #3
David G4EBT
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Default Re: Equipment for testing capacitor Direction

As you don't have a scope, if you have an amplifier you can easily check which is the outer foil like this:

https://www.bing.com/videos/search?q...3&&FORM=VDRVRV

I don't think many people bother or are even aware it might be an issue and few report any problems, but when I get a new batch of caps (usually yellow poly ones), it only takes a few moments to check them on the 'scope and just mark the outer foil end with a Sharpie. Scores of videos on youtube about outer foil identification, including Mr Carlson and his gadget being demonstrated in his informative video at the command centre of the 'Starship Enterprise!'

https://www.bing.com/videos/search?q...3&&FORM=VDRVRV

Hope that helps a bit.
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Old 17th Oct 2018, 3:29 pm   #4
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Default Re: Equipment for testing capacitor Direction

I can't see it mattering at all unless it is a particularity badly designed circuit, off hand I can't think of a way to do such a bad one.
 
Old 17th Oct 2018, 3:46 pm   #5
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Default Re: Equipment for testing capacitor Direction

When replacing old waxies and the like with yellow Polyesters, I never bother faffing around trying to work out which 'end' is which. Modern yellow polyesters are significantly smaller than legacy paper/waxie capacitors so the chance of any inadvertent feedback/noise issues is smaller too!

As mentioned, a circuit which is so critical as to need capacitors installed a particular way round is probably going to have other issues too - though I will admit to having 'tamed' an instrumentation biomedical amp which oscillated at about 75KHz by fitting an earthed sleeve made from a length of coax-cable braid over the 6BR7's input-coupling capacitor because there was no room to fit anything else.
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Old 17th Oct 2018, 6:11 pm   #6
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Default Re: Equipment for testing capacitor Direction

Quote:
by fitting an earthed sleeve made from a length of coax-cable braid over the 6BR7's input-coupling capacitor
And that would have still oscillated with the outer foil on the other end as it is a coupling capacitor, outside coupled to the inside.
 
Old 17th Oct 2018, 6:22 pm   #7
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Default Re: Equipment for testing capacitor Direction

If the capacitor is earth sleeved, the outer foil won't radiate to whatever part of the circuit that is forming part of the oscillation loop, seem to remember some domestic receivers used shielded capacitors to reduce stray pickup.

Lawrence.
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Old 18th Oct 2018, 6:43 am   #8
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Default Re: Equipment for testing capacitor Direction

I watched Mr C's video and not knowing better decided to go about ID ing the outer foil, I'm bu**ered if I could tell the difference, had one scope on 10mV and another on 1mV/div. I've not noticed any issue's either when I've built amps, so now I bung em in any old how as long as they look right, IE neat.

Mr Carlson does know his stuff, but I think he has some odd idea's, like retro fitting soft start circuitry for ECC** type heater's.

Andy.
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Old 18th Oct 2018, 12:12 pm   #9
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Default Re: Equipment for testing capacitor Direction

The Marconi TF2603 RF millivoltmeter manual makes a reference to such capacitors in its maintenance section. It expressly points out that if such a capacitor is replaced, the 'outer foil end' should always be connected to earth, and, IIRC, is also states why that is necessary.

Al.
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Old 18th Oct 2018, 12:47 pm   #10
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Default Re: Equipment for testing capacitor Direction

If there is significant voltage across the capacitor (such as a decoupling capacitor, or a capacitor in a filter or a resonant circuit) then it may be important. But if there's almost no voltage across the capacitor - such as a coupling capacitor - then it won't matter.
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Old 18th Oct 2018, 2:21 pm   #11
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Default Re: Equipment for detecting capacitor's outer foil.

Depends on whether the outside foil is meant to shield incoming or outgoing radiation. Most of the times it won't matter but in some cases manufacturers relied upon characteristics like this.
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Old 18th Oct 2018, 2:41 pm   #12
David G4EBT
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Default Re: Equipment for detecting capacitor's outer foil.

We've been here before.

Here are as couple of pics of a 'scope test I did back in 2015.

It's a quick enough test to do.

The first pic is with the outer foil grounded (as identified by the test).

Second pic is with the outer foil not grounded.

I guess it might matter if, say, a cap is located next to AC wiring, such as valve heaters which might induce hum.

Until 2015 till the topic cropped up on the forum, I was ignorant of it and never wondered what the black line meant on unpolarised waxy paper caps. I can't recall that lack of awareness ever causing a problem by not knowing which was the outer foil. On the other hand, given how quick and easy it is to check, maybe it's not bad idea, though I dare say some will scoff and feel that it's along the spectrum towards audiophoolery.
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Old 18th Oct 2018, 2:43 pm   #13
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Default Re: Equipment for detecting capacitor's outer foil.

This topic came up in another thread only a few weeks ago. It only matters under certain circumstances, such as low capacitor value and high strays.
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Old 18th Oct 2018, 3:18 pm   #14
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Default Re: Equipment for detecting capacitor's outer foil.

First time I’ve seen Mr Carlson videos - thank you David for link - interesting but what does he use all that gear for? Must have cost a lot.
Cheers
John
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Old 18th Oct 2018, 4:41 pm   #15
CuriousNerd
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Default Re: Equipment for detecting capacitor's outer foil.

Many thanks everyone for the response i will carry on with the project and will look into investing in scope for the future as it is very handy to have for other tasks as wlll
cheers
D.
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Old 20th Oct 2018, 8:25 am   #16
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Default Re: Equipment for detecting capacitor's outer foil.

If you have a cheap new multimeter with capacitance range down to pF level, then you may be able to measure a difference in capacitance between a terminal and a metal foil 'sleeve' wrapped around the cap casing. I haven't tried that, but will as soon as I get to my bench.
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Old 20th Oct 2018, 10:50 am   #17
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Default Re: Equipment for detecting capacitor's outer foil.

I would just put it on my bridge and put my hand on it and then turn it round and try again.
The bridge will go off balance when the outer is on the active terminal.
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Old 20th Oct 2018, 11:17 am   #18
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Question Re: Equipment for detecting capacitor's outer foil.

Quote: The bridge will go off balance when the outer is on the active terminal. Unquote.

That doesn't sound right. Presumably, the 'active terminal' refers to the bridge. Since the 'outer' of the capacitor will have the foil, then surely the bridge will go off balance if the capacitor is connected to the bridge the other way round?

However, have I misunderstood your remark?

Al.
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Old 20th Oct 2018, 11:28 am   #19
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Default Re: Equipment for detecting capacitor's outer foil.

The outer foil will be more sensitive to touch when it is not on the earth side of the bridge terminals.
Slipping the capacitor in a short bit of metal pipe and connecting the pipe to an extra terminal should also work.
For a go/no go test touching the capacitor should be good enough.
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Old 20th Oct 2018, 11:42 am   #20
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Arrow Re: Equipment for detecting capacitor's outer foil.

O.K. - thanks for your reply. I'll have to dig out some old caps. with the type of construction referred to here and experiment.
Why? Just curious.

Al.
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