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Old 19th September 2007, 02:53 AM   #1
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Default Testing for cap leakage

Forgive the elementary nature of this question...

I just inherited a handful of really old film caps from my father's TV repairman days in the 50's & 60's. I seem to remember he had a little box that when he pressed a button a light told him whether the cap leaked or not. I'd like to test these caps to see if they're still okay. I can measure their capacitance, but I don't know much about leakage.

Is there simple way to measure leakage, or can I just recreate my dad's box by using a 9-volt battery, an LED and limiting resistor. Press the button to throw DC at a cap in series with an LED (and resistor) and judge by whether the LED flashes or stays lit, or is that an over-simplification.

..Todd
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Old 19th September 2007, 03:59 AM   #2
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You need something a lot more sensitive to test for leakage than an LED. A meter that measures milliamps down to microamps would be about right. Film caps rarely leak, and I never bother measuring them. Electrolytics are the usual suspects. You apply a voltage and monitor the current, usually with a shorting switch across the meter, so you don't damage it during the initial charge. On a good cap you'll see the current drop for quite a while- hours to days, before it stabilizes at a very low value. On older or fake caps you might just see a continuous draw. If you have old paper and oil caps, those can be leaky too. If used for coupling to the grid of a tube, they can cause all sorts of subtle or serious problems.
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Old 19th September 2007, 05:15 PM   #3
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Interesting. Thanks Conrad.

I wonder how that old box with the button & light on it worked.

I wouldn't know a paper/oil cap if I saw one. Are they the rectangular metal flask-shaped can type?

Most of these are molded plastic axials (Big Chiefs, Imps) but some are waxed axials (Arrow). I just assumed they were all wound film caps.

Measuring these old caps is interesting. Their tolerance is upwards of 50% and even higher in some cases. I compared to some modern film caps which are consistently quite tight to their labeled value. Much better than quoted tolerance in my experience.

I suppose it's not surprising that they've made a few advances in capacitor manufacturing during the past 50 years.

..Todd
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Old 19th September 2007, 05:54 PM   #4
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I expect the button and light was an HT supply with neon lamp in series that glowed when current flowed.

A component bridge will tell you if there is leakage in a capacitor and an ESR meter will tell you if an electrolytic is failing.
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Old 19th September 2007, 07:02 PM   #5
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I don't know much about 'em, but black plastic caps are often "Black Beauties", which I believe are paper/oil or something similar. They were junk back then, having a high failure rate, but today they go for big bucks on eBay for people doing restorations of guitar amps. They also are reputed to have good sonic characteristics. IMO, you should find or build a capacitance bridge that shows you not only value, but dissipation factor. The modern meters are nice, but usually don't give the DF, and aren't nearly as much fun as balancing a traditional bridge.
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Old 19th September 2007, 08:13 PM   #6
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Quote:
Originally posted by Conrad Hoffman
The modern meters are nice, but usually don't give the DF, and aren't nearly as much fun as balancing a traditional bridge.
Yes, you want one with lots of knobs and dials, the bigger the better.
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Old 19th September 2007, 10:50 PM   #7
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Well, as mentioned earlier, I'm pretty new at this and haven't got a clue what a capacitance bridge or component bridge is. The only bridges I know of are bridge rectifiers, musical bridges, and the kind water runs under (which is hopefully not happening to a bridge rectifier).

What is dissipation factor? What is dissipating in a capacitor?

..Todd
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Old 20th September 2007, 04:00 AM   #8
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You're onto what I consider a fascinating subject, but I'm not sure where to start. Google all the terms mentioned. Go to the BAMA mirror site and download the manual for a GR 716-C capacitance bridge. Some of those old manuals were a goldmine of fundamental info. Google and go to the General Radio Historical site to see various capacitance/impedance bridges with lots of knobs and dials. Go to my pitiful web site and look at the brief piece on capacitor losses here. Go to a used bookstore and pick up a 1950 or so basic EE textbook like the ones by Timbie and Bush. Go to the various capacitor manufacturer sites because they usually have a basic cap tutorial. That will get you started, then ask a bunch more questions!
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Old 20th September 2007, 05:40 AM   #9
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Good stuff. That aught to keep me occupied.

..Todd
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Old 25th September 2007, 12:49 AM   #10
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Default Leakage

Iím ignorant about leakage. I thought old caps had potential for an ESR problems.

Is leakage a problem in the 20-20kHz range?


Jon
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