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
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
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.
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
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
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.
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
What is dissipation factor? What is dissipating in a capacitor?
..Todd
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!
A small amount of leakage isn't an issue for most low impedance solid state circuits, but when the cap is used for coupling to the high impedance grid of a tube amp, a little bit of leakage is a disaster. It changes the bias point and can result in high distortion, excess dissipation, and general bad behavior. It's not an AC issue, but failure to block the DC, as a cap's supposed to do. IMO, higher than normal leakage is a sign of a defective cap, so I don't want 'em in any of my circuits, even solid state.
For anyone interested in capacitor leakage testing, there's a guy name Tom Bruhns, IIRC, who seems to have done a lot of work in that area. You can find most of his posts on the subject by searching at http://groups.google.com (the Usenet newsgroups message archive from about 1981 to the present; a goldmine!).
While reminiscing with my dad recently about his TV repairman days (he's in his mid-80's now), we went through his old junk and found the old capacitor tester box he used to test for cap leakage. He said it worked pretty darn well.
Amusingly, it's built like a brick outhouse. I'm sure it would withstand a nuclear blast.
I reverse engineered it and came up with the following schematic.
I don't get what the 100R resistor does. It seems to me if it was across the DUT with the button in non-test position, it would discharge the DUT caps a little more gently than it would currently, but in its current spot, I don't understand what its purpose is.
I think I would add a power switch and fuse.
..Todd
Amusingly, it's built like a brick outhouse. I'm sure it would withstand a nuclear blast.
I reverse engineered it and came up with the following schematic.
I don't get what the 100R resistor does. It seems to me if it was across the DUT with the button in non-test position, it would discharge the DUT caps a little more gently than it would currently, but in its current spot, I don't understand what its purpose is.
I think I would add a power switch and fuse.
..Todd
Attachments
That's neat! The 100R almost has to be a mistake, either in tracing it out, or in the original construction. Discharging the caps through the switch without it would soon wreck the switch contacts. I'd be wary of using this on modern low voltage caps, as the voltage has to rise to the neon trigger point, before any current will flow. That could easily be 80V. I think the neons with a bit of radioactive material trigger lower, but can't remember how much. Lack of a mains transformer also gives one pause!
Bad clouds!
Conrad: I looked at your website, forwarded a link on the GR bridges to a friend who has one, and looked at the bad science. Hmmm. I was told about the "holding capacity" of air for water at school - must have been after puberty as simultaneous condensation and evaporation makes much more sense.
Conrad: I looked at your website, forwarded a link on the GR bridges to a friend who has one, and looked at the bad science. Hmmm. I was told about the "holding capacity" of air for water at school - must have been after puberty as simultaneous condensation and evaporation makes much more sense.
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