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Inexpensive Capacitor Leakage Tester
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Tac Eht Xilef:

--- Quote from: PA4TIM on February 20, 2018, 07:01:06 pm ---This is not the leakage you worry about in old tube gear that is long not used. That has to do with the leakage of the degraded oxidelayer on the working voltage. So a "defect" but often "repairable"  You test that at the working voltage so you know for sure the oxide layer can withstand that. Just for safety. This way you can find defects in the isolation. If have seen caps working perfect at some voltage (far beneath the working voltage)  but  if you increase the voltage, nothing happens other them more leakage current. Almost like it is a zener. (until it heats up to much and then poefffff )
--- End quote ---

^This^. Old paper caps in particular can often test OK at a lower voltage (or even their in-circuit operating voltage) but start leaking heavily between that & their rated voltage - they're faulty ones that just haven't failed yet, but soon will ;).

I normally replace most paper caps on sight, but right now I've got a couple of unusual radios in the queue that deserve to be kept as original as possible. Original caps that test OK or come good after drying / reforming will be kept, & faulty ones will be re-stuffed where possible.

At the moment I'm building a leakage tester based on the supply in the 3rd schematic here, current-limited to 10mA and should be good for ~700V. I'll wait until it's built & tested before I post the schematic - most of the parts are from my junkbox/ebay, but I'm waiting for Mouser to get their finger out & send the MOSFETs & caps I ordered last week...
PA4TIM:

--- Quote ---I normally replace most paper caps on sight, but right now I've got a couple of unusual radios in the queue that deserve to be kept as original as possible. Original caps that test OK or come good after drying / reforming will be kept, & faulty ones will be re-stuffed where possible.
--- End quote ---

Do you reform paper caps ?  (reforming only works for electrolytic caps)
And what do you mean by drying ?
Are you talking about caps from very old radios (1920's or so) like the electrolytic cap on my picture (from a 30's Philips 836A that was damaged by a granate during WW2. the radio was from his parents)

I restored a only a few radios (maybe 15 or so), but never replaced all caps. Only the ones that tested bad. The good caps worked fine for 50-60 tears and they all still work fine now.
I know, this is a big nono in the audioworld where recapping is the magic word.  >:D
(But I'm for most into testgear restorations, there the quality of components is probably just better)


Calambres:
I built a very good working Capacitor leakage tester / reformer following the instructions from M. Caldeira's very detailed youtube series:

https://www.youtube.com/playlist?list=PLy5M_tA9ux7KWexvEjLB0SANSxQXi0zTS


I also wrote an article on my build in my own web page. For those who can read spanish and may be interested, click here.


My tester:



jsi:

--- Quote from: jsi on February 20, 2018, 12:38:57 am ---
--- Quote from: bitseeker on February 19, 2018, 07:01:45 am ---Nice ammeter, jsi. Be sure to bring caps up to voltage slowly to minimize the risk of pegging the meter.

If you want to get an actual leakage tester, don't limit yourself to the Heathkit ones. Many Heathkit devices go for too much money on eBay. It's also a good idea to get one that discharges the cap, either automatically or with a button/lever, prior to removal.

Be sure to check local sources, too. I got a Sprague TO-6 locally for significantly less than they go for on eBay.

If you're comfortable with high voltages (I assume so, since you're going to work on tube stuff), you could make your own tester. Have a look at the schematics in the operating/service manuals for some of the vintage testers.

--- End quote ---

Great! The Sprague TO-6 is officially on the hunt list.  :-+  My wife loves to hang out at estate sales and she always calls me when she comes across any electric stuff.  The Simpson ammeter is an example of such a find. 8) 

The idea of making a tester has crossed my mind.  But, as I think this through, buying a used meter might be the best way to go.  I've got 2 projects: a radio / record player from my youth, and a Hammond Organ from my wife's childhood.  After that I'm going to be done with this old stuff.  I much prefer working in the digital realm.  If I pickup a used Heathkit or Sprague tester I can resell it when I'm done.  I've done that with other tools that were needed for a project and in the end they cost me next to nothing.

--- End quote ---

SCORE!!!!!!   :clap: :clap:
All I can say is make sure you marry a good woman.   I told my wife to be on the lookout for any capacitor checkers at the estate sales.  She then proceeds to tell me she'd bought some electrical "stuff" without telling me about it.  Danged if she hadn't bought a Heathkit C-3 condenser checker and a Heathkit radio frequency generator.  And, nobody wanted them so they were half price.  They need to be rebuilt, but that's going to be far easier than starting from scratch. 

bitseeker:
Nice score! Congrats! :clap:
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