The AM Forum

When testing filter capacitors:

What do you feel is the maximum allowable leakage at the capacitors normal working voltage?

How does this apply as the test voltage is decreased?

For instance, if the capacitor has a DCWV of 600 VDC and you decrease that to 300 VDC or even 50 VDC does the maximum allowable leakage remain the same or is there an adjustment needed?  Even though this would not be as accurate an assessment of the capacitor would it suffice in a pinch?  Sometimes it's not possible to test the capacitor at full working voltage.

Rick/K5IAR 

Rick, either a cap is a resistor or it aint. A good electrolytic will show a rapid and steady charging action from a low resistance to a value of infinity or close to it on a ohmmeter, and do the exact same thing when the leads are reversed. If you see any lowering of resistance in either direction from infinity, especially a repeated discharge - charge cycle the cap is bad.

Generally, any electrolytic older than 20 years is on borrowed time.

So the actual voltage applied is not that important?  I was of the understanding that a "good" filter capacitor, whether it be electrolytic or other, would behave exactly as you stated, Derb.  However, is it possible that when full working voltage is applied it will demonstrate leakage that would not show up on a simple resistance test?  I may be way off the mark on this, so any advice and help is much appreciated.

Rick

as long as the voltage is under the cap's rating, I don't think so, but I've lost a lot of knowledge of basic theory in the last few years while I went AWOL. I just never feel the need to hot test filter caps and actually measure the leakage current. if they're more than 20+ years old, it's cheap and best to replace them.

you could do it, but why? It's like pullin a old car our of a barn and expecting it to start. Simple cold VOM test will tell you what's up enough to judge if the cap is ok or not.

I usually test any junkbox electrolytics by slowly running the voltage up to maximum rated, or by using a current limiting resistor of several kilohms.  If I can get to full working voltage with no more than a milliamp or so of leakage current, I won't hesitate to use the capacitor.

I test them by putting a milliammeter in series with the capacitor, and putting the whole thing across the power supply.  I run a preliminary test by charging the cap through a current limiting resistor, to see if it will hold a charge.  If so, I let it cook  for several hours at near full working voltage, and if it hasn't shorted out by then, I connect up the miliammeter to read leakage current.  In all the years, I have never blown a meter doing this, although I have blown plenty of multimeter by forgetting to re-set the range, and trying to measure several hundred volts from a power supply or the a.c. mains, with the meter set to read current instead of voltage.

Thanks guys.  I guess if I use both methods I can't go wrong.  I have an old can paper capacitor that is supposed to be 10 mfd at 600 vdc, non polarized.  When I test it for proper capacitance with the Fluke it reads a little under 7 mfd, however, it does charge and discharge when testing the resistance with a VTVM.  I think I'll cobble together a little power supply and microamp meter to see how much leakage there is.  I can always restuff it, but I don't want to unless absolutely necessary.  Thanks again Derb and Don for the words of wisdom.

Rick

How about large high voltage oil/paper type like in the plate supply of the BC-610. I've got a couple of replacements for leaky ones and a few more for a new maul?

Run them up slowly to close to the working voltage too? I've got a variable supply that's good for 1,000 volts @ 500ma, but some of these caps go to 4,000 volts.

I think those should be treated differently, and you may not be able to tell how. There's 2 types of oil caps - oil filled and oil impregnated, and the impregnated kind is nothing more than a paper cap that uses oil as the electrolytic between the paper. They can and do dry out
just like a regular paper cap.

if the cap actually says oil filled, i'd run it to half rated voltage and let it cook a while. see if you see any evidence of distress like bloating or oil seepage.  Then if nothing's amiss, get out yer bangstick and hit that puppy. A cap does nothing but store energy. That's its job. make it show that it's storing that energy. Think of yourself as a capacitor dominator wearing leather, a spiked collar, and holding a whip.

ride herd on those electrons. just be careful.  8)

Something that shows up once in awhile at hamfests is the ZM-11U RLC bridge, I inherited mine (made by Clough/Brengle) from my father.  Among its many functions is the ability to test capacitor leakage at various voltage levels and the really nice feature is the high voltage is derived via the rectified output of an RF oscillator.  So if the cap suddenly develops very high leakage or shorts the oscillator (and thus voltage) simply stops without smoke or fireworks.  It also measures capacitance value, dissipation factor, ohms (well into the megs range), insulation leakage, inductance, and transformer turns ratio.  Here is a link to a description on N6GCE's website:  http://www.prc68.com/I/ZM11.shtml

One of my oldest and most used pieces of specialty test gear.

Rodger WQ9E

I'd Just like to add, I agree with alot of the above here..But Prior to any Testing servicing etc....and you new guys out there coming in...picking up some of these old xmitters getting into these old circuits...First thing you do is Unload the Filter condensers...Build yourself a heavy resistive load and Discharge these systems....PLEASE...one hand in Pocket one hand in movement...ALWAYS...

hmm. there is no data behind the files. I do have one of those. It is excellent.

The site used to work but you are right, the links are dead.  The manual is on Bama:  http://bama.edebris.com/manuals/miltest/zm11u/

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hmm. there is no data behind the files. I do have one of those. It is excellent.
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I agree with derb ... I had a long conversation with a components engineer when I worked for TI in industrial systems ... He said the start of reliable 'jellybean' components resulted from the US space program research into (spectacular) failures of electronic components...more reliable components started surfacing in the 1970's ... unless such subjective measures as euphony or restoration correctness drive the use of 'older' parts, I would not use them unless adjacent circuit components were protected from their failure....for example the much abused 32Vx lv transformer failing due to rectifier/filter cap failures ....I'm not familiar with that particular Collins tx but I bet you could stop that by adding a fuse just for that  xfmr ...thats what I'm doing as I redux my JVII project...hope this helps...73...John

My experience with new electrolytics is the capacitance of a new cap more likely will read slightly higher than its stamped value.  I'll always put a cap on a cap meter and test it.  If it reads below its stamped value, it gets used in the trash can.

I'll also say this: for new electrolytics; buy only Sprague, Nichicon, or Rubycon.

Chinese lytics are always suspect.