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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Tue 14, 2017 7:21 pm 
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analog.tv wrote:
Instead of that battery, I ran across a small homemade power supply made with a radio transformer, 6X4, and some filters mounted next to the high voltage box from a defunct Motorola set. This and a multimeter seem to be able to show leakage (if any) on capacitors. I would not recommend this to beginners, though....one wrong move and you're looking for a new multimeter. Craig


It will not damage a DVM that has auto ranging.

I use a 400 volt regulated supply and a DVM with 10 meg. input impedanceto test caps, and it works fine.

Pete

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Thu 16, 2017 1:46 am 
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More Cap Data for a 0.002uF, 600V Wax paper cap removed from a Zenith radio

The chart shows the leakage current in microamps versus voltage applied to the cap. The data shows that the leakage current vs voltage is fairly linear.

This data is for 1 leaky cap , other 0.002uF, 600V leaky caps with will have very different leakage current. The goal is to get an idea of the variation vs applied voltage.

Attachment:
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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Thu 16, 2017 6:53 am 
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It would be interesting to know what kind of caps the OP was referring to.

Electrolytic capacitors are normally tested for leakage current. Other kinds of caps are tested for leakage resistance.

The amount of leakage current that is acceptable in a used electrolytic cap depends on its size (in uF), and its voltage rating. There are various formulas around that one can go by, but for the sizes typically found in tube radios and similar equipment, anything over 1 mA is suspect. Note that one must not make the mistake of testing electrolytics for leakage right out of storage; they will nearly always fail if this is done! Electrolytics have to be given a chance to reform either by bringing them up slowly with a power supply or allowing the equipment to operate for a time in order for a leakage test to be realistic. Since electrolytics tend to adjust their dielectric layers to the voltages they are operated at, used capacitors should be tested at the voltages they run at in serivce. New capacitors should be tested at rated WV.

With other kinds of capacitors, the accepted minimum leakage resistance for used components is 100 megohms, a value often cited in literature because it is high enough that it will not affect the operation of practically any circuit. Note that it is not necessary to use full rated voltage; insulation tests can be performed at any convenient voltage as long as it is high enough to evidence any faults in the dielectric. 50 volts is usually considered the minimum test voltage, but 500 volts is more common for things which can withstand it. By Ohm's Law, if you are testing at 500-V, 100 megohms corresponds to 5 uA of current; at 50-V, 100 megohms corresponds to 0.5 uA.

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Thu 16, 2017 12:10 pm 
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There is an article in the current issue of ER that talks about this subject.

Interesting!!

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Thu 16, 2017 5:02 pm 
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A grid coupling capacitor with 100 M ohms leakage resistance would be a fail in many cases. Circuits that rely on grid leak bias usually have a 5-10 meg resistor to B-. If the preceding plate has 50 to 100 VDC on it, its easy to see where 100 M leakage though the coupling cap would completely destroy to bias voltage. (Correct grid bias is usually somewhere around -2 V to -15 V). With 100 M leakage resistance the bias would now be around +5 to +10 V, totally ruining that stage.
On the other hand if the cap is used as a cathode resistor bypass then even a very leaky cap (<10 K) could still be acceptable.
So its not correct to generalise and say that any cap greater than 100 M ohms is acceptable. Maybe OK in bipolar transistor circuits, but not for tube or FET circuits.
Tom


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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Thu 16, 2017 8:59 pm 
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Mikeinkcmo wrote:
There is an article in the current issue of ER that talks about this subject.

Interesting!!


I would be interested in the article, but what is "ER" ?? Economics Review, Elementary Radiology or ??

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Thu 16, 2017 10:15 pm 
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Eddy wrote:
Mikeinkcmo wrote:
There is an article in the current issue of ER that talks about this subject.

Interesting!!


I would be interested in the article, but what is "ER" ?? Economics Review, Elementary Radiology or ??

Electric Radio ?? https://www.ermag.com/

Dave


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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Thu 16, 2017 10:18 pm 
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Thanks Dave.

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Fri 17, 2017 2:42 am 
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The amp in question is a silverface deluxe reverb 1970s.
The caps in question are .1uf 600v Pactrons blue film caps.
With everything connected I was probing the DC on both sides of the caps and some I was getting the .05vdc and .6vdc on certain cap. On those caps one at a time I unsolder one leg and and checked the dc coming through the capacitor. I measured almost 10vdc coming through the questionable caps.
Thanks,

jason


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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Fri 17, 2017 2:47 am 
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I would recommend you replace any of the wax caps with film. It is very likely all the wax caps leak Are you sure the pactrons are film? Is there a part number on them? Can you post the schematic and let us know which cap you are discussing.

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Fri 17, 2017 3:54 am 
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http://vintagefenderamprepair.com/wp-content/uploads/2011/04/Deluxe-Reverb-AB868-schematic.pdf
Pretty much most of them were passing DC except the silver spragues
Above is the schematic.

Thanks,
Jason


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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Fri 17, 2017 2:55 pm 
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reeves03 wrote:
?......I unsolder one leg and and checked the dc coming through the capacitor. I measured almost 10vdc coming through the questionable caps.
Thanks,
jason

There is a potential test error here. You would not measure voltage coming through anything....that would be a current measurement. Suppose your meter has a 1 megohms input resistance. If you measure 10 volts "coming from a capacitor", that is a current of 10 uamp. If the input impedance is 10mohm**, then it only takes 1uamp to get that 10 volts.

To measure leakage in capacitors, you need to measure current., not voltage.


**this article states that the cheaper DVM are typically 10megohms, but that some models can be much higher:
https://en.m.wikipedia.org/wiki/Voltmeter

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Fri 17, 2017 7:13 pm 
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I use my digital voltmeter and a highly regulated high-voltage DC supply.

I place the cap under test in series with the regulated DC supply voltage and one of my Fluke digital meters. The meters have an 10 megohm input impedance... any voltage results from leakage resistance in the caps. A good mylar or plastic cap shouldn't show any leakage current worth noting. Using Ohm's laws you can calculate the current based on the meter load resistance and the measured voltage.

For electrolytic caps I measure DC current, if the cap is showing over 500 uA leakage I usually chuck it.

Pete

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Fri 17, 2017 7:31 pm 
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So...I should have mentioned that you can measure current, if you know the input impedance of your meter?

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Fri 17, 2017 7:48 pm 
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Just showing how I was doing leakage measurements. Trying to measure in the lower uA range usually is difficult.

Pete

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Fri 17, 2017 8:03 pm 
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If you know the voltage of your power supply and the input resistance of your meter you can easily determine a pass-fail point and not need to do any further calculations to decide if the cap is good or not. Many years back this was published in one of the major electronic magazines, perhaps more than once. With about a 300 volt supply and an 11 meg VTVM, I used to reject anything measuring more than about half a volt on the free end of the capacitor for paper and film types.

Whoever set that arbitrary 100 Megs as an acceptable amount of leakage is completely wrong, but it was widely published over the years. As already pointed out, that much leakage can be fatal in a coupling capacitor application, and can cause weird symptoms in many other circuits. Any professional technician who has done enough troubleshooting for unusual symptoms can verify that. 500 Megs or 1000 Megs is a much better standard for acceptance and new film caps always measure much higher than that. I would not be comfortable using an old mica or disc ceramic cap which measured as low as 500 megs or 1000 Megs.

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Fri 17, 2017 8:38 pm 
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One thing I didn't see mentioned. When first connecting a cap to voltage it acts as a dead short. Leakage is only measured after the cap is charged. A sensitive meter in series with a cap and power supply can easily be destroyed unless enough resistance is in the circuit.

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Fri 17, 2017 9:44 pm 
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Peter Bertini wrote:
Just showing how I was doing leakage measurements. Trying to measure in the lower uA range usually is difficult.

Pete

Amen--I like your technique.
MY only point was perhaps nitpicky---you can't measure the voltage coming through a capacitor.

In the Wikipedia article, note the comment about some DVMs having input impedance up to 1 giga-ohm. The overly-belabored point? You have to know your test equipment.

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Sat 18, 2017 12:22 am 
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Quote:
Whoever set that arbitrary 100 Megs as an acceptable amount of leakage is completely wrong, but it was widely published over the years. As already pointed out, that much leakage can be fatal in a coupling capacitor application, and can cause weird symptoms in many other circuits. Any professional technician who has done enough troubleshooting for unusual symptoms can verify that. 500 Megs or 1000 Megs is a much better standard for acceptance and new film caps always measure much higher than that. I would not be comfortable using an old mica or disc ceramic cap which measured as low as 500 megs or 1000 Megs.


The 100-megohm "throw-out" number was mentioned in a number of old military manuals and it worked its way into other literature from there. For conventional tube circuits, it is generally okay, even in coupling applications. If a cathode resistor is used, self bias is self adjusting. Since most grid bias resistors are one megohm or less, 100 megohms or more of leakage in a coupling cap forms a 100:1 voltage divider which causes a small shift in operating point. Most circuits simply adjust to it and continue on.

Of course if you are dealing with a situation where the grid circuit resistance is considerably more than a megohm, where the circuit design does not allow for automatic adjustment of operating point, or where that is not tolerable, then 100 megohms of leakage resistance in a cap may be too much. Just as there is no tube tester that can say with absolute certainty that a tube is good, but they all can tell you when one is really bad, a leakage reading below 100 megohms on an electrostatic cap meant "fail." But 100 megohms or more did not mean that such caps were okay to use anywhere in any circuit; you had to use your own judgement on that.

With regard to the Fender amp at the center of the discussion, getting 10 VDC on the open ends of plastic film caps with B+ applied to the other ends is a bit unusual, even if you were using a VTVM with a 11-megohm input resistance. It is possible that there is some leakage through the epoxy coating on the outsides of the caps; some of the early formulations had contaminants or underwent chemical changes after many years. One simple thing you could try is to replace one of the caps with a new 0.1-uF, 630-volt film cap and then repeat the test on it. If there's no voltage, you know something is wrong with the old caps in the amp. But if you still see a voltage, then it is likely there is something else going on, perhaps some electrostatic effect on an overly sensitive meter.

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 Post subject: Re: Capacitor Leakage question
PostPosted: Feb Sat 18, 2017 1:17 am 
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Norm Leal wrote:
One thing I didn't see mentioned. When first connecting a cap to voltage it acts as a dead short. Leakage is only measured after the cap is charged. A sensitive meter in series with a cap and power supply can easily be destroyed unless enough resistance is in the circuit.


Auto ranging Fluke. And, electrolytic caps can take a long time to charge with a 10 Meg source impedance, precharging helps.

Pete

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