Re: {Collins} NOS domino mica capacitors and failure modes



Hi Gaylord,
I believe that testing caps' leakage at or near (or slightly above) their voltage limits is essential. You need a test that actually tells you if the caps will work properly in use, especially in tube gear with high B+. The high precision of digital meter capacitance values is not very useful with ordinary coupling and bypass caps: it almost never matters whether a .01uF bypass is .005, .01, or .02uF, but it often matters a great deal whether it is leaky at 200 VDC. At the low voltages used in digital meters, leakage is often very different than at high voltages: lots of bad caps have zener-like behavior: little or no leakage at low voltages, and suddenly show serious leakage or shorting at some higher voltage.

I think the best testers around (outside of HP and other high grade stuff) are the Sprague TelOhmike (the Cadillac) and the Heathkit IT-11 and IT-18. They are the best because they can impress anywhere from 3 volts to 600VDC on a cap under test, and the degree of closure of the 6E5 green eye tube gives you an estimate of the cap's leakage at the applied voltage. In the mica/paper cap switch position, if the eye is wide open under voltage testing (open circuit angle), the resistance under volt stress is > than about 500 Mohms with my IT-11. The eye will be completely shut below 100 Mohms, so it is a very stringent test. Almost all paper caps will fail this test, and all new plastic, ceramic and mica dielectric caps will pass easily if good. First, you have to be certain that your unit (IT-11 or whichever) has good, accurate non-leaky bridge caps and accurate resistors in its voltage test decade, so that the impressed voltage is what you think it is. There are also a couple of mods that make the IT-11 and 18 give an easier to detect null (open eye). I just checked on Ebay and there are 2 IT-11s for sale presently, one at $9.95 and one at $15. The 6E5 eye and the numerous available voltages also allows you to reform electrolytics...and watch the dynamic process: the eye goes from closed to fully open as/when the dielectric re-forms at a given applied voltage...when it's fully open you step the applied voltage up to the next value and the process repeats:...fun to watch!
Fred


At 08:01 PM 28/01/2012, you wrote:
Hi Fred,

I measured with a DMM, so very little voltage. I have an old Heathkit capacitor tester I may drag out to do more testing. It does leakage tests at various voltages.

Gaylord

----- Original Message ----- From: "Fred & Helen Archibald" <hfarchibald@xxxxxxxxxxxxxxx>
To: "Gaylord Hart" <gahart@xxxxxxxxxxxxx>
Cc: <collins@xxxxxxxxxxxxx>
Sent: Friday, January 27, 2012 2:32 PM
Subject: Re: {Collins} NOS domino mica capacitors and failure modes


Hi Gaylord,

Many thanks for the interesting results..done like a proper experiment!

What voltage was across the caps when you measured their leakage? I use 500 or 600VDC, as many caps not leaky at low voltage become so at 500V. Also, if a cap isn't leaky at 500V is not going to leak in any receiver circuit.

I've got a cigar box full of old dipped SM caps, mostly used. I check every one at 500V before using...and 99% pass.

Fred VE1FA



At 04:42 AM 27/01/2012, you wrote:
OK, much measuring and re-measuring on the NOS silver micas I received from Dave, with a few of my own silver micas thrown in. These are all molded micas, postage stamp / domino capacitors. I removed any paper capacitors in the group (noted by the upper left dot on the domino indicating paper). BTW, there are too many "standards" for the dot-labeled micas, including how many dots there are and what they represent.

I only measured the NOS silver mica capacitors in the group. Sample size is 126. Rated capacitance values for the lot range from 5 pF to 10,000 pF. The measurements I took were with an Agilent U1732A LCR meter and a Phillips PM 2535 DMM. I measured each capacitor with the Agilent LCR meter calibrated and set for the optimal settings for accuracy for that capacitance value. Measurements were taken as is, no voltage soak and assuming these caps have never been in service. All lead lengths were factory original, and no signs of solder were observable on any leads. Manufacturers range from El Menco, Sangamo, Aerovox, Micamold, Cornell Dubiler, Sprague, and Solar. Most were El Menco and Sangamo. Some of these are bakelite encapsulated, some in other plastics. Without wanting to cast aspersion to Dave as to his age, the source for most of these caps, these are OLD caps. I can only assume Dave is in his 20's and enjoys visiting flea markets and has an eye for value.

Quite frankly, I was surprised at the results. All of the capacitors measured over 300 Megohms leakage resistance (the limit of my DMM), so I do not believe leakage performance deteriorates over age for these caps sitting in storage (although if these caps started at 500 Megohms and dropped to 350 Megohms, my tests would not show this). From a capacitance perspective, 96.8% of these caps were within their rated tolerance based upon measured versus labeled capacitance. 81% were within 5% of the labeled capacitance (regardless of specified tolerance). 92.9% were within 10% of the labeled capacitance (regardless of specified tolerance). And 96.8% were within 20% of the labeled capacitance (regardless of specified tolerance).

Over the entire group, the average measured capacitance was within -.01% of the rated value (similar number of caps measured above rating and below rating). The worst-case measured positive tolerance was +20.7% (a large number of the caps have a +/- 20% rating). The worst-case measured negative tolerance was -18.0%. 46.8% of the caps measured below their stated capacitance. 43.7% of the caps measured above their stated capacitance. 9.5% of the caps measured exactly at their stated capacitance.

My conclusions. NOS silver micas retain their leakage resistance performance exceptionally well over time. NOS silver micas also are very stable over time for their rated capacitance values. I would not hesitate to replace a silver mica cap with a vintage silver mica cap, but I would not do so without confirming leakage resistance and capacitance values first. For non-critical applications (where capacitance tolerance is not an issue), you are probably safe using an NOS cap if you cannot measure the actual capacitance. I would never make this claim for paper caps, which seem to be more prone to fail over time.

Based upon my own experience (limited and anecdotal), I think a high B+ DC voltage applied over time contributes to the deterioration of silver micas in-circuit. The 2 bad caps I found in my 75A-1 were both exposed to the full B+ across their leads, and both showed low leakage resistance (20-40 Megohms or so). I will replace both of these with NOS vintage replacements.

I will try to take a sampling of the caps I measured and provide a voltage soak over time to see if that changes anything.

If anyone wants a copy of my spreadsheet, contact me off-line and I will forward.

Gaylord WB7ODD



----- Original Message ----- From: "Gaylord Hart" <gahart@xxxxxxxxxxxxx>
To: <collins@xxxxxxxxxxxxx>
Sent: Friday, January 13, 2012 12:38 AM
Subject: Re: {Collins} NOS domino mica capacitors and failure modes


Got the 2nd mica cap out last night, pretty much the same measurement results:

Bad C69 (original spec = 100 pF +/- 20%, 500V)
Leakage resistance (bench DMM, 300 megohm max scale): 14.6 megohms or 15.1 megohms, depending on DMM polarity to cap.
Capacitance (Agilent LCR meter): 92.4 pF, Q = 12.0

Whopping big difference on leakage resistance and Q from a bad to a new good capacitor, even though the bad caps still have pretty high resistance.

Both of these caps came from circuits where they had the B+ voltage across them, so I suspect moisture ingress coupled with high DC voltage or electromigration contributed to the degradation. An alternate hypothesis is that the noise problem only shows up in caps in the B+ circuits because a high voltage is needed to induce the noise voltage across the capacitance leakage resistance. The caps still test good for the rated capacitance, and for a circuit with no substantial voltage across it, would probably perform OK. At this point, however, I think the HV contributed to the failure.

Gaylord WB7ODD




----- Original Message ----- From: "Gaylord Hart" <gahart@xxxxxxxxxxxxx>
To: <collins@xxxxxxxxxxxxx>
Sent: Wednesday, January 11, 2012 11:23 PM
Subject: Re: {Collins} NOS domino mica capacitors and failure modes


I just pulled the first of the bad silver mica caps from my 75A-1. The second will take more work since it is buried in the 1st LO compartment on the underside of the receiver. Here's what I found:

Bad C28 (original spec = 100 pF +/- 20%, 500V)
Leakage resistance (bench DMM, 300 megohm max scale): 24 megohms or 19.5 megohms, depending on DMM polarity to cap.
Capacitance (Agilent LCR meter): 83.2 pF, Q = 9.7

Brand new 120 pF silver mica reference capacitor
Leakage resistance: >300 megohms (DMM max reading)
Capacitance (Agilent LCR meter):  124.5 pF, Q = 66.8

Two interesting things to note about the bad cap. 1) When I connected the DMM to the bad cap, the leakage resistance slowly rose over time. This correlates with my experience that the noise problem in the receiver occurred less frequently and went away over time as I left the rig on. 2) The capacitor has developed a polarity sensitive leakage resistance, which would suggest a leakage path with diode-like attributes as part of the model.

I also measured several old silver mica caps that have been in a parts bin since I cut them out of old radios I took apart as a teenager many years (decades) ago. These were all in service in a circuit at one time, and usually have leads too short to be of much value, but all of them measured over 300 megohms resistance and had Q's over 65. I've taken Dave Harmon up on his offer for a shoebox of old NOS silver mica's, and when I receive these I'll do more exhaustive measurements on a larger sample size.

Gaylord



----- Original Message ----- From: "Fred & Helen Archibald" <hfarchibald@xxxxxxxxxxxxxxx>
To: "Gaylord Hart" <gahart@xxxxxxxxxxxxx>
Cc: <collins@xxxxxxxxxxxxx>
Sent: Sunday, January 08, 2012 9:23 PM
Subject: Re: {Collins} NOS domino mica capacitors and failure modes


Hi again, Gaylord,

The two approaches I find most useful for locating noisy caps are the following.

1. Following the schematic with the radio on (no antenna, but speaker connected) and the volume up to where you can hear the noise easily, use a 0.1 or 0.47 uF capacitor (250+ WVDC) with clip leads to short various parts of the circuit to ground. Following the signal path, you can usually localize the noise to a particular circuit pretty quickly. Pulling tubes, starting at the antenna can do it too, but I find the AC shorting cap faster and more specific when you get close.

2. Turn on radio (no antenna, but speaker on) and use a high Z probe on a scope to follow the circuit signal path. Use a millisecond sweep (audio range) and you should be able to see the cracks and pops of the rogue noise. You can combine this with pulling tubes and/or using the shorting capacitor.

Testing NOS caps for noisiness isn't always easy. Some you can see by a flickering or closing 6E5 magic eye tube in a cap tester set to apply 5-600 VDC in a leakage test, but other noisy caps will give an apparently steady dark wedge in the eye. Substitution in the circuit is the best test.

Good Luck!

Fred VE1FA


At 08:37 PM 08/01/2012, you wrote:
Thanks for following up Richard and Fred.

You are right, the problem is noise, static crashes, and it comes and goes. The longer I leave the rig on, the less frequent and intense the noise, which has made it extremely difficult to trace to the component level. These are silver-mica caps, have the colored dots on the face, red case, manufactured buy El Menco, and are typically 10-100 pF. I know I have a bad C28, so I'll replace that one and look for others. I think I have one other acting up. Consensus seems to be to replace with modern dipped mica, which I will probably do, but I would still like to find a NOS source and do some testing on resistance values.

Thanks, Gaylord


----- Original Message ----- From: "Fred & Helen Archibald" <hfarchibald@xxxxxxxxxxxxxxx>
To: "Gaylord Hart" <gahart@xxxxxxxxxxxxx>
Cc:
Sent: Sunday, January 08, 2012 10:57 AM
Subject: Re: {Collins} NOS domino mica capacitors and failure modes

 <collins@xxxxxxxxxxxxx>
Hi Gaylord,

Are they "EL-MENCO" reddish domino-style caps? I've replaced lots of them that were noisy (frying bacon), leaky, or shorted in 51J, R-390/R-392 receivers of the same vintage. The mica dielectric should last forever, so I suspect (but don't know) that the silver is migrating/growing crystals. The caps are typically noisy and often act like a zener. That is, they don't short until the voltage across them reaches a certain value, which I've measured as anything from 2-150 VDC. I always replace with the maroon dipped silver micas, often used. I've almost never found a bad one, and I pre-test them all at 500VDC to have
>100Mohms resistance.
Personally, I wouldn't use even NOS EL-MENCO domino-style silver micas, as I think failure is mostly age-related.

Cheers!

Fred VE1FA


At 04:37 AM 08/01/2012, you wrote:
Wrapping up a 75A-1 restoration, and I need an NOS source for the domino mica caps. Anyone know of a source for NOS here?

Also, can anyone enlighten me on the failure modes for these caps that makes them noisy? I'm a bit particular in replacing failed components with originals when I can, but if the failure mode is simply "aging," then replacing a bad component with one of the same vintage is questionable. I am assuming that having a high voltage across the caps leads over a period of time is the cause for the breakdown, not just aging, so an NOS part should be a reset on the next time to failure. Insights?

Gaylord WB7ODD

PS--many thanks to those who provided feedback along the way on my 30L-1 refurb. I put the chassis back in its cabinet last night and it's good to go.
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