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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