NOS Silvered Mica Capacitors  To Use, or Not to Use..

Replacing silvered mica capacitors in Collins (and other) radios is a routine task during restoration.  The question often comes up, however:  what to replace them with?  It's clearly ok to use modern epoxy-dipped silvered micas, but what about brand-new, but old- stock, capacitors that one picks up at hamfests over the years?  Some restorers are fine with these NOS capacitors, while others won't use them on any account.  To try to answer the question of whether or not it's ok to use NOS capacitors, Gaylord Hart WB7ODD tested a large lot of these devices.  Here's his account of the experiment:

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.  The capacitor samples are shown below:

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.

Several requested I re-test the molded mica capacitors for leakage under a HV load. I finished refurbishing my Heathkit IT-28 Capacitor Checker and calibrated it. Tonight I ran leakage tests on the original sample lot of molded micas previously tested and reported on (n = 126 caps) . I tested all caps at 300 volts since some of the capacitors are rated for 300V. The checker was calibrated for full eye closure at 2 uA leakage current. Not a single capacitor failed the leakage test. I'm still convinced NOS molded mica capacitors are in general good to use as replacements: 1) only 4 of the 126 were out of their rated tolerance for capacitance (and three of these were only out by .7% or less), 2) none showed a static leakage resistance less than 300 megohms, and 3) none showed a leakage current >= 2 uA at 300 volts applied voltage. Having said that, I would not install an NOS molded mica cap without first measuring capacitance and leakage performance. I'm done testing here--on to the original task, restoring radios.

As an aside, there were about a dozen NOS paper capacitors in the pre-test lot in the same molded package style, and I decided to test these as well. Every one of these tested bad.

Click here to see a copy of my spreadsheet.

Gaylord WB7ODD