Silver Mica Capacitors

Discussion in '"Boat Anchor" & Classic Equipment' started by WA1GXC, Aug 23, 2019.

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  1. WA1GXC

    WA1GXC Ham Member QRZ Page

    Silver mica capacitors can be prone, over years, to failure issues due to the phenomenon of

    "silver migration". I just had a catastrophic failure of a 1973 Drake T-4XC with a New-Old-Stock (NOS)

    replacement S.M. cap. While running down a low injection level problem, I pre-emptively replaced

    a handful of RF circuit caps, thinking perhaps they were RF-lossy. After maybe 5 minutes total of

    transmit testing, the grid-coupling cap to the sweep-tube P.A. finals failed to a dead-short. Instead of

    -55V bias, the tubes had +235V on the control grid (G1) during key-down transmit. Think for a moment,

    and let your imagination do the rest.

    These were not old, crappy WW II-era postage-stamp micas. They were modern-era brown dipped-mica

    MIL-style manufactured by General Instrument probably in 1970's.

    You all have a deep well of experience and knowledge out there. I'd be interested in your experiences and

    comments. Should I stick to brand-new manufacture in circuit-critical capacitors, or are we all screwed once

    in a while by a random component failure?

    Happy ending--after changing-out a handful of resistors nuked by the event, I ran-down the underlying

    injection problem (weak PTO output) and the unit is putting out power like the day it left Ohio--using

    the same stressed final tubes. Vacuum tubes rule!

    TKS
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yes and yes.

    Using brand new components is generally wise but some percentage of components, even brand new components from reputable vendors, will be bad or fail prematurely (infant mortality). There's not much you can do about that but testing components prior to using them is sensible and even then some will fail for unexpected reasons though statistically the vast majority will work just fine.

    FWIW, it's unlikely that your new silver mica cap to have failed from actual silver migration which requires a voltage differential, a moist environment and time to occur. It's more likely you had a cap that was shorted in the factory from excessive plating, cracked mica insulation or some other manufacturing glitch. That's assuming the component was new old stock and not salvaged from an unknown piece of equipment. It's likely you could have measured that short with an ohm meter prior to installing the cap into the circuit for it to fail in such a spectacular way and take out a bunch of other components.
     
    WA7PRC and N2EY like this.
  3. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    Yes! My old boss at Ablestik Adhesives was a co-discoverer of silver migration...and it is very well understood what conditions cause it. (By the way, you do NOT need any moisture for it to happen....silver will creep across a perfectly dry glass slide! And rather quickly too!). But you do definitely need a DC potential between the conductors.
     
  4. KK4NSF

    KK4NSF Ham Member QRZ Page

    Well maybe, maybe not. ;)
    I've used a LOT of old silver mica caps in the gear I've built, and have never had a failure. The majority of the ones I've used were pre-WWII vintage... and to be honest, some of them have withstood a lot of higher-than-rated voltages / currents. BUT I must say that I examine and test each one carefully before I use it..... and if it shows the slightest corrosion at the juction of the leads and body, OR if there's a gap there, OR if it shows any DC leakage on the meter at all, I assume it's either bad or going bad.

    So... you are right. Eventually, all of the old ones will be gone, and we will be forced to use new ones. However, that does not keep me from using the good ones while they last.
     
  5. WA1GXC

    WA1GXC Ham Member QRZ Page



    Thanks to everyone so far. Out of curiosity, I later checked a large number of S.M. caps from the same lot using a 20 MOhm function. All show undetectable

    leakage. Of course 2 volts thru the path under test probably won't show much. Other than building a test jig and putting 300V on the component under test

    to measure leakage, I guess we're back to faith-in-numbers. I'm intrigued to know if the failed device would have shown anything significant on a DMM. Only similar

    failure I've experienced was 3 decades ago. A 20 year-old Collins 75S-3C from 1966 had a S.M. cap in the BFO circuit with only 150 volts on it (500V rating),

    went to dead-short and wiped-out a bunch of stuff. S t u f f happens.
     
  6. K9STH

    K9STH Platinum Subscriber Platinum Subscriber QRZ Page

    Basically, any silver-mica capacitor with more than around 75-volts on it is prone to silver-mica disease. They can acquire the disease with even less voltage on them but such is fairly rare.

    When silver-mica capacitors are used in most, but certainly not all, tuned circuits (excepting i.f. cans), the applied voltage is, generally, pretty low. As such, those capacitors are usually OK. But, when used as coupling capacitors (i.e. in the Collins 75A-4 receivers), or when used as bypass capacitors, the silver-mica capacitors are failing on a regular basis.

    Glen, K9STH
     
    KA0HCP likes this.
  7. K1APJ

    K1APJ XML Subscriber QRZ Page

    We are all screwed once in a while by random component failure.

    I sometimes experience similar issues with NOS parts. Then I realize that NOS from the 1970s is going on 50 years old, whereas old crappy stuff from WWII is 75 years old.
     
    N2EY and KA0HCP like this.
  8. WR2E

    WR2E Ham Member QRZ Page

    If your meter has a " Siemens " function on it, use that. It's the reciprocal of Ohms also know as Mhos. Measure of conductance rather than resistance. Much more sensitive when looking for leakage, sometimes TOO sensitive!
     
  9. N1OOQ

    N1OOQ Ham Member QRZ Page

    I've used many silver micas over the years, never had a problem. Which, of course, does not mean it never happens.
     
    KK4NSF likes this.
  10. K9STH

    K9STH Platinum Subscriber Platinum Subscriber QRZ Page

    OOQ:

    The newer, "dipped mica", capacitors, so far, are not failing that often from "silver-mica" disease. It is the older postage stamp / domino silver-mica capacitors that are now problematic.

    Glen, K9STH
     

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