Thoughts on using old mica capacitors

Discussion in 'Homebrew and Kit Projects' started by K8AI, Jun 5, 2017.

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  1. WA7PRC

    WA7PRC Ham Member QRZ Page

    It appears the first requirement is CHEAP:
    The capacitors I was going to suggest, while temperature-stable and rated for RF current, cost MUCH more than a buck each.
     
    K8AI likes this.
  2. G4COE

    G4COE Ham Member QRZ Page

    Dubilier and Micromold comes to mind, referring to the old ones of course with the coloured dots.

    Dave
     
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  3. K8AI

    K8AI Subscriber QRZ Page

    Well, that's why I was asking specifically about this particular type cap. I'm HOPING to put them to use and I was just putting out a friendly request for input from some of you more knowledgeable and experienced radio folks. I think that's a reasonable and prudent thing one can do, wouldn't you all think?

    Hey, if I get a bunch of "DON'T USE THAT CRAP" suggestions, I'll throw them in the trash and spend the money on some new, high current/voltage, low loss, stable silver mica chips or something. It's that simple. Thanks.
     
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    I would test them. If they test good then use them.

    I have 50 year old caps that work just fine. Even some older than that.

    I never did a shotgun repair on my radios replacing all of the caps just because 1 went bad.

    Have Fun.
     
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  5. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    If I were you, I would go ahead and test and try them.

    First check their value with a capacitance meter for sure. This would be one indication that they are still good; if they match their coded value that a good sign. Once you know that value, you should also be able to tell from the color code chart whether they are mica or paper too.

    If you have an ESR meter, you could test them with that. If you don't have one, they are pretty easy to build (look up the plans on http://www.ludens.cl/Electron/Electron.html that's what I used to build mine).

    But you could use them even if you don't have a meter. Once installed, you could test them with a 100W signal through the filter for an extended keydown. Afterwards you can check the temperature of the caps. If one gets hot, that would indicate a problem capacitor with high ESR.

    This website seems to indicate that if they are mica, they would still be fine to use: https://www.antiqueradio.org/recap.htm

    What 'cha building?

    73,


    Mark
     
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  6. K8AI

    K8AI Subscriber QRZ Page

    One thing I'd like to use them in is a 10/15/20m "triplexer" for field day. Also, maybe some more bandpass filters. I made a set of BPF for last year's field day but I used some silver micas from the junk box.

    I have checked a bunch of these old postage stamp micas with a BK precision capacitance meter and they're, for the most part, fairly close to their marked values.
     
    KA9UCN likes this.
  7. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    I might check them for temperature stability then. You could put the caps in the refrigerator, then once cold, put them on the meter. See how the value changes as they go to room temperature. Then put a hair dryer on them to check the value as they get warm. The mica should give them good temperature stability, but they are not as good as silver mice ones.

    73,


    Mark.
     
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    Silver fish ones are good too. :)
     
  9. G4COE

    G4COE Ham Member QRZ Page

    I think I'd be being careful where I'd be using them, not because of their age but because of their voltage ratings..... i.e triplexers, harmonic filters. Capacitors, their materials and their construction methods has changed considerably during their time.

    Dave
     
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  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    When I worked in the RF Lab of the world's largest OEM of RF-driven sealed CO2 Lasers, we used LOTS of porcelain dielectric capacitors from American Technical Ceramics (ATC). Their 100-series have a P90 temperature characteristic (they're quite stable) and are rated to handle RF current (see datasheet). Digi-Key has a pretty good selection in stock: http://tinyurl.com/y8nndenn
     

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