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Super-Drifty Silver Mica Capacitors

Discussion in '"Boat Anchor" & Classic Equipment' started by KW4H, Oct 18, 2021.

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  1. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Posting this separately as it's an issue that could be of interest regardless of the boat anchor involved. The previous owner of a radio I'm restoring at the moment (National NC-125) removed a 4300 pf mica from the front end tuning section, and I had no choice but to find a new mica. Bought a small number of them off of eBay and the drift is off-the-charts. No matter how long the radio is left on, that mica drifts, and drifts, and drifts. Verified it was the mica by warming it up slightly with a heat gun and sure enough -- it rocketed into orbit.

    That capacitor is obviously going to have to go. When replacing the old mica "domino" style capacitors in tuning sections, what is the best type to use?

    73 - Steve, KW4H
  2. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    After doing some more research, and based on the price I paid, I'm pretty sure that mica capacitor was cheap crap from China. I've ordered a new 4300pf, 2% mica from Mouser, and also a 2% multilayer ceramic (C0G) (also from Mouser). Will try both and report back on which one is the most stable. The MLCC ceramic was about a third of the cost of the mica. Now, I'm going to go put myself in the doghouse for ordering parts off of eBay. Yeeesh!
    KA0HCP and KP4SX like this.
  3. K8CCA

    K8CCA Ham Member QRZ Page

    Didn't they use temp compensated caps in applications like that. I read that they may have even been hand selected during build. Is there a modern temp compensated mica these days?
  4. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    It would be good to hear from others who may be better versed than myself -- however, I believe a good choice is C0G (C-zero-G) (Class 1) ceramics. They have zero drift with a tolerance of plus/minus 30 parts per million (that's 0.3 percent), and are considered temperature compensating. Temp range is -55C to +125C. The mica I ordered today is also rated C0G -- it should behave identically to the ceramic, and it will be interesting to test each. The MLCC ceramic was 4 bucks, the mica a truly outrageous 11 bucks. I'm hoping that the test will show that the much cheaper ceramic will perform just fine at a lower cost.
    Last edited: Oct 18, 2021
    KA0HCP likes this.
  5. K6BSU

    K6BSU Ham Member QRZ Page

    The recommended capacitor for maximum temperature stability is the CoG or "NP0" in the dipped silver-mica package. Most stable VFOs use these with outstanding results.
    KW4H likes this.
  6. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    That's one of the two capacitors on the way from Mouser. The POS dipped mica I bought through an eBay seller is going into the trash. Lesson learned.
  7. VE3EKJ

    VE3EKJ Ham Member QRZ Page

    Actually, C0G might make it worse! When I was a lad a century or so ago, temperature compensating caps could go down or up in value. With most circuits it is not the mica capacitor that is causing the drifting. Other parts cause the drift as the rig warms up!

    Compensating caps were used that would change in value opposite to the direction of the existing drift. You would take a couple of caps that would change by differing amounts and by trial and error find one that would counter the drift.

    So unless the mica cap was the specific cause of the drift, which is unlikely, all it will do is follow along with the drift that is caused by something completely different.

    Bill V E 3 E K J
    SWL37632, KP4SX and KW4H like this.
  8. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Thanks, Bill. Well, in this situation pretty much all causes and options are on the table, because someone who owned this radio in the past removed the original mica and put a polystyrene in its place (and it was also entirely the wrong value). It had to come out and be replaced with something more appropriate. I ordered a silver-dipped mica off of eBay and it obviously has terrible drift. No matter how long the radio is left on, it just drifts and drifts. It drifts so much you barely have time to peak the signal when doing an alignment. To verify it was probably the capacitor, I hit it with a little bit of focused heat from a heat gun and it went into orbit. Even touching or moving the capacitor makes the circuit unhappy. BUT -- we shall see.

    73 - Steve
  9. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Is this affecting all bands?
  10. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Seems to be the "A" band only.

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