Super-Drifty Silver Mica Capacitors

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

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  1. SWL37632

    SWL37632 QRZ Member

    I agree with EKJ.

    As I was writing this I remembered conversing with some folks on one of the old net reflectors and the word "Oxley" popped into my head.

    I was spending a lot of time and resources in stabilizing FET vfos some 20 or 30 years ago...before the Si570XX chips made their appearance. Huff-n-Puff, ovens, all kinds of different exotic wire for the coil, temperature compensation, etc., etc.

    Anyway, just did a quick search and found something interesting that may be helpful:

    Thermotrimmer® - ngunn.nethttp://www.ngunn.net › _Oxley_Trimmers

    Bill

    20+ WPM 1970's Extra
     
  2. KP4SX

    KP4SX Premium Subscriber QRZ Page

    The schematic I'm looking at is terribly blurry but it looks like it is used on the B band.
    Anyway, I wouldn't expect to see a 4300 in a tempco scenario.
     
    KW4H likes this.
  3. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Some of this is definitely up in the air at the moment. The capacitor was definitely missing and the replacement a bad choice, so what's coming in from Mouser will resolve that. However, other issues remain. For example, I was attempting to align the A band today and L-1 would not produce a peak. Signals were getting through, but L-1 appeared to be inoperative. I looked closely at the coil and couldn't find any technical or physical issues -- but there's a reason it won't react. It could be a shorted coil, and I might have to pull it out for further investigation. It could be rebuilt.

    73 - Steve, KW4H
     
  4. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Have other components been replaced? Resistors? Often times designers would use resistors that would drift in the opposite direction of the caps when heated to compensate for the drift.

    Carbon resistors have a negative temp coefficient whereas modern film resistors are typically positive. They shouldn't be swapped in sensitive circuits.
     
    KW4H likes this.
  5. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Any drifted resistors in the entire radio have been replaced -- and the recap is finished. I've now replaced that capacitor with the C0G and the crazy drift stopped. There are other issues getting it aligned (will post separately) but things are looking up.

    73 - Steve
     
    PY2RAF, KM1H and KP4SX like this.
  6. K0OKS

    K0OKS Premium Subscriber QRZ Page

    My point was... were those replaced resistors replaced with models of a similar temp coefficient (at least negative or positive)? If carbon resistors were replaced with metal film or metal oxide resistors in a sensitive circuit it might make it hard to stabilize.

    I am talking about drift in value as the temperature changes (temp coefficient), rather than drifting in value due to age.
     
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  7. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Ah! I get your question, then. Well -- in the front end of the NC-125 there are no resistors, it's all coils and caps. The first resistor in the radio sits at the grid of the RF amplifier. However, as far as the circuits in the rest of the radio go -- for better or for worse, I did in fact use metal oxide. It's a subject of great debate sometimes and a very interesting one at that -- and maybe I should pay more attention to that detail of when carbon comp should really be used. Do you have any rules of thumb?

    Thanks and 73 - Steve, KW4H
     
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  8. K0OKS

    K0OKS Premium Subscriber QRZ Page

    If the circuit is influenced by temperature drift of the resistor then I will try to use whatever had been in there, otherwise I use metal film for everything else because they are accurate and cheap.
     
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  9. KM1H

    KM1H Ham Member QRZ Page

    I dont know of any silver mica caps with a NPO spec which is limited to ceramic caps AFIK.
     
  10. KM1H

    KM1H Ham Member QRZ Page


    The first rule is that there are no resistors such as mentioned:

    I am talking about drift in value as the temperature changes (temp coefficient), rather than drifting in value due to age.
     

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