On Tue, 2008-03-04 at 07:45 -0800, Jim Brown K9YC wrote:
> On Tue, 4 Mar 2008 09:55:27 -0500, Jack Mandelman wrote:
>
> >However, instead of using
> >doorknob caps I replaced the originals with 6KV 470pF Y5P ceramics.
>
> It's no surprise that the original caps, which have a fairly high
> negative temperature characteristic, change value as they warm up --
> they are SPECIFIED to do that. :) I'm wondering if it was a pricing
> decision by Ten Tec to use them, or if the designer was attempting
> to compensate for heat-related drift in the other direction by the
> inductor?
>
> 73,
>
> Jim K9YC
>
>
No coil is that temperature sensitive. The change in capacitance by
ordinary (bypass and coupling grade) ceramic capacitors is dramatic, and
much as an 80% loss of C going from room temp to 85 C. I learned about
that long ago, tucking a little GP dog bone against an IF tube socket.
Then I took it out and put it on a bridge and applied heat. I learned
that the compact capacitor tended to have a very high temperature
coefficient, a characteristic of the high dielectric constant ceramic
used to make it small, and also was piezo electric.
73, Jerry, K0CQ
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