[TenTec] 238 Tuner Mod

[Dr. Gerald N. Johnson]

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