[ARC5] R-25 dial nonlinearities

[Richard Knoppow]

    Keep in mind that the variation of frequency with capacitance is 
inverse square law. Resonance is f=1/(2pi*L*C)^1/2  This is not linear. 
The cap may have plates that are shaped to vary the C faster as it is 
reduced and so expand on the high end but most caps were compromise 
shapes with only some expansion because the plate shape becomes extreme. 
You can try bending the plate but once bent it hard to get them back. In 
most superhetrodyne receivers the tuning capacitor for the local 
oscillator must be set up so it tracks the RF and antenna stages. 
Usually done by some combination of trimmer and padder caps maybe on 
both sets of caps.  Look on tubebooks.org for the third and forth 
edition of "The Radiotron Designer's Handbook" which explains how to do 
this. It takes some mental juggling.

On 1/7/2017 4:57 PM, J Mcvey via ARC5 wrote:
> I got around to marking the dial on the "bandspread" R-25 .
> (This is the one that someone had pulled the plates out of the variable
> cap.)
> It tunes from just below 1700 to a bit over 2000.
> The 100khz spans on the dial increase in size as it progresses to 2000
> to the point where the distance between 1900 and 2000 is about twice the
> 1700 to 1800 span.
>
> Is there an easy way to correct it or it that just the way it's going to be?
>
> The interaction of L and C vs dial calibration has always been my nemesis!
>
>
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL