[ARC5] R-25 dial nonlinearities

[Kenneth G. Gordon]

On 8 Jan 2017 at 17:26, Bruce Long via ARC5 wrote:

> Some of the old 1930 vintage radio design handbooks have useful information on 
> that topic but I have not seen any stand alone written treatments.

I have seen such info over the past 60 years, but didn't save the info anywhere that I can 
find it now.

It was exactly as you describe: scribe the plates in the shape arrived at by forumla, etc., 
then file them to that scribe.

If one looks closely at the plates of the tuning capacitors in the ARC-5 receivers, one will 
see that their shape is "modified" from a straight-line capacitor shape already.

The non-linearities with the subject modified R-25 dial spread also occurred with the 10 
meter conversion of a BC-454 I discussed here some time ago. The difference in dial 
spread between the low end and the high end of the 10 meter band is significant, but not as 
drastic as the R-25's. By hand-calibrating my 10 meter receiver dial, I can easily live with it, 
and it has not been that difficult to achieve.

My 10 meter receiver has exactly one rotor and one stator plate left in it (I did not do this, 
BTW), so that may be one reason the issue is relatively small in this case.

Even so, I have always been VERY interested in how to accurately modify the plate-shape 
of tuning capacitors to achieve an accurate dial.

I am very grateful to the group for looking into this issue.

I may dig into my voluminous files to see if I can, eventually, find the info I have stashed 
away some where.... :-(

Ken W7EKB

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