[ARC5] R-25 dial nonlinearities

[Kenneth G. Gordon]

Thanks, Bruce. Well, I most certainly would be intereted in your finished article. I am sure 
others would be too, although maybe not enough of us.

God Bless and vy 73,

Ken W7EKB

On 8 Jan 2017 at 21:59, Bruce Long wrote:

> 
> Kenneth
> When I first got interested in hm radio and especially in receiver design as a 
> teenager in the late 60's and early 70s it was fairly common to read in the 
> amateur literature about building a receiver and filing the tuning capacitor plates 
> to achieve linear dial calibration, but I had never saw any good explanation of 
> how to do it.  
> 
> When I got out of college and had some time and some money and access to a 
> very simple desktop computer at work that ran visual basic I worked out the basic 
> principals myself and gathered notes for an article that I never finished.  Just a 
> few years ago I found the equation I derived for straight line frequency capacitor 
> plate shape in a 1935 radio engineering handbook published in Australia.
> 
> I never finished my article in part because digital dials became easy and 
> affordable and amateur interest in the topic and indeed in equipment design and 
> building seemed to have dropped off.
> 
> Now perhaps there is an audience among the vintage radio crowd, for a tutorial to 
> understand how it was done and to appreciate some of the receiver tuning tricks 
> the old receiver design guys developed and put into use.
> 
> 
> 
> From: Kenneth G. Gordon <kgordon2006 at frontier.com>
> To: ARC-5 List <arc5 at mailman.qth.net>
> Sent: Sunday, January 8, 2017 1:37 PM
> Subject: Re: [ARC5] R-25 dial nonlinearities
> 
> 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 --- This email has been 
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