[ARC5] R-25 dial nonlinearities

[Richard Knoppow]

    There have been all sorts of methods of getting linear calibration 
for frequency. This is of course the reason for the Collins permeability 
tuned oscillator. The coil is wound with square law spacing and the 
inductance is varied by moving a slug of ferrite or powdered iron in and 
out of it.  In the Collins set up there is a "corrector" consisting of 
an adjustable cam that give fine variation to the slug as it is moved. 
The corrector appears in a Collins patent c.1940s and was evidently used 
in some WW-2 vintage Collins equipment. I can't dredge the details out 
of my memory.
     General Radio had a patent on an arrange where both C and L were 
varied simultaneously.  This was used on a VHF wavemeter to get a very 
wide frequency range and approximately linear calibration. I think it 
was also used on a GR signal generator. The square sliding cap has been 
mentioned. It can be made to have square law capacitance thus yielding a 
linear frequency calibration without being too delicate as a 
conventional straight line variable air capacitor is. There are probably 
other methods I don't know about or have forgotten. I think I ran across 
an RCA patent for a mechanical linkage to accomplish the same thing. 
Most of the time a compromise capacitor was used, which explains the 
crowding at the high end of most radio dials.

On 1/8/2017 9:55 PM, Jim Falls wrote:
> I had trimmer cap fits when aligning my BC-312-L. All the RF stages use
> the same style 4-vane semicircular straight line trimmer. It's bloody
> murder getting the uppermost band set: the cap is far too sensitive to
> minuscule shaft movements. I ground down a screwdriver so it fitted the
> slot precisely, then used a mini vice grip with a popsicle stick handle
> (torque wrench style) to get close. I wonder if a custom tool was used?
>
> Even then, I had to leave the "clamp collar nut" merely snug. Tightening
> it threw everything off. I finally had to walk away before I took a
> hammer to it. It's reasonable at 16-18 MC, but not worth bald spots to
> get any closer.
>
> Cheers!
> Jim K6FWT


-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL