[ARC5] R-25 dial nonlinearities

[Richard Knoppow]

    Capacitor plate shape is covered in many older engineering books. I 
will see if I can generate a list.  Many older books are available on 
line from tubebooks.org and archive.org   These are free.
    Variable caps used to be available in three or four standard types:
Straight line capacitance, straight line wavelength, straight line 
frequency and compromise. Straight line frequency requires that the plat 
shape be rather extreme so were not very often used.  Receivers mostly 
used "mid-line" caps which provided some extension on the low 
capacitance side in order not to compress the high end dial too much but 
still retain a more practical plate shape.
     One thing to look for when the dials seem far out of calibration is 
the centering of the rotor.  The variation in capacitance changes when 
the rotor is off center and is not linear.  At the center the 
capacitance is minimum. Remember the capacitance is a square law 
function of area so the change in variation with spacing is somewhat 
critical. If you can measure the capacitor on a bridge you can adjust 
the end bearings to get it exactly centered.  This is assuming that no 
serious plate bending has been done. Plate bending is legit on some 
devices and not on others. For instance, the handbook for the 
Hewlett-Packard 200CD oscillator gives specific instructions for bending 
the plates (but the factory bending should usually be left alone) while 
in the Hammarlund SP-600-JX the plates should NEVER be bent. If they are 
you can never get the dial to track.
     I would suggest looking at the cap in the R-25 to see if its 
correctly centered. I don't know whether they are supposed to have bent 
plates.

On 1/8/2017 10:55 AM, Mike Everette via ARC5 wrote:
> Gordon White wrote:
>
>
>     Unfortunately I did not hear that he had died until his heirs tossed
> out his design notebooks.
>
>
> And so, more knowledge and history is lost forever.
>
> Guys, if you have stuff like this, please please PLEASE make sure
> someone in your family knows its value and significance.
>
> 73
>
> Mike
>
> WA4DLF
>
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL