[Johnson] Invader VFO Modification

[k2cby]

For what it's worth the differential capacitor method of temperature
compensation was written up in QST (I think by Doug DeMaw) some time in the
1970s - 80s when he did a feature on VFO design.

 

As best I can remember he said it was dirt simple to adjust. I believe the
procedure was: (1) fully mesh the differential capacitor on the NPO side.
(2) Set the regular trimmer & coil to frequency. (3) Let the VFO warm up.
(4) Then adjust the differential cap to yield frequency initially set (i.e.,
to cancel the drift).

 

As I also recall, the fixed capacitors (one NPO and the other N750) were
identical in value and on the order of hundreds of pF while the differential
cap was small (maybe 15 or 20 pF).

 

I was tempted to try this but I could not find (afford) a butterfly
capacitor at the time.

 

Miles B. Anderson, K2CBY
16 Round Pond Lane
Sag Harbor, NY 11963
Tel.: (631) 725-4400

Fax.: (631) 725-2223
e-mail: k2cby at optonline.net