[Johnson] Invader VFO Modification

[james.liles at comcast.net]

Good afternoon Miles:

If you are interested, you can do the same thing with a pair of piston
trimmers.  Simply make 1/2 or 1 turn at a time forward on one and
reverse on the other until the original frequency is reached.

Another note, the adjustment must be made in the center of the VFO tuning
range because the positive temperature comp personality of the tuning cap
varies as you change frequency from top to bottom of the tuning range.
If you measure the drift at the low, center, and high end of a well balanced
CTO VFO, you will find the the drift to be negative at the low end as
compared
to the center, and positive as compared to the center on the high end.

Kindest regards Jim K9AXN


Message: 4
Date: Tue, 06 Aug 2013 09:25:31 -0400
From: k2cby <k2cby at optonline.net>
To: Johnson at mailman.qth.net
Subject: [Johnson] Invader VFO Modification
Message-ID: <7619AB6863594E74AE75F9FE7858AE3A at milesdesk>
Content-Type: text/plain; charset=us-ascii

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