Larry East, W1HUE
Tucson, Arizona
(contact author)
As is common with varactor tuned VFOs, the MFJ Cub tuning is not very
linear; frequency spacing (i.e., kHz/degree of tuning pot rotation) is
quite a bit finer at the low end of the VFO tuning range than it is at
the high end. The tuning linearity can be greatly improved by simply
placing an appropriately sized resistor from the wiper of the tuning pot
(R4) to the "cold end" of the pot (the end that connects to R5). This
connection is shown in the diagram at the right. The "appropriate size"
depends on the particular band model that you have, but a good starting
point is 4.7K. There are extra holes for the pot connections, so it's
very easy to solder a resistor to these holes on the bottom of the board.
Probably the best approach is to tack-solder in a 10K pot, adjust it
until you get approximately the same frequency spread on the first and
second half of the rotation of R4, measure the pot value, and replace it
with a resistor that comes closest to the pot value.!
The shunt resistor will reduce the total tuning range somewhat, perhaps
10 kHz or so for a resistor in the 3-5K range. (There will be less effect
on the 30M model since it has a smaller initial tuning range.) The tuning
range may be further reduced, if desired, by paralleling a resistor
across all of R4. My 15M CUB had a "stock" tuning range of 62kHz, which
caused the tuning to be much too sensitive for my liking. By paralleling
R4 with a 10K resistor and placing a 4.2K resistor from the wiper to the
"cold" end, I obtained a 41kHz tuning range; 22 kHz are covered by the
first half of the pot rotation and 19 kHz by the second half.
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Copyright © 2005, 2012 by Larry East, W1HUE ————
Page last updated on June 17, 2012