RF Gain Control
by Don Cantrell, ND6T
I've grown used
to RF gain controls, especially on my home brewed receivers. Often it is the
only gain control and serves to adjust audio output as well. When I first put
the BITX on the air I called a friend who lives within shouting distance
(literally!) for a test. That's when I began to miss that RF gain control on
the BITX.
During my
experiments with the analog VFO I installed an additional pot on the front
panel that provided a precision 1 kHz tuning. The new DDS system, of course,
didn't need that knob and it became available for re-purposing. What better use
than a RF gain control?
My plan was to
insert a 10K ohm variable resistor (rheostat) in series with the power feed to
Q1, the receive RF amplifier. Original design integrity is maintained by placing
it between R15 and R16. This was accomplished by “tombstoning [1]” R15 (10
ohms) on the pad next to the supply via. I attached one lead from the control
to the top of the (now upright) R15 and the other lead to the (now vacated)
solder pad. Done! That's it!
I now have my
beloved RF control with about 17 dB of
adjustment range, and the feng shui of the front panel is restored.
de ND6T
[1] “
Tombstoning” is the term for tipping up a surface mount component. It now
stands like a headstone over the site of its original position. This is the
normal method of opening a circuit during trouble shooting. Very handy and one
of the many reasons that surface mount construction is so much easier to work
on and modify. Thanks Farhan! The BITX40 is beautifully laid out large,
logical, and well-spaced. A true joy to work with. Those relatively large
components make it easy compared to through-hole.
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