Friday, January 13, 2017

ND6T's RF Gain Control (with "Tombstoning" and Feng Shui)

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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