Adding an RF Gain Control to the μBITX

Although the μBITX has a very wide dynamic range, there is no AGC. Not a big deal, the volume control does nicely in most instances, but I find an RF gain control almost a must. Especially when in conversation with stations within a mile or less!

Fortunately, the μBITX is built with conductor traces easy to access and modify. Most any potentiometer, from 500 ohm to 5 Kilohm will make a nice addition to the transceiver. If you have an audio taper, that’s even better.

Locate the trace using the attached photo. Use a knife ( I like the X-acto number 5 ) to carefully scrape the coating from the trace and to cut a small 1mm (or so) separation where the connector header will go. I cut off a section of two pins from some .1” right-angle header stock and used pliers to form the short pins to contact the newly bared copper trace while the plastic portion of the block was flat against the printed circuit board.

Carefully tin the trace where the header pins will connect to it. A bit of alpha cyanoacrylate gel glue will help fix the block in place in a few seconds. Then solder the pins to the board. This makes a very convenient access point, one that can be easily disconnected just as you would the other plugs that connect this board. If you don’t want to use it, just place a standard shorting plug on it and you are back to normal operation. Want to add an AGC circuit? This would be a place to plug it in.

Use small shielded coaxial cable to connect the control to the board. I use common RG-174 type. Tie the shields together at the control end only, not the plug end.

I use a 2-conductor section of female .1” spaced header stock for the plug. I strip the shield on each of the cables back about half an inch at the plug end and bare about an eighth of an inch of center conductor. I slip an inch length of heat shrink tubing over that end of the two cables and slide it back out of the way for the moment. I then solder the bared center conductors to the plug pins, test them, and then cover the plug and cable end with the heat shrink. I apply heat to shrink the tubing, making a nice form-fitting cover and strain relief.

Unless the "cold" end of the control potentiometer is solidly connected to common ground, attenuation will only be accomplished by the serial resistance. So a good CLOSE ground connection is very important. A long wire to some ground on the Raduino will not be effective. A shaft-mounted ring lug, bent up to solder directly to the ground lug on the pot is the preferred method. To insure a solid connection to the panel (which should be grounded also), scrape any paint from the area that the lug will contact. A star lockwasher there is even better.

This arrangement gives nice, smooth control of 40 dB on my build. My original installation (in a BITX40) wasn't making a good ground connection and measured 26 dB control range. I was quite pleased with it but noticed better performance of the μBITX build. Measurement revealed the problem.

I designed an Automatic Gain Control system that will attach to this modification. You can see that project by clicking here.

de ND6T

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