Adding an RF Gain control

Don Cantrell ND6T has posted details of how to add a simple RF Gain Control mod to the uBITx on his website.

Almost any potentiometer, from 1 Kilohm to 5 Kilohm, can be used to add an RF Gain control.  This will make a nice addition to the transceiver. If you have an audio taper pot (logarithmic pot), that’s even better.

Locate the trace (assisted by the photo above). Use a knife ( e.g. 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.  Cut off a section of two pins from some .1” right-angle header stock and use 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.

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.

About 26 dB of control is achieved with this arrangement.

 

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