I did some mods to my BITX40. I added a 10 turn 10K pot for the tuning, to get better granularity, and I added UpperSideband (USB), so i could use it to work digital modes.
The 10K pot was straight forward, but the USB mode introduced clicking when tuning across the band. So, I added a 1000uF Electrolytic and a 100nF cap, in parallel, across the digital VFO’s power regulator. I also added a 56ohm resistor between rig power and the digital VFO power. Clicking was still there. I discovered I had linked with version 2.0.3 of the SI5351 library, and saw a note on the BITX40 Facebook group, that one should link with version 2.0.1. I did that, uploaded the sketch and viola! The clicking was gone. In fact, adding the filtering got rid of a number of other things I was hearing when I tuned. Pretty cool.
So my next mods will be to put a 8 pin microphone connector in, add in a mode switch to get the rig into calibration. Also, adding a voltage divider to put the current input voltage on the display and last but not least, a S meter circuit, to use the bottom left row of the display as an S-meter.
The BITX40 is a lot of fun. And modifying it so I can do JT65 and such will be a blast.