Audio Pop mod from QCX submitted by Greg N3NW

This audio pop mod from Greg N3NW appeared on the BITX20 lO Group list before this website went live in January.   The implementation may not look that elegant (sorry Greg), but this was about an experiment to reduce pops not an entry in a beauty contest!

Greg “borrowed” a rather elegant circuit snippet from the QCX transceiver that takes care of the pop completely.  This is effectively the same mod as that submitted byJoe VE1BWV (and no doubt taken from the same source) who confirmed that it works.  The mod has now been confirmed independently by both Pete W8KS  and John AD0RW.    So this mod, is known to work, however, it may come with one downside, for which John AD0RW has a work around.

The circuit diagram for the popular QCX transceiver (developed by QRP Labs) can be found near the end of the very well documented assembly guide for this single-band CW rig.  An excerpt that contains the TX Mute circuit installed by Greg can be seen below:

The TX mute switch in the QCX involves just 4 parts – a BS170 (Q7 above), a 0.1µF capacitor (C52 above), a 1N4148 diode (D5 above), and a 120K resistor (R60 above).

Greg used a 300K resistor for R60,  and a 440nf capacitor for C52 to adjust the timing, which keeps BS170 open just long enough when switched back to RX to remove the pop.  The CW sidetone gets weaker with this, however in my uBITX board 10K R253 was replaced by a 1K resistor to make the CW tone just loud enough with this mod.

Pete W8KS has, however, found that the CW sidetone (even after replacement of R253 with a 1K resistor) to be insufficient and asked for suggestions to address this mod’s shortcoming.

John AD0RW found a much better place to connect the drain of the MOSFET. First he replaced R70 (in µBITx circuit diagram (see circuit excerpt above) with a 510 ohm resistor.  Then he connected the MOSFET drain to the relay side of that resistor, i.e. to the point M2 on the schematic above.

This setup still shorts the incoming audio to ground and kills the pops, but leaves a voltage divider of R253 and R70 connecting the sidetone to the volume control, increasing the amount of signal injected.  Depending on your value of R253, this may be perfect as is. The value of R253 varies (some early production kits had a 1K resistor and more recent kits get a 220K resistor).  John’s was 220 k and he needed more volume, so he put a 50 k trimpot across R253. It is just about right at maximum resistance, but can be adjusted downwards if more volume is required.  If your R253 is a smaller value and the sidetone is too loud, just replace it with something of higher resistance.  The volume control now has final control over the sidetone level and can be tweaked in operation if needed.

Reference 1
Reference 2