Definitive audio pop fix

With the release of the v4 board from HF Signals, we finally have  a design for an audio pop fix that can be applied to the v3 µBITx board with few additional parts.

Mike ZL1AXG has completed this mod and can vouch for it have removed all unwanted pops (on both transitions from RX to TX and TX to RX). It doesn’t kill the CW sidetone.  His application of the mod is described below.

The mod can be placed in the same position on the v3 board as in the v4 design,  using a simple Dupont header as shown below, but there are other ways of achieving the same result.

This mod only involves 5 parts.   It is now the recommended mod to fix the audio pop.  The audio pop fix summary will be updated shortly.  All other fixes are now effectively redundant because they are more complex.

NB – This is an extract from the circuit diagram on  There is an error with the numbering of the leads on Q74.   Check first, before wiring up!

Parts required are:

  1. 2n7000 MOSFET or similar (Q74)
  2.  1 µF ceramic capacitor (C79)
  3. 1N4148 or similar silicon signal diode (D14)
  4. 100K resistor (R78)
  5. 1K resistor (R70)

Complete the following steps:

  1.  Locate R70 (100 ohm) resistor on the right hand side of the board when looking from the front panel and remove this resistor.
  2. Drill a small hole through the board roughly in line with the two solder pads for R70 in front of the relay 7/10 of an inch to the right.
  3. Install a standard dupont female header with 7 sockets (spacing 0.1″ per pin) on to the board (see first photo above).    The first two pins are bent over at 90 degrees and solder to the pads for R70.   Pins 3-6 are removed.  Cut them off underneath and then pull them out with a pair of pliers.    Solder the final pin 7 underneath the board
  4. Install parts on the  plug in board as per the circuit diagram below, and use a male dupont pin  to connect to the T-R line.  Plug the board in to its socket, connect the T-R line, and you should have a nice quiet transition going to TX.
Step 1 & 3: Install the dupont header to replace R70. Pins 1 & 2 on the left are bent over at 90 degrees and soldered to the pads. Pins 3-6 are removed. Pin 7 passes through a new hole drilled in the board
Step 2 – Reverse side showing pin 7 soldered to the underside of the board to securely attach the female dupont header above
Step 4: Install the components on a small piece of perf board. The purple wire connects to the T/R line on the raduino.

New graphic for

Some of you will have been sufficiently observant to notice that the graphic at the top of the webpage has been updated.  This is because Mike ZL1AXG has installed a Nextion 3.5″ touch screen on his own µBITx.    Careful work with a drill and file will yield satisfying results.

He also installed the latest pop fix from the v4 board (see the separate article).

BCI mod from ZL1AXG

Mike ZL1AXG (editor of has taken a different approach to mounting a BCI filter.  The high pass filter design and mounting point are the same as that documented here.  However, the mounting system has been modified to make the filter easily removable. 

Mike has other filters that plug into the QRP-Labs LPF filter board so his filter takes the same dimensions (1.5 x 0.5 inches or 38.1 x 12.7mm) and uses the same mounting system (two four pin male dupont header connectors.) The protruding centre pins of the DuPont connector were cut off, and the outside pins  bent over (in opposite directions) to improve stability of the mount.  The pins were soldered to the empty pads to ground on C216 and C210 on one end (RHS in the photo below)  and to the RX signal rail on the other end (LHS below).  A cut was made to the track mid-way between the dupont connectors (not shown here).

The BCI filter as mounted can be seen below.  It makes a tidy looking unit that can be substituted or bypassed later.