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 uBITx.net 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 hfsignals.com.  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.