This mod realizes a CPU controlled switchable low-pass filter which is inserted between the output of the audio DAC and the circuit which combines microphone and CPU generated audio signals.
The following components and equipment is required:
The pictures show the state of the PCB and the schematic diagram before modding (note: The DAC-Mod is already employed in the PCB picture).
Connect the 22 kOhm resistor across the cut in the DATAMOD audio path. Connect (one side of) the 10 kOhm resistor and (one side of) the 100 nF capacitor in parallel to the node connecting R603 and the new 22 kOhm resistor.
Connect the other side of the 100 nF capacitor and the 10 kOhm resistor to the collector of Q724 using the wire (cut the wire to the necessary length).