...turns your Motorola "MC micro" into a fully-fledged amateur radio transceiver.


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:

  • Soldering iron
  • Resistor 22 kΩ
  • Resistor 10 kΩ
  • Capacitor 100 nF
  • 10 cm wire
  • DAC-Mod

PCB & Schematics Without The Mod

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).

Location of the audio path in the schematics
Location of the transistor switch
These locations need to be modified
Cutting the DATAMOD-audio-path

Disconnect "DATAMOD" audio path

Cut the connection between R835 (DATAMOD-pot tap) and R603 (120 kOhm).

Cutting the connection to the collector of Q724


Cut the connection between the collector of Q724 and the node connecting R755/R785.

The modified circuit
PCB after the complete modification

Creating the low-pass-filter

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.

Connecting the switch

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).