Modification of Motorola PMR microphone for use with amateur radio equipment

I was solving what microphone to use with my new SunSDR2 DX TRX. In my drawer was a Yaesu fake microphone from Aliexpress, whose only advantage was that it cost almost nothing. The modulation from it was flat with lots of distracting noises caused by breathing and PTT button squeaks. That didn’t make me satisfied, but I remembered that somewhere in my inventory I would surely find another microphone for Motorola radios from my recent radiocommunication job.
The advantage of these mics, apart from their robustness, is mainly their modulation presentation. I’m not overstating it when I say that for our purpose they will put in the pocket of any of those Shure and Heil polished legends for exorbitant money. And they’re widely available.

Schematic diagram of the original microphone (MDRMN4025, but older types use the same circuit):


The circuits marked in yellow are specific to MOTOROLA radios, they will go away. Only the circuit for powering the electret microphone insert and the coupling capacitor at its output will remain.

Unfortunately the cable used is unshielded because Motorola uses a pretty high signal level from the microphone. It has a preamplifier built right into the microphone and the input in the radio may not be as sensitive, hence no requirement for cable shielding. However, this does not matter in our case, so I didn’t use the original cable and instead took the shielded cable from that Chinese junk from Aliexpress.

I removed the red marked unnecessary components from the PCB, including the connector:

From the other side of the PCB you need to connect one pin of the PTT button to ground (a drop of solder will do a good service):

I soldered the cable directly to the PCB to the marked points:

And it’s almost done. Don’t forget to put back the rubber microphone cone when you assemble the case. The result is perfect as expected.

The power supply voltage for the electret microphone capsule doesn’t have to be exact 5 VDC of course (it’s available on the SunSDR2 microphone connector), Motorola themselves use 12 V and it works just as perfectly with the 8 V available on the microphone connector of the ICOM transceivers.

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