Another RFI mod of a speaker mic (41-80K) for DMR use

I bought a remote speaker-microphone (RSM) for a MD-390 DMR portable from, a 41-80K.

They assured me it was compatible with the radio in digital mode, but it turned out to be lousy with ‘motorboat noise' on tx audio due to RF ingress tot he electret capsule.

Since the RSM was otherwise a good rugged and economical product, it was worth trying to rectify the RF ingress problem.

Above is a pic of the electret. Two fine tracks can be seen bonding the metal can of the electret to the -ve pin, so that is good… the can showed low resistance to the -ve pin. The +ve line is bypassed to the -ve line about 12mm from the electret with an unknown capacitor, but it was clearly not effective at 440MHz.

The fix involved cleaning the mask off +ve track and cutting it to accept a SMD choke (10µH) right near the electret pin.

Above, the choke in place and a 100pF SMD capacitor is soldered directly to the electret pins. Note that connections are very short, much shorter than original bypassing, and it has a hope of working properly. In fact the circuit around the original bypass cap, and connecting tracks forms a small loop with the electret at the end… why did they do that?

On test with a 5W MD-390 with short rubber ducky antenna, it was not possible to induce motorboat noise at any location of the RSM relative to the radio or its antenna.