Another speaker mic modification to reduce RF interference

Remote speaker-microphones and DMR portables discussed RF ingress to Speaker Mics (RSM) used with DMR radios in digital mode.

The problem

I purchased a RSM branded Kenwood but obviously a Chinese fake for an MD-390 for about $5 posted, but it turned out to be lousy with RF interference in the form of the ‘motorboat noise’ on transmit audio.

Dismantling the RSM  I found there is precious little RF filtering, just a single SMD cap near the electret capsule.

A solution

Above is the modified RSM.

I cut the track between the existing bypass cap (which they did not fit… these things cost tenths of a cent each) and the electret and installed a 10µH SMD choke, and soldered a 1000pF SMD cap across the electret terminals.

It is the best of the RSM I have now (ie least RFI… can’t hear any).

Works well on my MD-390.