Retevis MD-380 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 Retevis SM that was advertised as original equipment for the MD-380, but turned out to be lousy with RF interference in the form of the ‘motorboat noise’ on transmit audio.

Dismantling the SM (fighting the way through TORX with PIN screws, what are they trying to protect) I found there is precious little RF filtering, just a single SMD cap at the end of a long (wrt 500MHz) branch track.

A solution

Above is the modified SM.

I cut the track from the red wire to the electret and installed a 10┬ÁH SMD choke, and soldered a 1000pF SMD cap across the electret terminals. I also cut the track near the electret to disconnect unwanted track (aka antenna).

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

Pity it does not suit my MD-390, see TYT (Tytera) MD-390 speaker mic plug compatibility issues.