On page 22 it says:
4:
Control-RRC only: JMP-1 is placed behind the TTL connector. With the strap in place a DC voltage is feed to the microphone element. All ICOM microphones should be DC-feed.
Dynamic microphones like the ones used by Kenwood should not be DC-feed. HEIL microphones should not be DC-feed either.
Heil microphones aren't all dynamic and some are electret and require the DC bias. I found this out the hard way when trying to debug a non-operational microphone.
