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Does anyone know if a ICOM SM-20 mic will work OK with a ICOM 761 and / or if rewiring the mic will be needed? I am trying to get my first station assembled and air worthy.

first you need the mic wireing from the schematic for the 761.
If you dont have it go to ICOM on internet and download it.

also get the diagram for the mic which may or may not be available on the net.

If not check icom brochures ofr the mic which might at least indicate
the impedance of the mic.

Worst case is open up the mic and the use ohm meter to wring out the wireing from mic, PTT , common wire. shield, possibly 8 volts for an electret mic. Up/down switch etc.
If the sm 20 is a desk mic it will be easier.
If its a mobile mic with phone dial and lights and pf keys it gets complicated unless you can get the schematic downloaded.

Then match up the pins to their label and match mic inpedance or have the mic impedance lower than the rig input impedance..

If everything is ok except the pin out of the connector, do not change the connector wireing....
make a small jumper with plug and socket to change the wireing from mic to radio.. Thus the mic will always be original wireing.

I have found in my case that Icom mikes work with all/most Icom radios if they both have 8 pins.
I have used the SM-5 with the Icom 761, 765, 781.
Maybe I was lucky