I have a couple of them and mine work find with my Icom 756 Pro III and the 746 Pro. I have the switches in series so I can connect 3 rigs and I have a Jetstream 220 rig all running through the switches no problem. I am using a Heil PR-40 for the mic not the original Icom mic but I have plugged in my hand mic that came with the rig to test with and it worked on the Icoms just fine (I did not try it on the Jetstream).
There is a pamplet that comes with the switch that explaines how to set it up inside. It should be available on MFJ's website. If you dont have the paperwork on it and cant locate it on the web let me know and I will copy it and snail Mail it to you.
Delbert McCord KA8OCN
PS if you need it please drop me a note at my email firstname.lastname@example.org