Heil Pro 7 IC with Flex 6600/6600M working AOK now.

Bob KW4CQ

In order to make my Heil Pro 7 (Electret element) work with my 6600M I had to order a Hosa YMM261 Stereo Breakout Cable ($5.00) from Amazon.  Just plug the mono plug into the Black plug on the Hosa adapter cable and leave the Red plug unused.  Plug the TRS plug from the Pro 7 into the Headphones jack on the back of the Flex.  Plug the TRS plug on the cable adapter into the MIC jack on the back of the Flex.  Solves the problem beautifully. 
Bob KW4CQ

Steven WA8Y

At Dayton, I bought the Heil pro micro for use with my Maestro. I found the same problem. Heil wanted me to buy the dynamic version. I wonder if it would have worked out of the box.

Steven WA8Y

I tried your idea of using the Tip Ring Sleeve adapter but it didn't work on the Maestro. The ring is used for PTT it puts the rig entrancement the moment you plug in the mic. Instead here is what I did. I crosswired the mic's Sleeve to the Tip. In my junk, I found a small box with a 1/4" stereo jack wired to a 3.5mm plug. I removed the Tip wire and soldered the Sleeve wire to the tip. It is not very small or elegant but I definitely don't want to cut the plug off the pro micro headset. I will eventually cut into a stereo extension cable and crosswire it.

AA5NM

INTERESTING LEAVING ME TO WONDER HOW TO FIND HEIL PRO 7 FLEX6400 SOLUTION