I recently purchased an Icom 720A on eBay, but the radio is missing the
microphone. I understand that the early model of Icom transceivers had a
preamp in the microphone itself, and you have to have either the original
mic that came with the radio, or a similar microphone with a preamp to drive
the audio in the transceiver.

Mike-

I was under the impression that the older Icom transceivers used low impedance
dynamic microphone elements, maybe 200 Ohms. However, I'm not familiar with
the 720A.

If you don't already have documentation for the radio, go to
http://www.mods.dkand click on ICOM. Then click on Manuals & Other on the left
side of the page. There are both service and instruction manuals listed for
download. I did not download them so can't say how good they are.

There should be a diagram included in the service manual, that will show how
the microphone is wired. I would expect that if a pre-amp was required, you
could take any microphone and replace the cartridge with one of the Radio Shack
electret cartridges that come with the built-in transistor. I found one in an
old cordless phone that would probably work as well.

Heil should know what you need, and have a model that will plug right in.

73, Fred, K4DII