Replying to my own question here!? I don't think the Icom 705 microphone can be made to work as the PTT and microphone output are connected together.? My guess is when you press the PTT on the microphone this applies the 3.3V feed to the microphone and powers it up.
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However, I have discovered that a Baofeng* micriophone works perfectly and has the connections in the correct order on the 3 pole plug.? I say "perfectly" in the sense I can set a suitable gain using the microphone test.
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*? and possbly a Yaesu microphone as well but I haven't tested this but I can find micriophones for sale which say they are suitable for Baofeng and Yaesu.
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John
M0WIV
John Laidler