[Icom] ICOM HM-12 Microphone repair

Mon, 17 Mar 2003 23:02:27 -0800

Hi Jim,

Good show! That was a nice, clean, cost-effective repair. It was very
enterprising of you to try different circuit configurations until you
obtained the best results.

Cheers for now, 73,
Adam VA7OJ/AB4OJ

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Jim Isbell
Sent: Monday, March 17, 2003 17:03
To: mail list ICOM Radio
Subject: [Icom] ICOM HM-12 Microphone repair

A while back my ICOM 725 quit transmitting.  The receiver was fine, but
no transmit.  At the time I had only CW and SSB capability.  I have
since added AM and FM with the UI-7 module.  But when it quit I found
that CW would put out a carrier but SSB didn't work at all.  It turned
out to be a simple thing, the microphone was kaput.  After looking for a
replacement and finding them in the $50 plus range I decided to fix it
myself. At the local Radio Shack store I found a package that contained
one condenser microphone element (RS#270-090) for $1.99.  It might not
work, but for $1.99 it was a cheap fix.  I found that the element had
just two leads instead of the three on the ICOM mike.  But I combined
the power and the + audio lead together and tacked them to the + lead on
the RS element.  The ground lead went to the - lead on the RS element.
 The first test showed that I was not getting full modulation so I went
back in and moved the power lead from the 1K resistor in series with the
power  to the other end of the resistor where there was already a hole
in the circuit board.  This removed the 1k resistor from the circuit and
fed the power directly to the mike.  That solved the problem.  I am back
on the air at a cost of $1.99 plus tax.

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