[Icom] SM-20, HM12 and HM36

Mon, 23 Sep 2002 22:12:12 -0700

Hi Johnny,

Actually, the preamp is integrated into the electret element, as the lead
length between the electret condenser capsule and the gate of the FET in the
preamp must be as short as possible. This minimises stray capacitance, which
could shunt the capsule output. (The capsule needs to "see" a very high load
impedance.)

There is an official IC-706 mic gain mod on the Icom support page:

http://www.icomamerica.com/support/troubleshooting/ic-706.html

Symptom: Low mic gain.
Cure: Install field modification on main unit: Change value of R236 resistor
from 27 k, to 68k-82 k. The higher the resistor value, the greater increase
in mic gain.

This is much more practical than attempting to add a preamp to the
microphone.

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/
Note new e-mail address:
mailto:[email protected]

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Siu Johnny
Sent: September 23, 2002 21:36
To: [email protected]
Subject: RE: [Icom] SM-20, HM12 and HM36

Hi Adam,

Thanks for your mail.  I did open up the HM12/36 and all I found are only
resistors and capacitors.  Have I missed anything (or oversighted)? There
is no semiconductor devices inside the hand mic.  Your advice, please.

The idea of addding a pre-amp is originated from my operation of IC706(1st
version).  I always have the feeling that this mobile rig does not have
enough mic gain so that I would like to modify either HM12 or HM36.  My
IC575A and IC756 do not have any problem with both the hand mic and desktop
mic

Thanks a lot for your help.

73

Johnny Siu VR2XMC

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