RadioBanter - View Single Post - icom HM-7

Fred McKenzie · December 6th 12, 01:02 AM posted to rec.radio.amateur.homebrew

In article ,
(Mark Zenier) wrote:

If it's an electret microphone element, you could check out the datasheet
and see what it recommended. (The kind with the built in FET, and about
the only kind that Japanese have seemed to have used for decades).
I have a vague memory of the Panasonic listing in the Digikey catalog with
recommended resistors in the 2k to 5k range.

Mark-

The Icom HM-7 is at least 30 years old. It has a 600 Ohm dynamic
element with a 2SD661 silicon transistor amplifier. The HM-7 design is
similar to a transistorized carbon microphone replacement. The resistor
equivalent to the one you refer to, is inside the radio.

This discussion is about a bias resistor. Picture a 10K Ohm resistor
connected between the transistor collector and base, with another
resistor connected from base to ground. This second resistor is either
1800 Ohms or 18,000 Ohms. (Actually the microphone element is connected
between the resistor junction and transistor base.)

I found the diagram in an Icom IC-22U owners manual, but it is hard to
see if the value is 1.8K or 18K. There is a tiny fly-speck where a
decimal point could be.

I think the only way we will know for sure, is if someone has an HM-7
and will open it up to see.

Fred
K4DII