In article ,
Kosmas Anastasiadis wrote:

In article ,
Kosmas Anastasiadis wrote:

I found the schematic of icom's HM-7 hand powered mic.
One of the resistors inside is 18KOhm or 1.8KOhm ???

Thanks
SV2BFI

Kosmas-

I found that same schematic on the web. I did not take my microphone
apart to verify, but assume the resistor is 1800 Ohms (1.8 KOhm). If
you look closely, you can see a speck where the dot should be.

That resistor, along with the 10K Ohm resistor, determines Voltage at
the transistor base, which is about 0.65 Volts higher than at the
emitter, which sets the transistor current.

Suppose the collector Voltage is about 5 Volts. Base voltage would be 5
times 1.8/(1.8+10), or 0.76 Volts. Emitter voltage would be about 0.1
Volts, so current would be about 0.1/22 or 4.5 ma if I did the math
correctly. If the resistor were 18 KOhms instead, current would be
unreasonably high.

You can get a better idea if you look at the circuit of the rig the
microphone goes to. I'm fairly sure the HM-7 was used with an IC-22U,
which is where my microphone came from. (Mine is not marked as HM-7.)


Thank you for your time Fred.
I suppose that's correct.

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 Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)