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.)

Fred
K4DII


Thank you for your time Fred.
I suppose that's correct.
73s
SV2BFI