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.

Kosmas-

I dug out my old Icom microphone and found it was the one for an IC-22S,
which does not have an amplifier. So far, I have not found a copy of
the IC-22U diagram, so do not know what the amplifier load resistor is.
It appears the HM-7 would work with newer radios such as the IC-718,
which has a DC load resistance of about 1200 Ohms (1K AC + 220 DC).

I assumed that the radio has a regulated voltage to feed the microphone
amplifier, probably 8 Volts like the IC-718. The Base-Emitter junction
of the transistor might be around 0.6 volts for silicon.

If the unknown resistor is 1800 Ohms, current in the transistor would be
relatively low, and collector voltage would probably be a little higher
than 4 Volts depending on the load resistor.

If the unknown resistor is 18,000 Ohms, current in the transistor would
be higher, and collector voltage would probably be lower than 4 Volts.
For this value, the circuit might work as well without it!

Perhaps it would work with either value resistor, but I prefer the 1800
Ohm choice. I wish someone would come along with an HM-7 who can open
it up and check resistors.

Fred
K4DII