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

Thanks
SV2BFI

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

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

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)

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

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

On Dec 3, 8:22*pm, 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


Hello Kosmas, I looked inside my HM7 mic. which
came with my Icom 740, the resistor is one point eight K.
(1.8k), and is in parallel with the 4.7uf. capacitor as shown
in the schematic. I checked the value of this resistor with
digital meter,and it confirmed this resistance value.

HTH, Doug. GM3OAV

Στις 06/12/2012 03:07 πμ, ο/η doug Î*γÏ�αψε:
On Dec 3, 8:22 pm, 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


Hello Kosmas, I looked inside my HM7 mic. which
came with my Icom 740, the resistor is one point eight K.
(1.8k), and is in parallel with the 4.7uf. capacitor as shown
in the schematic. I checked the value of this resistor with
digital meter,and it confirmed this resistance value.

HTH, Doug. GM3OAV


I found a schematic at an japanese site too.
The resistor is 1.8Kohm.
I finish it and it is working loud and clear.
I have an icom ic-738 transceiver. I have both
hm-36 and sm-20 microphones. The hm-326 has very
low output and can't drive the transceiver to full
output on ssb. The sm-20 has enough power but it
picks up any surround noise (blowers, my children,
telephon rings, TV etc etc). So i need a
dynamic microphone with enough power to drive well
my transceiver without too much sensitivity.
I have an old dynamic handy microphone from a CB
transceiver and I put inside a preamplifier on a
small pcboard and it works fine.

Thanks to all for your help.

73
SV2BFI

Can anyone tell me what will replace a hm7 i have a icom 720a that is 8 pin