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Old December 3rd 12, 09:22 PM posted to rec.radio.amateur.homebrew
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Default icom HM-7

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

Thanks
SV2BFI

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Old December 4th 12, 09:20 PM posted to rec.radio.amateur.homebrew
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Default icom HM-7

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
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Old December 5th 12, 12:26 AM posted to rec.radio.amateur.homebrew
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Default icom HM-7


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

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Old December 5th 12, 05:57 PM posted to rec.radio.amateur.homebrew
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Posts: 237
Default icom HM-7

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)


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Old December 6th 12, 12:02 AM posted to rec.radio.amateur.homebrew
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Default icom HM-7

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


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Old December 6th 12, 02:07 AM posted to rec.radio.amateur.homebrew
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Posts: 4
Default icom HM-7

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
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Old December 6th 12, 08:45 PM posted to rec.radio.amateur.homebrew
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Default icom HM-7

Στις 06/12/2012 03:07 πμ, ο/η doug *γραψε:
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

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Old November 25th 17, 05:37 PM posted to rec.radio.amateur.homebrew
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Posts: 1
Default icom HM-7

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


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