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KF4UBK
12-17-2007, 07:18 PM
Have a Icom-720, It came with a HM-7 mike that has a amp built into the mike. The mike does not work.It will put out 50 watts on am with no modulation. is there any other mike that can be modified to work with the 720?

AG3Y
12-18-2007, 02:46 AM
The SM5 and later SM6 are both desk mikes that were made to work with that radio. I have been running a 720a with an SM5 here for YEARS, now, and never had a complaint about the modulation characteristics.

I believe there is a later model SM20 that had a few more features built in, but all of these microphones are probably not available as new stock any more.

The Heil series of microphones will work with the newer Icoms, and I am quite sure that there are some made specifically for the legacy rigs as well. You might go directly to his website and get your answers there.

Actually, the 720 is a wonderful rig for running soundcard digital modes. The interface is extremely simple to build, because since the input to the rig is more line level than mic level, the issues of ground loops, RF feedback, etc. are reduced to almost insignificance !

You could conceivably run any type of audio console being fed with the mic of your choice, into an Icom 720 and the only thing you would have to watch out for would be to isolate from a DC standpoint, the phantom power line which also carries the audio output from a standard Icom microphone's built-in head amp (preamp) .

BTW, I don't understand what you mean by "it will put out 50 watts on am with no modulation" The 50 watts indicates the carrier level. What do you think is wrong with that ? Are you not hearing any audio in a monitor receiver? What happens on SSB ? Do you get any ALC action or fluctuating collector current or RF output in any of those metering positions ?

73, Jim

KF4UBK
12-18-2007, 05:46 AM
its puting out 50 watts dead carrier on am ,but no wattage on ssb.

K0CMH
12-18-2007, 03:22 PM
The IC HM7 mic was common to many Icom radios. #My IC-730 also uses the same mic.

I have had good success using a number of "powered mics" with my 730.

My best success, and the mic I am now using, is an old Turner +3. #I get great audio reports and it easily modulates the SSB to full power. #I had good luck with an old Astatic mic (the version that was powered). #I tried a Shure studio mic that had adjustments for the impedance, but it never adequately modulated the radio.

You will need to get the "pin out" diagram for the Icom mic jack in the radio, but these were all the same. #Then you will need to get the wiring scheme for the mic you plan to use. #These can be found on the internet for free.

Just rig up the mic to match the appropriate pins:

Pin #1 = audio input
Pin # 5 and 6 = Push to talk (connect the PTT switch across these two pins, one wire to one, and the second wire to the other #-- #there is no polarity to this)
Pin #7 is the shield ground.

This is how both the MH7 and the SM5 are wired on the IC-730. #The IC-720 manual will also have the wiring scheme for the mics, and I believe it is identical to the IC-730.

I have to have the battery in my Turner +3 mic for it to modulate the rig, but I only have to turn the "volume" knob up a little, about 20 - 30% of its full deflection.

Good luck.

EDIT: My IC-730 manual says that the MH7 has an impedance of 1300 ohms. This may help you to find a mic that will better match the radio. I believe the 720 manual will also state the mic impedance (it is in the specifications section of my 730 manual, and Icom made most of the manuals very similar back then).
Also, I did not make it clear before that the 730 does not use any + voltage. The HM7 has a small circuit inside, but the manual should show that it is just a few resistors, no amplifier. But your manual should indicate just what is going on in your 720.

AG3Y
12-18-2007, 06:32 PM
An unamplified microphone will NOT work with an Icom 720 ! My service manual shows a single transistor amplifier built into the hand mic. Beside that, the SM5 had a two transistor amp, and the 6 a three transistor one, if I recall correctly.

The sensitivity of the 720s mic gain stage is at least 20 db lower than a conventional mic amp stage, and the difference is made up in the microphone's electronics in any case. I don't know why Icom chose to do it that way, but that is the way it was done!

73, Jim

K0CMH
12-18-2007, 11:00 PM
Very Interesting, Jim; and I am coming to you for information again.

I guess Icom wires up the HM7 mics differently for different rigs.

I have not open up my HM7, so I can't say what is inside for sure.

But I can email a copy of my Ic-730 manual to you. It shows some resistors and a capacitor inside the mic (a schematic of how it is wired for the IC-730). It only uses the element and a PTT switch. The +5 volt pin is "not connected" and the +5 voltage is not used. There are no transistors in the schematic ofr the mic in the 730 manual.

Now I am curious just what is inside an HM7. I did try once to open it, and it appeared I was going to break the case if I kept trying what seemed the way to open it.

Possibly there are more components inside it that just don't get used by every Icom radio.

Anyone know the proper way to open up a HM7 mic? Anyone know where a complete schematic of the mic can be found?

Jim, I would assume your 720's manual would show the "pin out" configuration for the HM7 and the SM5. From what you say, I would think they are using more pins on the 8 pin plug/jack than a 730. Can you share what the 720 manual says?

This is very interesting. I made my ASSUMPTIONS based upon the fact that my IC730 and my IC718 had the exact same "pin out" configuration, even though they used different mics. And yes, I confirmed that I cannot exchange the mics between the two rigs. Neither will function on the other. But since the pin out configuration was the same, I kind of though it was a very common configuration for Icom. Well, that is what I get for Assuming.

Hope you can enlighten me on this.

AG3Y
12-18-2007, 11:25 PM
On the hand mic that is shown in the service manual, Pin 1 is connected to the collector of an NPN transistor through the center lead of a shielded cable, while Pin 7 is connected to the shield. Pins 5 and 6 are connected to a N.O. "PTT" switch. The mic element is tied to the base of the transistor, and to a tap point between some bias resistors, and a few caps to establish an AC ground point that is actually a small DC level above ground to power the electret element ( ? ) or at least establish an operating point for the transistor.

As mentioned before, the SM series mics are wired a bit differently, but this appears to be the handmic that came stock with the radio.

Hope this helps. I could try to scan it, but you need a magnifying glass to see it!

73, Jim

K0CMH
12-22-2007, 02:07 PM
AG3Y:

Yes, you are correct. I kept missing the emitter/base/collector in the diagram of the HM7 (wow - I have been out to lunch on that for a few years, I guess I missed it because ICOM forgot to put the circle around the symbol - yeeks, talk about missing the obvious, I have to eat a big crow pie on this one).

I measured the potential between the two pins on my 730, and there is a potential difference.

Also, I should have picked up on where the input was connected, as you stated. Another crow pie to eat. I looked at the schematics of my 730 many times and missed that also.

Thanks for clearing this up.

Changing subjects: I see that the IC-720a manual shows only 40 watts output on AM. If the original poster was getting 50 watts on AM, then he was doing very well (20% above factory claim). I guess a better thing for the original poster to check is the output on SSB.

AG3Y
12-24-2007, 03:34 AM
There is an internal adjustment for AM carrier level, as well as peak modulation level in the 720. It would be quite possible to adjust the levels differently than the factory specs and end up with a higher ( or lower ) reading compared to the factory tolarances. That is not to say that the radio would operate properly, but it would be possible to see a difference on meters, or with a scope.

The difference at the receive end between a 40 watt signal and a 50 watt signal would be virtually indistinguishable !

Glad to have been some help! 73, and Merry Christmas. Jim

KA4DPO
12-24-2007, 04:16 AM
If you want to replicate that circuit a 2n2222 will work just fine.

WB0GAZ
05-22-2010, 11:16 PM
Very Interesting, Jim; and I am coming to you for information again.

I guess Icom wires up the HM7 mics differently for different rigs.

I have not open up my HM7, so I can't say what is inside for sure.

But I can email a copy of my Ic-730 manual to you. It shows some resistors and a capacitor inside the mic (a schematic of how it is wired for the IC-730). It only uses the element and a PTT switch. The +5 volt pin is "not connected" and the +5 voltage is not used. There are no transistors in the schematic ofr the mic in the 730 manual.

Now I am curious just what is inside an HM7. I did try once to open it, and it appeared I was going to break the case if I kept trying what seemed the way to open it.

Possibly there are more components inside it that just don't get used by every Icom radio.

Anyone know the proper way to open up a HM7 mic? Anyone know where a complete schematic of the mic can be found?

Jim, I would assume your 720's manual would show the "pin out" configuration for the HM7 and the SM5. From what you say, I would think they are using more pins on the 8 pin plug/jack than a 730. Can you share what the 720 manual says?

This is very interesting. I made my ASSUMPTIONS based upon the fact that my IC730 and my IC718 had the exact same "pin out" configuration, even though they used different mics. And yes, I confirmed that I cannot exchange the mics between the two rigs. Neither will function on the other. But since the pin out configuration was the same, I kind of though it was a very common configuration for Icom. Well, that is what I get for Assuming.

Hope you can enlighten me on this.

I just successfully opened my HM7 microphone --- turns out you need to pry off the rear circular label (with HM7 on it); this isn't easy because the glue is aggressive, however, behind that label is hidden the screw needed to open the microphone. Unfortunately, I damaged the label in the process of removing it, however, I'm not concerned about appearance. Anyway, hope this helps (somewhat belatedly).

W0LPQ
05-23-2010, 02:46 PM
Scroll about half way down to Icom and the HM-7 .. it is a single transistor amp.

http://www.qsl.net/g4wpw/date.html

W0LPQ
05-23-2010, 04:11 PM
0GAZ ... try the xyl's hair dryer ... it will get the glue hot enough to just peel off the sticker. Did just that with the Drake 7075 Mic, to replace the cartridge. The access screw is behind the little aluminum sticker ... get it hot and the sticker just peels off..

AG3Y
05-23-2010, 05:46 PM
its puting out 50 watts dead carrier on am ,but no wattage on ssb.

I hope that UBK has figured out this situation by now, but I thought that his question did deserve an answer.

SSB stands for Single Sideband Supressed Carrier modulation ( It would be SSBSC if all the letters were included ! ) A properly adjusted radio will probably have the carrier, and certainly the opposite sideband to that which is being transmitted, suppressed by at least 30dB or so. Thus, it is very unlikely that you will see any indication on a relative output meter, until ( and this is very important ! ) you speak into the microphone!

And then, you shouldn't expect to see more than a 30% of "dead carrier" indicating on your power meter. The reason for that, is that most meters respond to the average, not peak, power of the modulating waveform. And since the human voice power is around 30% of its peak power, that is what you are going to see on the typical power output meter. Any higher reading will probably be a result of distortion and over-modulation.

I know this answer is late, but hopefully will be helpful to someone.

73, Jim

KJ4VPJ
06-09-2010, 12:52 PM
Thank you Jim for the clarification.

I just ran into this myself. I just acquired my ticket, and bought a 720a. I hooked everything up and my PS barely pulled 1 extra amp when I keyed the mic. Oh boy, I bought a bum radio. Then I read the manual.

The 720a has a built-in SWR meter, and the instructions said to switch modulation to RTTY. That was curious I thought. Then it hit me. I'd be in trouble if that amp meter did move on SSB. Sure enough, flip it to RTTY and my PS was reporting a 14A draw.

Now I'm running into my next problem (which is also related to this thread's subject) which is the generic mic I bought doesn't have the built-in pre-amp. I've tried several circuits based on what I have in my junk box, but very little in the results column.

I'm able to run a mic signal through my PC's sound card. With the mic gain and volume controls on the PC cranked up, I can make the needles bounce, but not very much. And I'd imagine that if I can be heard (noone has answered as yet), I don't sound good.

So now I'm ordering a mono pre-amp from Jameco Electronics (Velleman Electronics K1803). We'll see if that does anything.

While I'm impatient to try and make my first contact, I realize this is a hobby. Had this radio came with a microphone, I wouldn't have learned as much as I have already.

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