706: Microphone Tips

© 2003-2011 by Harold Melton, KV5R. All Rights Reserved. Rev.06/11/05

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Micorphones and the Icom 706MkIIG

If you really want to connect another mic:

The 706 uses an 8-pin “modular” RJ connector for the microphone. This is the same as used by CAT-5 computer LAN (network) cables. The connector costs $4+ each and requires a rather expensive assembly/crimp tool. My way is, of course, much better.

  1. Go to office/computer supply store.
  2. Buy a nice soft CAT-5a LAN patch cable, one with nice molded strain relief plugs. Get a 10-footer.
  3. Cut off one end, or cut in half to make two mic cables.
  4. Ring out and document pin numbers to wire colors.
  5. Solder the conductors into your mic or other audio accessory.


You can find an old, classic mike and replace the element in it with a modern electret condenser. (See photo) ElectroVoice Mercury and Shure Unisphere A. The Shure has been modified with a dual-element electret, PTT switch, and CAT-5a cable.. I don’t use it because the sound quality is inferior to the stock mic…


  1. The supplied HM-103 hand mike is the best one you can run on the 706. I do NOT recommend using other mikes on the 706, except the Heil iCM series. Other mikes, including Icom desk mikes, will have a sound clarity that is inferior to the stock hand mike (yes, really)… The HM-103 sounds best when you talk across it. Never place it flat against the mouth as this will cause considerable distortion.
  2. Rewire the mic’s on-off switch for PTT.
  3. Wire the element always “hot” or you won’t be able to use VOX.
  4. On the other hand, a mic off switch is useful when running SSTV where you need to talk then transmit SSTV.
  5. Obtain a quality electret condenser mic assembly, designed for 8 volts DC phantom power. A setreo mike element, used on cam-corders, may be wired in parallel, and you’ll get lots of output that’s needed by the “pre-Pro” Icom radios. These will not sound as good as the stock hand mike.
  6. Bob Heil will not sell you a bare iCM element, even if you beg!


17 thoughts on “706: Microphone Tips
  1. I’v been using the Heil iCM desk top mic with the Heil HSTA-IM 8 pin to RJ connector for almost 15 years. Works like a champ. I’v done several audio test between the stock HM-103 hand mic and the iCM mic. Hands down, the Heil iCM desk top mic has the better audio. The iCM works the 706MKIIG’s VOX with no issues.

    I originally bought the Heil iCM mic for my IC-761, which works like a champ and twice on Sundays. I use the 706MKIIG the majority of the time on 6 / 2 / 432 side band. Having just 1 mic for both radios works nicely and keeps the desk top clutter down to a minimum.

    Thanks – KD5ITM

  2. I wll never buy another Heil product because of an issue I’m having with their hs706 headset and they could not tell or would not tell the rj45 connector wire color codes and what pins to use. I even talked to Mr Heil himself and was of no help, If that is how they treat their older customers Then I’m done with them.

  3. Hi KV5R,

    I’d be very grateful if you could help me with this.

    My lovely Icom IC-706 MkIIG has developed a problem when I transmit audio.

    I have done some testing, using a shortwave receiver in the shack.

    When I key-up the carrier is present, but my audio is not… or I should say it’s there’ish… but very, very faint, scratchy and unintelligible.

    My mic gain is 5, and I can see the ALC defecting fine when I speak.

    I’ve tried this on HF, VHF and UHF and using the stock mic, with the same results.

    Now… I’ve been using a headset with the rig, together separate remote PTT switch. The headset is a PC headset with an electret microphone, and it’s been working just fine.

    However, I’ve read your article stating the audio line has 8-volts phantom power on it, and shorting it will blow the 8-volt regulator’s tiny fuse (inside).

    I don’t think I’ve “shorted”, not knowingly anyway.

    Can you suggest what’s gone wrong? If I have blown the 8-volt regulator fuse, can I replace it? If so, where is it in the rig?

    Thank you very much, James.

    PS. I have posed this on qrz.com question on qrz.com.

    • If you lose the 8V reg the radio won’t power on. Sounds like it’s developed a problem in an audio stage. But if you’re getting ALC then audio is working to some point.
      Make sure you’re not overloading the SWL receiver; transmit into a meter & dummy load and let the SWL receiver just pick up the leakage. Check your SSB output power with meter into dummy load while talking. Or better, use an RTL-SDR dongle with a short antenna to receive, so you can see the spectrum the 706 is putting out.
      73, –kv5r

      • Thanks very much for the information.

        Maybe I don’t have a problem. I’d not thought about overloading the SWL receiver.

        I have an SDRPlay. I will have a look at the audio spectrum it produces.

        73, James.

  4. My Icom 706mkiig cannot seem to connect to irlp enabled repeaters using the HM-118TN microphone I purchased. Is there some setting that I need to change to make it work? Or am I missing something? I’m rather new to this again after being vacant for 42 years please advise. As a secondary question hopefully not related to the first question is it possible to hook into Echo link with my present radio Hardware?

      • EchoLink is used to connect to a repeater using your Windows laptop. It replaces your rig completely. It is great to listen to your local club’s repeater at work. 🙂

        I am also having problems with microphone HM-118TN with a IC-709 MKIIG rig. The mic audio works and the channel up/down works, and the DTMF button light up, but they do not make any sound when the DTMF buttons are pushed. It seems to be a MKIIG problem because HM-118TN’s DTMF buttons work with my buddy’s IC-706 (non-MKIIG) rig. Anybody know why?

        NOTE: The MKIIG anual says use mic HM-118T, but they cannot be found. Only the HM-118TN is available.

  5. The IC-706 mic does NOT use a “standard” CAT-5 cnnnector. It LOOKS like the standard one, but has “wire guide holes” internally to accept the individual wires when putting a NEW connector on the cable. Often, operators put quite a strain on the mic cable when using the radio and the connector DOES provide a “catch” to hold the outer sheath, but does nothing to prevent the wires being pulled. As a result, the most common failure point is the shield (wound tightly and put into shrink-tubing, before insertion, prior to crimping). I’m tracking down the proper connector at this time, as the standard CAT-5 connector is designed for FLAT RIBBON type cable, and NOT individual wires. …and yes, a CAT-5 Male/female extension is a GREAT idea – I used the RJ-31X jacks in a standard wall box.. works swell in the mobile.

  6. A few months ago I found a way of turning the mic audio off as I was leaving for a while…now upon returning I have forgotten how I did it but pretty sure I did it with the external buttons on the Icom IC 706 MK2…(getting old ain’t fun)(done in case kids listened while I was gone)…when I did it I noticed I had output power but no audio, as in AM or SSB…I have the CI-V cable but can’t remember if I used that or not…ny help would be greatly appreciated…Phil !!!

    • If I remember, there’s a Lock switch on top of the mic, between the two buttons. But the best and easiest way to keep little fingers from transmitting is just unplug the mic and hide it, or take it with you… 🙂

  7. can you or should you use both a FL103 narrow SSB filter along with a
    fL223 wide band filter together in the Icom 706MKII
    g? will it work well or should i not??

  8. I found your 706 site very interesting. I’m sure you are very busy but hope you could help me with connecting a Kenwood MC60A mike to a IC7000 radio.

    Thank you, George KF5NAJ

    • Easiest and safest way is to buy an adapter pigtail (Heil, MFJ, etc). That way you don’t mod your mic, and don’t need an RJ45 modular crimper.
      If you decide the hard way, you need the schematic for the mic, and for the radio’s mic jack, and an RJ45 crimper — then you end up with a MC60A dedicated to the one radio, and an RJ45 with no rubber strain-relief.
      I’d just buy the pigtail, get one with a nice strain-relief on the RJ45, and not mod your mic.
      73, –kv5r

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