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HM-36 mic trouble

Discussion in 'General Technical Questions and Answers' started by N9IZ, Sep 14, 2009.

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  1. N9IZ

    N9IZ XML Subscriber QRZ Page

    Greetings. I've been refurbishing an older rig for a co-worker (IC-736). It came with the original hand mic (HM-36) and seems to have very low audio in TX. While watching the ALC meter and adjusting the Mic Gain I see no movement. Finally, with the gain fully advanced and speaking quite loudly into the mic I see the needle move. I engaged the compression and it got a little better. I swapped it with a spare HM-36 from my extra parts box. I was able to back the gain off about a quarter of a turn and still see some deflection on the meter. On air testing with hams in VA and AR confirmed low TX audio. I turned on the compression and they both said audio was perfect. That's fine with me as long as the audio is clear but I'm not used to using so much gain, or compression for that matter. Anyone have any ideas as to what would cause the original mic audio to be so low? I know it's fairly old. Maybe the ICOM hand mics are just weak? Incidentally, the meter works as expected when tested with my spare SM-8 mic and Heil iCM. I hate to give up my spare hand mic in case I want to go portable. Thanks for any suggestions.

  2. W0LPQ

    W0LPQ Ham Member QRZ Page

  3. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    If you can find one replace the HM-36 with an HM-7. The HM-7 has a built in pre-amplifier and was supplied stock with the IC-730, IC-740, and other Icoms tranceivers of that era. You should not need to re-wire it as it is 100% compatible with the IC-736.
  4. W0LPQ

    W0LPQ Ham Member QRZ Page

  5. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    No, the HM-36 only contains a bias network for the electret mic element but there are no active devices in the circuit. The HM-7 provides 6 db of gain and uses a single transistor to to do that.
  6. KD8GFC

    KD8GFC Ham Member QRZ Page

    I use the IC-718 and it came with the HM-36 hand mic it workes ok but no real punch. I also use the SM-20 desk mic that workes great but picks up the power supply fan so I put a Astatic 575-m6 hand mic amplified and have had great reports with it you just got to remember not to crank it up to much it has plenty of punch with out picking up back ground noise or sounding to hot. Try some different mics you might like the results. Jim !!
  7. W9GB

    W9GB Ham Member QRZ Page

    Jeff -

    You said that the radio's audio is fine with your spare HM-36.

    SO the issue (problem) is with the original HM-36 microphone.

    Here is the check list.

    1. Check the +8 VDC from radio with a VOM or DVM.
    CAUTION: DO NOT short pin 2 to ground as this can damage the radio's internal 8 V regulator.

    2. OPEN UP the HM-36 microphone. Check the continuity (VOM or DVM) from the microphone plug (end of microphone's coiled cord) and wires on inside of microphone. Wires inside coiled cords can and do break!
    I repaired the original dynamic microphone for the Icom IC-211 due to a bad microphone cord -- took me longer to fine a suitable (exact match) coiled cord!

    3. The HM-36 uses a 3-WIRE elecret element.
    Go to the Parts Drawers at your local RadioShack and look for the 3-WIRE elecret microphone elements (< $5) -- exact replacement. While the 2-wire elecret versions can be used, would require re-wiring changes (avoid more work/changes/mods).

    Using the HM-36 schematic and Radio Shack diagram as guides -- changeout the elecret element.
    Reassemble the microphone taking care not to pinch small gauge wires.

    Tryout the HM-36 microphone with the new element!
    You could add a one transistor pre-amp inside this microphone (like the HM-7) powered from the +8 VDC line --
    but double check wiring before powering from the Icom radio (only 10 mA current draw).

    Last edited: Sep 14, 2009
  8. N9IZ

    N9IZ XML Subscriber QRZ Page

    Very good ideas. Thanks for the suggestions. The schematic will be helpful, too. Will be looking for those replacement electret elements in the future. For now I'm fine with the desk mic. Will give my friend the good, working hand mic with his radio.

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