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Thread: Inter Com Noise

  1. #1

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    Inter Com Noise

    My super Cub has a Sigtronics SCI4H that began making constant noise at some point but I never heard it because I kept a cord plugged into the music jack, then others used my Cub for testing and the complaints started

    Bringing it home from Texas the other day I tried everything I could isolate and found the noise only goes away if you

    A. Flip the switch to either of the isolation options

    B. Plug a cord into the Mini-port for tunes, doesn't matter if there is a devise connected to the cord, its turned on or if there is music playing, the noise goes away.

    I'm thinking maybe the plug is the culprit but not sure how to test that.

    For now I've back to keeping the plug in it.
    Remember, These are the Good old Days!

  2. #2
    wireweinie's Avatar
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    I'd suspect that music jack, too. What type is it? Can you see the contacts inside or is it enclosed? If you can see the contacts, see if they touch each other when a plug is not installed.

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  3. #3

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    Quote Originally Posted by wireweinie View Post
    I'd suspect that music jack, too. What type is it? Can you see the contacts inside or is it enclosed? If you can see the contacts, see if they touch each other when a plug is not installed.

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    I'll do some excavation and get a look at it... thanks.
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  4. #4
    hotrod180's Avatar
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    Quote Originally Posted by OLDCROWE View Post
    ... the noise only goes away if you ....B. Plug a cord into the Mini-port for tunes, doesn't matter if there is a devise connected to the cord, its turned on or if there is music playing, the noise goes away.
    I'm thinking maybe the plug is the culprit but not sure how to test that.
    I'm thinking maybe you solved your problem,
    if plugging a stub of a cord into the mini port makes the noise go away.
    Some might call that a band-aid fix, I'd just call it a fix.
    Cessna Skywagon-- accept no substitute!

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    Steve Pierce's Avatar
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    That was my torture device for the people using the airplane.
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  6. #6
    akavidflyer's Avatar
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    go here and search for 121g. will fix ya right up!

    https://www.oreillyauto.com/
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  7. #7

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    Quote Originally Posted by hotrod180 View Post
    I'm thinking maybe you solved your problem,
    if plugging a stub of a cord into the mini port makes the noise go away.
    Some might call that a band-aid fix, I'd just call it a fix.
    But then on a trip I can't listen to XM, when I move the antennae over form the 180...
    Remember, These are the Good old Days!

  8. #8
    sjohnson's Avatar
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    Make sure the grounds or shields for each of the headset jacks (the barrel connections), aux jacks, and mike jacks, are only connected to ground at the intercom. Do not connect the grounds to the airframe at the jacks. If the grounds are connected at the jack, you can get some unwanted current return paths (e.g. ground loops), leading to some weird, intermittent noise effects.

    You will probably have to use washers to isolate the ground rings at the jacks (between the jack and the panel). There are special washers for this, and they usually come with the jacks. They have a little shoulder to keep the jack housing from contacting the inside of the hole in the panel. A lot of intercom installation manuals overlook this.

    I don't have a Sigtronics installation schematic. Some intercoms provide a separate audio ground connection, some don't. If there is a separate audio ground, connect your jack grounds to that. If not, connect to airframe ground as close as possible to the intercom, e.g. use a jumper inside the connector housing. In any case, start looking at the jacks and try to solve the problem there, and only mess with the intercom connectors if you still can't fix it.

    I've chased this ground problem on several intercoms. Usually, its caused by someone forgetting to re-install the washers during maintenance.
    There are three simple rules for making consistently smooth landings. Unfortunately no one knows what they are.
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  9. #9
    wireweinie's Avatar
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    Quote Originally Posted by sjohnson View Post
    a Sigtronics installation schematic
    The source of oh so many noise issues.

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  10. #10
    CamTom12's Avatar
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    Inter Com Noise

    I had massive noise issues: alternator, strobes, heck even the wig-wags chimed in. Rewired (shielded!) my whole ics system and it’s as quiet as a Catholic Church now.

    I repeat the recommendation to get a hold of the wiring diagrams and re-wire appropriately.

    Then you can happily cruise to your favorite progressive rock in class G.

    EDIT: These are the washers sjohnson was talking about: https://www.aircraftspruce.com/catal...aneljacks3.php
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  11. #11
    wireweinie's Avatar
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    The manufacturer is Switchcraft. Shoulder washer number is S1029 and the flat washer number is S1028. They fit mono and stereo jacks and mic jacks.

    For the music jack use Keystone shoulder washer part number 7687. Use any 1/4" or 6mm nylon flat washer with it. The ones I use are from Ducati dirt bike fairings, as they are the thinnest I've found.

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    Last edited by wireweinie; 03-16-2022 at 06:15 PM.
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  12. #12
    wireweinie's Avatar
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    If you haven't heard me preach on the evils of Sigtronics, let me explain. Most Sigtronics diagrams show no shielded wires and the jacks grounded to the airframe. See the attached pic. Specifically the audio jacks. Both of these are major taboos when doing audio systems. Even music jacks need to be isolated from the airframe. The only time you can forego the isolation washers is if you mount the jacks to a non conductive surface (note that carbon fiber is conductive). But even then you need to use shielded wiring for the audio and mic circuits. I don't use Sig intercoms anymore because of this. If I did have to install one, due to a hostage situation, I'd fabricate a shielded wire harness patterned after NAT's diagrams.

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  13. #13
    Gordon Misch's Avatar
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    That's interesting, because when I installed Sigtronics intercom in my -12, the instructions and diagrams very explicitly required dedicated ground wires to the ground buss and forbade grounding directly to the fuselage. I called the company to ask about this, and they said to do it as specified due to potential for ground-loop induced noise. My intercom installation was 12 years ago and has worked just fine for the entire time. FWIW - - -
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