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Radio Signals in my System.

Discussion in 'Sound, Music, and Intercom' started by SweetBennyFenton, Apr 21, 2009.

  1. SweetBennyFenton

    SweetBennyFenton Active Member

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    All you sound-savy people were such a help last time, I thought I would run something else past you.

    The sound system in my theater has the nasty habit of picking up radio signals. I've done what I know of to minimise this, but even so when the system is powered up you can still hear the faint sounds of mariachi music in every speaker.

    Has anyone else had this problem? Do you think it's a problem with the microphone or cable that I use as a stage monitor? Any ideas?

    Thanks again...
     
  2. avkid

    avkid Not a New User Fight Leukemia

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    Are the cables to your amplifiers balanced or unbalanced?
     
  3. jwl868

    jwl868 Active Member

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    We had a similar problem at our church. (see this thread for the replies)

    http://www.controlbooth.com/forums/sound/5751-pa-system-am-radio-broadcast.html

    Shortly after that, a couple choir members tracked down the problem. I can't remember the details, but I there were some long, maybe duplicated runs of cable from the main PA in the front of the church and some another component in the balcony. They eliminated the extraneous run of cable (or maybe got lucky and eliminated a bad cable) and that was the last we heard of the radio.

    Joe
     
  4. KProductions

    KProductions Member

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    My guess is bad cables somewhere in the chain.
     
  5. SweetBennyFenton

    SweetBennyFenton Active Member

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    Thanks for linking me to the other thread. I searched for a thread on this before I posted, but I didn't come across that one.

    This gives me some good ideas. Thank you.
     
  6. BillESC

    BillESC Well-Known Member Premium Member

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    Balanced cables are your friend.
     
  7. museav

    museav CBMod CB Mods Departed Member

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    "Divide and conquer." The more you can narrow down where it is being introduced into the system, the easier to find the cause and a solution. Plug and unplug cables, adjust volume controls, etc. to see if you can isolate the problem to any particular device or cable paths. Also, knowing some details on the system might help, for example some older Mackie mixers were susceptible to RF on their inputs, for which Mackie introduced a fix that was incorporated into later models.
     
  8. Van

    Van CBMod CB Mods Premium Member

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    The Portland Area is so Innudated by High Power RF it's not too surprizing to get radio on a PA.
    You're gonna have to go over everything that is pre amplifier. My money is on it being the last cable you check.
    :twisted:

    Often an unsoldered sheild or ground wire, or a messy one < with wires sticking out all over, ( course those are kinda obvious) , tend to be the culprits. >
     
  9. SweetBennyFenton

    SweetBennyFenton Active Member

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    Yeah... and the in-wall wires seem to have been run by a monkey on speed. I have some good clues as to the problem now.

    I will kinda miss the mariachi music though. :)
     
  10. TimmyP1955

    TimmyP1955 Active Member

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    Listen to the noise on the console's headphone output. Starting with the local line level sources, disconnect console inputs until the noise goes away. If it does not, start disconnecting outputs.
     
  11. lieperjp

    lieperjp Well-Known Member

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    Do you know what model? Our church as a 15-yr old mackie console that has this problem with some of its inputs...
     
  12. museav

    museav CBMod CB Mods Departed Member

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    I believe that it was older VLZ series models and corrected in the VLZ Pro versions. Supposedly one of the design goals of the XDR preamps was much improved RFI resistance. Mackie did offer a modification for the units that had problems, you could probably contact them and ask if it might apply to your mixer.
     
  13. Anonymous067

    Anonymous067 BANNED USER

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    +1 for this....

    Although I start by muting all inputs, and if it goes away, unmute one at a time.
    Start unplugging inputs until the sound goes away...
    Make sure the sound is coming from all speaker outputs, this verifies its an input not at output. -What I mean is...-make sure that channel is in all auxs and mains, and if you hear it in all speakers, it's an input prob!

    I agree, its probably a problem pre-preamp. So start checking inputs.
     
  14. SweetBennyFenton

    SweetBennyFenton Active Member

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    Well, I re-built the entire sound system over the summer. For a while, it seemed like I had fixed the problem by not using any of the old cable runs that were inside the wall of the theater.

    Then last night during a nice performance of Endgame, we got to listen to talk radio for an hour through the house right speaker.

    I swear... I'm almost at my whit's end. The next time a director asks me about it, I may just say that the theater is haunted. :)

    (Update: I traced the problem back to the source following the steps y'all gave me. It's still not perfect, but I have been able to minimize it. Thanks everyone!)
     
    Last edited: Nov 5, 2009
  15. banjokeith

    banjokeith Member

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    I had a radio station coming through my system a few weeks ago and traced every cable I could find, with no results. It turned out to be the headphones on my board, or rather where I had them plugged in. The Allen & Heath GL2200 has a 1/4" jack on the rear of the board for headphones, as well as a very conveniently placed one right in the middle of the control surface. For some reason that day I decided to plug the headphones into the nice easily accessible jack and in doing so introduced a radio station into the system. I switched the headphones back to the rear, and 92.9 FM was gone.

    The moral of this is that no matter how carefully you choose the cables you use, and how much money you spend on well-shielded connectors, RF will find a way, and if you do end up having to track it down be sure to try everything!
     
  16. Lightguy48

    Lightguy48 Member

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    Also keep in mind it may be a cable that is the right length to be resonant at the frequency of the radio station you're receiving, basically acting like an antenna. Adding 25 - 50' of cable or removing 25 - 50' will change the length and therefore no longer act like a receiver.

    Another option is to install a ferrite or torroid choke, wrap the wire through the choke a number of times and it will help remove any RF on the line.
     

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