QU24 phantom power issue

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This topic contains 5 replies, has 3 voices, and was last updated by Profile photo of Chris93 Chris93 4 years, 11 months ago.

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  • #55275
    Profile photo of DSOM
    DSOM
    Participant

    Folks,
    I am helping run a Qu-24 board for a theater production. The board is about 1 1/2 years old and is only used for productions. I am using most of the channels for wireless packs but do have seven Audio Technica PRO45 mics to pick up the uncovered portions of the stage (hanging and on the floor). I have been having issues getting all seven mics up and working with phantom power. So, I thought I found a bad mic and replaced it. As I was hanging it, I was holding onto the xlr shell when my arm came in contact with a metal pole (ground). I received a shock and proceeded to get my multimeter out and I found 45v DC sitting on the cable back shell. Troubleshooting hasn’t pointed to a single channel, could it be the board? Maybe another mic I’ve missed? Any one experienced something similar? Thanks in advance for the help!

    #55276
    Profile photo of Andreas
    Andreas
    Moderator

    Sounds like pins 1&3 swapped in the cables.
    You could easily rule out the board and check dc level on pin 1 of your sockets, which must be zero (measured against the TRS socket). Pins 2 and 3 carry about +48V DC with phantom engaged.
    When you measured that voltage on the shell, what was your ground reference? Did you also check AC? With long cable runs you may compare different grounds (stage and FOH position). These are not necessarily identical, particularly with running dimmer packs and bad/old power distribution.

    #55279
    Profile photo of Chris93
    Chris93
    Participant

    Phantom power is supposed to be +48VDC on pins 2 and 3, returning via pin 1.

    Some people when assembling XLR connectors will solder pin 1 to the shell of the connector, which might be happening here too.

    Open up both ends of all cables in the chain and check that they are wired correctly as per this diagram:

    https://itmpmooc.files.wordpress.com/2013/03/xlr-pinout-ok.jpg

    It is likely that pins 1 and 2 have been reversed somewhere, pin 3 is always in the middle so people tend to get that one right. 🙂

    I suspect that pin 1 is missing at one end of one of the cables. The phantom power wouldn’t work as it has no return path, but you touching the connector shell would provide one, hence the shock.

    Chris

    #55282
    Profile photo of Andreas
    Andreas
    Moderator

    Indeed, broken ground connection would explain it as well. But then it should affect only single mics, which doesn’t seem to be the case here, right? Is there voltage on all remote cable shields? Does the voltage persist if you turn Phantom off on all channels?
    Hmm, maybe pins are not swapped but the shell is connected to the wrong pin.

    #55283
    Profile photo of DSOM
    DSOM
    Participant

    Thanks for the help folks. Gonna check things this afternoon after work. The voltage does dissapate as phantom power is turned off but it dissapates like a capacitor.

    #55289
    Profile photo of Chris93
    Chris93
    Participant

    Phantom power normally dissipates like that when turned off, no problem there.

    Chris

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