Strange issue with external monitor, only working when external speakers plugged in.

Discussion in 'Mac Accessories' started by JackTh, Sep 1, 2014.

  1. JackTh, Sep 1, 2014
    Last edited: Sep 2, 2014

    JackTh macrumors newbie

    Sep 1, 2014
    Hey – I am having a very strange issue with my external monitor; where it only works if my external speakers are plugged in and connected.

    I am using a late 2011 Macbook pro, running Mavericks 10.9.4, along with an MBox 2 (USB Audio Interface), and two powered speakers (KRK Rokit 5). This setup has had no issues. I have just acquired an external monitor (Benq T2200HD), which I am using with a thunderbolt to VGA adaptor (a cheap Chinese adaptor).

    To start - I plugged the monitor in with the thunderbolt adaptor, and it powers up and works fine. However, there is a nasty buzz coming from the speakers. I did some research and it looks like a simple ground loop issue. I did a bit of troubleshooting, and this is where things get weird.

    What I found is - If I unplug the MBox, the monitor loses signal from the computer (displaying ‘No Signal Detected!’). Further, if I keep the MBox plugged in, however unplug both the cables from the MBox to the speakers, the monitor loses signal. It doesn’t matter if the speakers are turned off, however they need to be plugged in (or the monitor loses signal). Weirder still, I can unplug the MBox, however the monitor will work if I plug in the speakers through my macs headphone jack.

    Simply put, the monitor only works if there is a direct connection between the computer, the speakers, and the power source. What is going on here? Is the monitor grounding through the computer port, to the speakers (I don’t know how this works, is this possible?). Could it somehow be the cheap VGA adaptor I am using?

    Some other information:
    - Everything is plugged into the same multi-box for power. I have tried plugging things into separate sources, and the result is the same.
    - If I unplug the thunderbolt adaptor, then the speaker noise disappears straight away.
    - The ground noise doesn’t appear in my headphones, which are plugged into the MBox.

    Any information would be great – am I missing something here, or is this genuinely weird?

    I've tested the monitor/adaptor with another computer, and they work fine. This means the issue is with my Macbook Pro.

    A few weeks ago I spilt some water on the keyboard, and it shorted some of the keys (I'm using an external keyboard and mouse now, which is why I also bought another monitor) - I'm still confused, but this could be the case - is it possible the macbook has a grounding issue, and is grounding the monitor through the USB port?
  2. waw74 macrumors 68030

    May 27, 2008
    sounds like something is grounding out.

    on the power brick for your mac are you using the flip down (ungrounded) or the long cord (grounded)
  3. JackTh thread starter macrumors newbie

    Sep 1, 2014
    I am currently using the long chord (three-prong grounded) power adaptor.

    However - I can unplug the macbook adaptor and it has no effect, so I'm not sure if it is involved with this issue. The two speakers and monitor are all three-prong grounded, FYI.
  4. JackTh thread starter macrumors newbie

    Sep 1, 2014
    An update - I have tried using another external monitor, and have had the same result. This means it is unlikely that the issue is with the monitor itself. I will do some tests tomorrow with an official apple thunderbolt adaptor, and see what the results are.

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