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Hardware Thread, Projector In One Of Our Rooms Is Picking Up A 'Hum' in Technical; One of our LCD projectors is displaying an image which is impossible to look at after just a few minutes. ...
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    DaveP's Avatar
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    Projector In One Of Our Rooms Is Picking Up A 'Hum'

    One of our LCD projectors is displaying an image which is impossible to look at after just a few minutes.

    There is a pulsing which moves down the screen. This is reproduced on the PC monitor. We have swapped the PC, the screen, all cables and the projector [The cable run to the projector is still in place]

    We have checked the room for a bad earth and eliminated that as a potential cause.

    We are now wondering if it is possible to get a Ground Loop Isolator [for video] Does such a thing exist? Where would I get one?

    What are other options that we have not thought of?

    Thanks.

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    jamesfed's Avatar
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    Is there any kind of large electrical sources near the room?

    Power transformers ect would be the kind of thing.
    Also it may sound weird but if you run an extension cable outside of the room and into another (where there is no problems) does the same image issue appear – would suggest problem with power going into that room.

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    DaveP (20th September 2011)

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    DaveP's Avatar
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    Quote Originally Posted by jamesfed View Post
    Is there any kind of large electrical sources near the room?
    Nothing out of the ordinary. The room is the same as the 5 other rooms on that corridor.

    Quote Originally Posted by jamesfed728744
    Also it may sound weird but if you run an extension cable outside of the room and into another (where there is no problems) does the same image issue appear – would suggest problem with power going into that room.
    We will give that a go tomorrow [should have thought of that myself ]

    Thanks.

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    Andrew_C's Avatar
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    You are looking in the right area with the ground loop isolator. Yes they are available, but ferkin' expensive! Canford Audio would be my first port of call, or any other professional presentation supplier. Hum Buckers.

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    DaveP (20th September 2011)

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    Are you using a vga splitter, or is the pc monitor daisy chained off the projector? Also, how long is the vga cable to the projector, does it run past anything else electrical?

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    DaveP (20th September 2011)

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    DaveP's Avatar
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    Quote Originally Posted by Chris_Cook View Post
    Are you using a vga splitter, or is the pc monitor daisy chained off the projector? Also, how long is the vga cable to the projector, does it run past anything else electrical?
    We are using a splitter. However we did put a VGA 'Y' shape in line with no improvement.

    The VGA cable from the splitter to the projector is about 10 m [much the same as other rooms but there are longer runs in school that do not show this effect]

    There is no significant power supply in the room. However we have found out from Site Support that there is a security device mounted on the outside wall of this classroom [not willing to go into any further detail here] but we do not consider that to be significant due to earlier electrical checks we carried out.

    Thanks.

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    DaveP's Avatar
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    We think we have solved it.

    It would appear that the projector is supplied on one phase and the PC is supplied on another with the difference causing the hum.

    [The electrical supply in the corridor where this projector is installed is a nightmare!]

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    That shouldn't make any difference, unless there are other issues too. After all, both devices turn the AC into DC as the first step. The earthing arrangements, however, will make a difference. You could have a significantly longer run on one giving rise to a small voltage between the two. This causes a current to flow in the screen of the signal cable. Voilą! Earth loop hum.

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