Fix a ground loop?

Discussion in 'Computers' started by Jeff W., Feb 9, 2004.

  1. Jeff W.

    Jeff W. Stunt Coordinator

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    How can I fix this ground loop problem? Is there a gizmo or something I can affix?

    Here's my setup -

    PC S-video out to Onkyo receiver video in
    PC coax digital audio out to receiver
    PC stereo L/R IN from receiver tape out (for recording LPs onto CD)

    TV has CATV of course, so does the PC (HSI)

    Problem is there is a ground loop when I connect the PC to the receiver, using any of the 3 cables... On the audio, listening to ANY input (DVD, TV, radio, phono, etc) I get a nasty hum, and when viewing the output from the PC on the TV, I get an upward scrolling light/dark line.

    If I unplug the PC from the receiver (all of the cables) it goes away, but how can I fix this? I have a GLI thing for the audio which takes out a lot of the hum, but only on that 1 input, and doesn't help at all for the video..
     
  2. Mike Voigt

    Mike Voigt Supporting Actor

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    Try disconnecting BOTH CATV feeds, and see if it still has the ground loop. Almost always, it is one of those.

    If it is, there is a cheap way to fix it: get a 75-300 Ohm converted and a 300-75 Ohm converter that you can attach to each other, then insert them into the cable. That may solve the problem. Radio Shack has them, they run maybe 3-5 bucks a pop. One looks like a CATV plug with two terminal screws, the other looks like two wires with a CATV screw-on end. Attach the wires to the plug, then CATV on one side and another CATV segment to the TV or the PC.

    If it is the CATV feed cables, but the above doesn't work, invest in a M.A.G.I.C. box (Acurus?), or something similar. They run about $100, but people swear by them.

    If it isn't the CATV feeds, check for any loose strands of wire. Beyond that, I can't help you - mine have ALWAYS been those, except for one loose strand...!

    Mike
     
  3. Glenn Overholt

    Glenn Overholt Producer

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    Are your PC and receiver plugged into the same outlet on the wall? Is it possible that your building wiring might not be totally correct?

    I'd go for the CATV first though. If the outside CATV ground is not tied in to your building's main ground, you've got the loop.

    However, your cable company should know better. They are violating building codes. If you can verify that, politely call them them back and let them know.

    Glenn
     
  4. Jeff W.

    Jeff W. Stunt Coordinator

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    I fixed it with a $7 "Calrad DC Ground blocker and isolation transformer" from here: http://www.cablesandconnectors.com/02100-02.HTM
    (P/N 75-504)
    [​IMG]

    The CATV was grounded and the house wiring is all perfect. The problem was that all the TV/HT equipment uses 2-prong polarized plugs while the computer equipment uses 3-prong grounded plugs, so the CATV from the TV tries to find a ground over the A/V wires, causing the hum and bars. The DC blocker breaks the ground from the CATV.

    I actually ordered 2 of those gizmos in case I needed one for the cable modem, but turns out I didn't. Putting one of them on the end of the CATV wire that feeds the TV/HT fixed it.
     
  5. WayneO

    WayneO Supporting Actor

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    I used that same product to kill my ground loop humming I'd get from my amp. Definately worth trying for $7.
     

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