My Sony Widescreen TV works well, but if I push the unit back against at living room wall a horizontal line appears on screen. The line drifts downwards until it reaches the bottom of the picture and then it rolls round to the top again and slowly drifts downwards.

I have tried moving the tangle of wires at the back in case the power cables are too close to the DSat box cable, but this has no effect. I'm surprised that the TV seems upset if it is in a confined space. Any tips?

Paul Michael, November 2004

Most likely you have an issue with something putting voltage on your house ground. A few things I've seen cause "hum bars" are:
Something putting stray voltage on the shield of the coaxial cable. The voltage could be from equipment in the home hooked to the coax, but it could also be (and often is) caused by something putting stray voltage on the house grounding system.

1) One of your TV's (or other equipment tied into the coax) has a "hot chassis" and is feeding voltage back down the shielding of the cable. You can check this by disconnecting everything from the coax and connecting one set at a time back up, and see if this corrects the issue.

2) An appliance in the home has an internal fault and is feeding voltage back to ground. A few ways you can check this is to unplug one appliance at a time (old washing machines are horrible about causing this) while the TV is on, and see if the hum bar goes away.

There are plenty of other things that can cause this, but those are the easiest two to isolate

CableGuy-MD, October 2006

ARE THERE ANY ELECTRICAL CABLES IN THE WALL? THIS COULD ALSO CAUSE A HUM BAR

TV INSTALLER, December 2004

Also a possibility that the DSat box is radiating interference. If possible, as a simple test, first disconnect the power to the cable box and then try running the TV with just a video recorder or DVD input.

Cobweb, November 2004

You're on the right track with power cables. The 'hum' bar is likely resulting from the 60Hz (US) or (50Hz UK) mains cycle. You are correct to first move all signal carrying cables away from power cables. Use good quality "individually screened" signal carrying cables. Otherwise, is there another appliance directly behind that wall it may be picking up interference from.

Cobweb, November 2004