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(Thanks for fmook for this one.)
Editor's Note: That horizontal bar is called a "hum bar".
Note by Tech3912 : Also bad crappy amps will do this as well. We see it alot as techs becuase ppl go to radio shack and get screwed... :)
cableguy78199- humbar issues can also be caused by an improper ground. the ground wire at the pre-wire should be on a ground rod or cold water pipe, nothing else!
Bad picture tubes is usually the main cause I run into. Try popping in a dvd or vcr tape and unhooking the cable line from wherever its going(into tv or vcr for example) if the lines are still running across the tv in anyway, up down, left right, then you know its not the cable line. I had a guy pay the cable company $75 for the service call for me to show him this and explain to him he needed a new tv or to get his fixed which I recommended not doing these days, unless its a pricey set.
humbars are caused by bad grounding (most of the time from what I have seen). In a modern house with 3 pronged plugs you will never see a humbar because the TV grounds out the CATV system, however if you have a non grounded drop running to a TV that has no ground you can allmost garantee that you will see a humbar. I run my arrial drops directly to power meters 90% of the time right along the threephase distro and even behind the meter base down to the power meter and never ever see humbars unless I haven't grounded yet.
Ground loop - ground from cable (coax) is using the same ground as the power to the TV. with both units plugged into the TV, there is a ground loop induced in the TV. Remove the ground for the TV, or reduce the size of the loop.
Also ......A bad electrical ground, via ground block will usually cause this. Make sure all ground wires are secure and proper. HD TV is usually the worst and first channel where you will see this. The ground should be attached from ground block to electrical ground rod, electrical service pipe or copper ground wire via rod. Grounding to inside copper or galvanized pipe is never a good idea but will work.
This can also appear as an annoying hum in the audio system. Home theaters and computers attached to cable modems are subject to this. In my case I have both horizontal bars and humming sound. I had a home theater system and the computer computer affected by this problem. Both the TV and the computer monitor were showing horizontal bars and the humming could be heard in the speakers of both systems. The computer was attached to the cable modem and then there is an audio cable from the computer to the home theater. The only solution was to install a ground isolator at the main cable coming from the outside.