I’m using RF Modulator to connect my DVD player to an old TV. Almost it's working fine. However this modulator generates noise and it displays cross line on the screen. When i touch the metal body of the RF in jack, those cross lines goes out. (Maybe noise is grounded via my body). Can you tell me a permanent solution for this? (Like putting a CAP)

 

Try using a large toroidal ferrite and loop several turns of the coax cable through it. This should kill the RF on the shield of the coax. You can salvage these from junked equipment like old satellite TV decoders, DVD players, etc.

See the Balun entry, scroll half-way down the page:
http://en.wikipedia.org/wiki/Balun

You also might find snap-on RF suppression ferrites. These won't be as effective, but should help somewhat.

Be careful to not make any kinks in the coax, as that destroys it's electrical properties. Once kinked, it's junk. Never bend a coax in a tight radius.
More info: http://www.picwire.com/technical/paper1.html
Basically, for TV cables (RG-59, RG-6) don't bend it into a circle that is less than 3" in diameter, or a 1.5" radius, or it's ruined.

Check your coax cable connectors very carefully. If you can see any part of the shield near the center conductor, it will result in diagonal lines or "ghosting" on your screen. A short from the shield to the center conductor often displays dots or heavy ghosting on the TV when tuned to channel 6, and a very weak signal with "herringbones" in the 60 to 67 channel range.

RG-59 coax is vastly inferior to RG-6. RG-59 is cheap; that is why many builders use it to run cable in homes. If you compare an RG-59 coax to an RG-6 coax, you will see that the center conductor is much larger and the shielding is generally much better.

If your coax cables are more than around 6-7 years old, they are due for replacement.

 

Is there a ground pin on your power cord? If so, make sure the PCB is attached properly to the case or buss and some of the noise can be sent via ground.

Sgt Wookies information is invaluable. The ferrite bead on the power lines can help in a variety of applications. With video / audio / data / and especially for test equipment they can cut down a significant amount of noise, and keep it from spreading via mains to other equipment.