If the white lines are overlaid on top of the picture, you're probably getting interference from some outside source. With cheap CRT TVs, one TV could be interfering with the other, and it now appears on both TVs because they've come to some sort of "mutual agreement," have signed a defense pact, and are about to attack you. You probably should get those particular sets out of the house immediately, before they have time to gather weapons.

If the white space is above the picture, and the normal picture just begins lower, it's probably a failure (reduction in peak value) of the vertical synch circuit. In older sets this could be adjusted with knobs on the back, but in newer ones it's probably "inside" the box and would require (semi) professional service.

If it's the latter, it could be caused by the "fried spider" syndrome, in which some critter has died where there's a "leakage short" in the circuit, but it's more likely due to "cheap components." Cleaning out any extraneous detritis can be hazardous, since it most likely is in a high-voltage area, and the high voltage often does NOT GO AWAY when the set is unplugged.

Use the warranty, if it's still good (or as good as a WalMart warrenty ever is).

John