Two things can cause the symptoms you've discribed...
A distorted or loose shadowmask or magnetism of the shadowmask.
It would normally take quite a large or firm physical shock to distort the tubes shadowmask and cause those symtoms.
If it has received a such an impact and the shadowmask has become dislodged or bent, you'd need a new cathode ray tube.
Otherwise,..the shadowmask also picks up residual magnetism from the earths magnetic field. Normally the TV demagnetises the tube shadowmask automatically at switch on.
If the bump was small, it's more likely just the degaussing positor has failed and the set is just unable to demagnetise itself. Maybe it's just coincedence you noticed it after bumping it. Replacing the positor is very simple and inexpensive.
The shadowmask, a thin sheet of metal with holes in it, is an integral part of the cathode ray tube fitted just behind the tube face to allow the electrons of each color gun to land on the correct color phosphors.
Naturally, any magnetic items eg. loudspeaker drive units, power tools, fixed magenets should not be placed near the tube of the tv as they will also cause the electrons to be deflected and land on the wrong phospors, resulting in these symptoms.
Cobweb
October 2004
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix televisions.