A very nice set, we sold JVC for about 8 yrs after all but
dropping Zenith. The only bad thing was the power supply,
way to complex & lots of SMD's.
First be sure its not near something magnetic.
Next check for PC cons in DGS, DGS relay if it has & be sure
the thermistor isnt broken.
Next give a real heavy manual DGSing.
If no joy get some small correction magnets off a junker
or even refrigerator door magnets. In TV's they are usually
small discs taped down or V shapes like on Thompsons.
Put up a pure single blank raster ( green usually used on in lines).
Place on bell near "blotch" move around AND rotate. Unless the
shadow mask has loosened that should do it.
BTW look in the set & see if one fell off ! You often find them
or yoke wedges laying on the chassis.
73 Zeno
GBA ! LFOD !