I would second the advice to try adjusting the ion trap magnet before doing anything else (especially before trying more rejuvenating procedures, which can kill an otherwise good tube if used indiscriminately).
The ion trap magnet is a thin circular assembly that fits around the neck of the picture tube between the yoke and the tube base. Its placement is critical -- if misadjusted, it can cause the picture to be very dim, or even completely dark.
The idea is to move the ion trap while watching the screen, and adjust it for maximum brightness. It can be slid forward and backward on the neck, as well as rotated around the neck.
In a normally working TV, you'll find an obvious "sweet spot" where the screen looks brightest, and many other positions where it's dim or completely dark.
Some ion trap magnets have a little arrow that should point toward the front of the TV, but many do not. If you have removed it during the course of restoration, it's possible that you reinstalled it backward. If trying the magnet in every possible position doesn't change your picture at all, it does no harm to flip it. Just mark one edge or take a photo to remind yourself how it was installed when you got the TV.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html