If one or more of the colors is displayed in a bow-tie rather than a full-screen rectangle, the STK392-nnn convergence amplifiers have probably gone bad (nnn is a three-digit type variant number). This is a common failure in CRT-type rear-projection TVs. There are two of these amplifier modules mounted on a metal plate or finned radiator. I recommend replacing both (one of the colors is shared between the two modules, or each color uses one amplifier from each module so both amplifiers could be damaged).
The STK392 has three power amplifiers inside. Each color needs two amplifiers for its convergence correction coils, so two modules are used. There are at least two different connection configurations among the different variants. Some variant numbers are merely higher-power versions of others, so they can be substituted (never sub a lower-power version for what is in your set - it will die rapidly and possibly with destructive fireworks). If you don't understand the datasheet for the part, don't try to substitute a different variant unless the dealer guarantees it is a suitable replacement.
The photos below show two typical package types close to actual size (the pin-to-pin spacing is 0.100 inches). It is best to get name-brand parts (e. g., Sanyo, Hitachi or Toshiba);
some of the Chinese off-brand replacements don't last very long. When I had a string of these jobs several years ago, I paid about $23-$35 each, but I saw some overpriced as high as $75 and some off-brand or no-name units as low as $12.
(Photo from MCM Electronics
(photo from electronic-spare-parts.com)
Often when the amplifier fails, some of the resistors connected to it are burned out. If all three colors are in the bow-tie shape, look for a small green fuse near the amplifiers and check whether is also needs replacing (don't change a blown fuse without also replacing the amplifiers or you'll waste a fuse).
It is essential to clean off the old heat-transfer grease from the heat sink surface and replace it. Apply a very thin, even layer of new heat-transfer grease to the metal plate on the back of the new amplifier so the entire surface is covered. Do not use too much; this will actually reduce the effectiveness of the thermal transfer. You want just enough to wet the contact between the module and its heat sink so any microscopic irregularities in the surfaces are filled in. Replacement should be performed by someone reasonably skilled at replacing soldered parts on a printed circuit board.
If nobody tried to adjust the convergence (see next paragraph) before the failed amplifiers were changed, very little if any adjustment should be needed unless there were burned-out resistors. If you still have a major misalignment, check the resistors around the module again to see if you missed a bad one.
STK392 failure is the most common electrical cause of mis-convergence. The most common mechanical cause is a shift in the optics due to a mechanical shock or twisting force on the TV. A stray magnetic field could also affect convergence. Minor
convergence problems can be corrected by using the remote control to go into Service mode, selecting the Convergence function to bring up a pattern of dots on a square grid, and tweaking the offending colors back into place a little bit at a time all over the screen (there is some interaction; moving the convergence pattern dots in one place causes them to shift elsewhere). Be careful to keep the dots square in the grid or the picture proportions will be distorted, and be patient - it can take a while to get it right. Complete instructions are available in a downloadable service manual located at a number of places on the Internet (sorry, it's probably not free, but it's still a lot cheaper than calling in a tech who may have little more experience at RPTV convergence than you do).