General color tube convergence question .. what gives?

[leonk]

This might not be the right forum, but this sure is the right place to ask this question, due to the caliber of individuals that frequent this forum. So here goes:

One of my hobbies is arcade video games. And I've had a chance to work on a few arcade machines, restoring them, recapping the chassis, etc. Most of the tubes I work on are designed for VGA input (mostly 15khz signal, some go up to 25khz). But it seems that no matter what vintage of tube I work on (late 80's to late 90's, 19" all the way up to 29") they all have the same problem. Terrible convergence!

When connected to a game PCB or PC, I'm able to put up a grid pattern to check convergence. In the center, the image always looks perfect (nice straight white lines both horizontally and vertically). Yet when you look at the pattern at the edges, 99% of the time, convergence is off (blue/red always a bit off .. and you end up seeing not a solid white line, but rather 3 thin different color lines).

What gives? Why are CRT arcade monitors always showing convergence issues? Is this common also for CRT tube TV's, except one never notices this because we don't look at grids on the screen? Is the quality of CRT's shipped to arcade monitor suppliers different than TV suppliers? (My 29" arcade monitor has a CRT tube made by Toshiba .. my 25" arcade monitor has a CRT made by Zenith)