CTC-5 21AXP22 convergence

[old_tv_nut]

I went back to fiddle with the purity today, because I noticed the lower center reds have some green impurity, enough to mess up skin tones. Went back to ground zero with edge magnets retracted, degaussed the tube, but I cannot get rid of it. Just above center is reasonably pure, but just below center is greenish.

I also noticed when adjusting the purity rings that when the purity towards the top edge goes out, it shows up in finger-width "waves" of impurity. This higher order effect is surprising and must be due to detailed yoke winding structure, I guess.

I'm also considering whether fiddling with some small added magnets (strictly ad hoc, have never seen it recommended) might help.

Question for those who have done more 21AX setups: When you degauss the tube, do you do it entirely from the front, or do you also run the coil into the back of the set and around the CRT bell?