"When adjusting the " purity magnets " on a conventional CRT, one sets the
test pattern to red and tries to get a even red colour all over the tube
face. "

A note about that. On a delta gun CRT it would be better to adjust the blue and on an inline gun it would be better to adjust the green but red was chosen because the eye can discern it better.

Actually on inline CRTs I learned how to adjust purity on a white field and convergence on snow (no signal). It works. However now it is just more useless knowledge. Now I am getting into the waveforms on the Ysus and Zsus. Tcon, logic board, LVDS. Where's that confounded yoke ?