[Username1]

Turning the color up high is not the way to do purity. Any noise will energize the other guns to want to fire, like color noise, bad idea.

You disable any signal, like setting the tuner between stations so the screen is white, no snow. turn color off. (The 2 drives set at mid-point if after doing gray scale once, you find the tube does not track well through brightness range using the untouched drive settings.) Then turn the screens down except for the one you are adjusting, adjust brightness control, turn contrast down all the way. Make your adjustment at a mid to lower brightness, then turn that one off, do the next color. When all purity is done, set the gray scale, then adjust the drives if needed as you check the gray scale as you change brightness.

This is the way I was taught in tv repair school, purity is done with no signal applied. Screen is white, no snow, tuner is between channels, brightness turned down to cutoff, and each gun turned down to cutoff, no retrace lines on any color should appear while making gray scale adjustments. The brightness range should have cutoff, but no retrace on full brightness.