Yesterday, I decided to put the original picture tube back in my 1968 Zenith 20Y1C50 set, and everything went perfectly. Not a single headache, and I had the thing back up and running in an hour, and only half an hour to do purity and convergence. So I get up this morning, and it still works, but when I go to adjust the brightness, the act of simply touching the brightness control kills the luminance. There is very little luminance/brightness. Almost like the potentiometer is stuck at 1/4 of the way up. Again, it was working perfectly when I put it back together yesterday.

Bad control, tin whiskers in control, bad connection to control...Those would be my first 3 checks.

Loss of luma can be caused by dirty service switch or H-K short in CRT. Hope it ain't the latter. Tap on CRT neck to see.

I moved the service switch back and forth and saw no effect, and tapping the CRT neck didn't do anything either.

Also tested the CRT and it tested "adequately usable".

Crisis averted. A wire, probably soldered in by hand, came loose from the brightness control. Set is working again.

So glad it was an easy fix!