Since I reported problems with my NAD MR-20 several weeks ago, the "patient" has pretty much stabilized.
A readjustment of the Screen and Focus controls restored the picture to its original quality. The black-level flickering and image jerking have almost completely stopped. I have not replaced any of the caps on the driver board, which might effect further improvement. Or not.
The only significant problem is that the CRT now takes five to 10 minutes to completely warm up. Once warmed up, color balance and scale are correct. There is no difference in the output among the three guns.
I'm trying to decide what to do next. I don't have access to a rejuvenator, and am wondering whether it would be worth the trouble, as I assume it will only push the CRT to an earlier failure.
Regardless, I'd like to sell this set to someone who collects classic TVs. (I won't give it away, because I want a "serious" collector to have it.) If I can't sell it to someone in this group or rec.antiques.radio+phono, I'll likely give it a shot on eBay.
Thanks in advance.