On Monday, January 20, 2020 at 4:59:07 AM UTC-5, Cursitor Doom wrote:
On Sun, 19 Jan 2020 22:44:02 -0800, John Robertson
wrote:

If the image grows/shrinks when you change the brightness, then there is
usually a fault with the HV diode, especially in B&W CRTs.

That might be considered to be cheating - actually providing useful
background information...

Since you didn't read the question in the first place I'll excuse
myself on that one. The issue was *not* about the image
shrinking/growing during intensity adjustment!!
--

But it's not uncommon for a low High Voltage circuit to cause what we call "blooming" when the brightness is advanced. If your example was also doing that and you didn't initially notice it, you would say "hey, mine is doing that!!! Thanks!!".

Look, it's still likely a weak CRT (depleted cathode), but it could be many other things as well. I remember 1980s era NEC projection TVs used a DC supply for the filaments, and a lazy bypass electro dropped the filament to about 4 volts. The three tubes looked like they were shot, but it would be unusual for all three to go that weak at once. New cap, filament back to 6..3, TV looked like new.