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!!
--

No deal? No problem! :-D