CRT Display Fault

Gentlemen,

I have a spectrum analyzer with a display problem. Forget the precise
make and model for once; this is a common display issue affecting
*all* CRT displays. Let me describe what I'm seeing...
At first switch-on, there's no picture at all; just an elongated,
fuzzy blob on the screen. Over the course of the next few minutes,
this fuzzy blog gradually expands and reveals the image it should
display. However, this image, even after an hour or more, never gets
beyond about a third of the size it should be. I can make out what
it's *trying* to display; all the information is present there, but
it's compressed and fuzzy (and a bit twitchy as well).
I'm sure I've encountered this issue with CRTs many times over the
past 60 years, but I'll be f***ed if I can remember what causes it.
Any ideas?

CD.