Particular CRT spot size

[Alex KL-1]

Hello folks of Videokarma,

Some time ago I got a strange old Matsushita RF equipment that has a 9" CRT.
Literally in NIB/NOS condition! No use! Zero dirt inside!
Since I already assembled a TV in the past, I thinking in use the CRT/yoke for making a little B&W TV or monitor with tubes where applicable (not in the H part, due do too low yoke inductance).
I tested it using it's own HV supply, and a spare chassis from a Sony TV-700U for feeding horizontal, vertical and video.
But, at testing, I discovered that the spot size varies too much, completely losing definition with bright image, blooming a lot, and bright and contrast are very poor (reminds a worn CRT). The auxiliary voltages in this flyback don't change (in fact, is stabilized).
In fact, the original equipment uses a variation of vector mode (x-y display), and is scanned only one line at maximum 20Hz (is easy to achieve bright this manner, like in a TV with inoperative vertical).

I don't have a HV proble yet, so I only wonder if this CRT is feed with too low HV, or is a CRT not optimized for TV using?
I had some hope, since some guys use oscilloscope CRT for making electrostatic deflected sets.

So, what is going on?

The CRT code is 23BLB4 (impossible to got info in the Net).