Checking hv

[Yamamaya42]

Well then, it's a step in the right direction.

What this will most likely take is someone who has intimate knowledge of this given set and vertical circuitry and how it may be better modified to the CRT you are trying to use.
Cause as it seems as for the vertical section, it's not quite working as well as the CRT that the chassis was designed to use.

This may be a result of load values, resistive and or capacitive, and is being seen more pronounced in vertical due to the lower frequency than the horizontal.
This is almost on the engineering level, and won't be easy, but I'm sure someone out there knows these things well and can offer some guidance.