[Test-Equipment] HP1740A Beam problem

[Darrell - W4CX]

Wondering if there's any 1740A expertise out there.  At first I thought I
had a blanking problem because the retrace was visable, but then the problem
worsened when the beam intensity became so high as to probably be damaging
to the phosphor.  Intensity control has a minimum effect, focus works.
First thinking it was a gating problem, I substituted the A3 PCB with one
from another unit I have...no change.  All DCvoltages are within spec.

Thoughts?
Darrell W4CX