On Nov 10, 1:31*am, "Phil Allison" wrote:
"Geoff"

I took the case off and no difference, your implied suggestion
that a change in magnetic field caused this matches what I
observe, ie no electronic settings change the waveform, it looks
like when someone shoves a magnet near a crt. I have not removed
the tube screen but I have shaken it and nothing like a magnet
bouncing around can be heard,

** The CRT is a **mesh PDA* type *- *right ?

There must be a chance the steel mesh has become magnetised.

Suggest you play around with a small magnet near the tube face and see what
happens.

FYI:

There is a fine steel mesh just behind the face of the tube that has final
acceleration voltage applied to it - *if you de-focus the beam you can
usually see the pattern of the mesh projected onto the phosphor.

... *Phil

Use a degaussing coil like you use on a color CRT TV set!!