"Jerry G." wrote:

It is possible that the G2 bias to the tube is a little out of spec due
to the wearing out of the high voltage multiplier, or some components
biasing the CRT have changed specs over time, or the CRT itself is
going weak. Proper verification would have to be done to determine the
exact fault. The most common cause is the CRT becoming weak with age.

It won't be an aged CRT. This monitor was new when I bought it and has had quite
light use.

Any advice as to who to entrust it to, other than Sony ?

Graham