Sony GDM-W900 diagonal line problem

I've just bought a second-hand Sony GDM-W900 24" CRT computer monitor and it's great apart from 3 very thin lines (red, green and blue) which run at about a 4 degree angle downwards from right to left then continue horizontally from left to right (but much fainter), in the bottom third of the screen. They are there at all resolutions and I have tried adjusting every setting in the menu to no avail.
I know about the damper wires but I've seen those. These are about the same thickness but at a slight angle and coloured. I found out how to do a monitor self-test (with no leads) and the lines are there. With no input they flicker up and down the screen so it's nothing to do with the input but a monitor fault unfortunately. I've also tried tapping the screen, degaussing etc.
Is there any hope?