Pentrantic CH-250 29 no output (appears to have no HT)

[Yeti]

I have just brought home a PGA Tour Golf machine that I was given, and it has a Pentrantic CH250 29 monitor in it (28 inch tube).

It doesn't work - the screen stays blank, no static builds up on the screen and although there is a noise from the back of it when you power it up (like the LOPT), there's nothing on the LOPT wire on the tube when I attempt to discharge it.

If it makes any difference, the HT type noise it does make goes away when I plug the VGA input into a PC.

I'm told the LOPT is very reliable on these - so can anyone here offer any guidance please? Thanks.