Yes, the CRT is physically broken. It's
interesting that the M20 will
drive this monitor; do you have the pinouts so I can make an adapter?
I'll try to get more than one 6300 monitor. Of course I'd love to have
the M20 all-original as well, but those monitors (as you know) are
pretty scarce so I don't have much hope for that.
And why not change just the CRT? You know how easy it is!