Common problem especially if the scope has been stored upright is small
flakes of cathode material shorting out from cathode to grid. If the
control grid is at the same voltage as the cathode, that's a likely
cause. An ohmmeter may not show this problem as it may not short out
until there's a bit of applied voltage or heat.

These can almost always be fixed by applying a charged 400 volt, 10uF
capacitor across cathode to grid (with the scope power OFF!, CRT socket
unplugged!).

And put a note on the front panel: "DON'T put this scope on it's
back!"