[dtvmcdonald]

I've tried grounding the collector ring with a screwdriver. I've also
tried putting + or - 9 volts on the unused pins which attach to
the Cesium dispensers, and they don't do much if there is no
hum on them.

I've tried using a magnet to move the electron beam over to the
side where it hits beyond the edge of the silver dot area.
This generates a very strong signal which lasts a lot longer.
Its currently cooling down so I'll try that again.


It does seem thermal, but why cooling down much faster than
warming up?

Edit: a new try shows that the actual picture lasts much longer,
at any setting of the bias, at the far left of the screen.