Ken G. wrote:
I have an old asteroids full size vintage video game .
I has a black & white monitor .

When it first comes on its fine . Within 10 minutes the whole image gets
bigger , things disappear off the edges . The center of the screen gets
a bit darker .
I suspected the hi voltage diode off the flyback and adjusting the hi
voltage pot but replacing that did not help .

I dont have any model #s the monitor has one medum circuit board and
one small board with the flyback in a metal box .

Thanks

Sounds to me, more like the Filament Voltage is dropping, cold solder
joint in filament circuit ?

This describes what happens to the image when I reduce the filament
Voltage on my Transmission Electron Microscope !

Yukio YANO