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