Canned air, your troubleshooting friend

After being on for about an hour, the intensity dropped to zero again.  After that I’d fire it up, and it would come on but quickly dim, and the intensity control wouldn’t effect the intensity, but move the beam slightly.

I traced the problem to the -15v supply, and finally the capacitor C143 (which I’d already replaced!).  I zeroed in on the problem by cooling areas using canned air with the bottle tilted to get a blast of frost.   The -15v supply would dip down to about 10v shortly after power up, but would rocket back up to -15v when I hit it with the frost, and I’d see the beam change.  A handy trick! With a fresh 10µF in it’s place, and it seems to be working for now.  I did need to hit the CRT bias trimpot to get the beam to extinguish with the intensity control fully counterclockwise.