The first step is to check the power supply. If it's doing something wrong then lots of stuff can be affected.
Lots of high voltages, and my meter read up to 1000VAC and 500VDC :-(
Not to worry, not everything needs to be measured with respect to ground. But placing the meter's ground lead into the -1000V rail does make one somewhat cautious...
Here's the results:
The unit is set for 250VAC and my Variac is set for 250VAC. The AC voltages in the secondary read a little low but not enough for me to worry about.
All the rails I can measure look pretty good. I measured the voltages across C409, C411, and C412 individually. With the meter's ground lead at point 41 I could measure C409, then I moved the ground lead to between C411 & C412 to measure them. Unfortunately (but maybe fortuitously) I left the ground lead in this position while measuring the grid and focus voltages.
The first odd thing I discovered was that the voltage at point 52 (grid voltage) did not vary. That's not too surprising because the "brilliance" control is totally ineffective. This was also the point I noted the ground lead was still between the two capacitors. A quick calculation showed the voltage at point 52 was really close to the -1000V rail, far closer than I would expect (and
that was odd).
On moving the ground lead to the -1000V rail, I discovered that the voltage across D301 was 0V! I also noted that there was a lot more muck around this pot than the others. (Perhaps this was a WD60 amount!!! :-D). It seems that someone had tried LOTS of cleaning of this pot when the problem looks to be a shorted zener diode (or maybe a shorted cap -- less likely).
I'm pretty sure I have some high voltage zeners, but the placement of this component is going to make replacement really hard. Perhaps this goes on top of the board like the caps too.
And I just happen to have some 100V 3W zeners. The original is a 330mW. I may be about to add some safety margin
Oh, and a good thing about this scope is that the caps discharge by themselves in about a minute.
Another thing of happened while I was checking voltages. At one point the trace suddenly shrunk to about half the width of the display. I may have a dry joint to find, it fixed itself up again a few minutes later.