In article ,
says...

thanks for all your efforts Rayner. I am using a 1:1 isolation
transformer when I power up the monitor and I do have an oscope. Ill
test the PSU although the lines are many more than two.. maybe 7-8
drifting to the bottom of the screen. Thing is that there are brand
new caps everywhere.

I read around the internet and there was this thread and just one more
guy with same monitor, same issue. He told me on eab boards via
private message that his monitor is with an expert at the moment.

it definetely looks like some harmonic but my knowledge in CRTs is
very little at the moment - learning every day.

Okay, if there are 7-8 lines it's not mains frequency. I'd say it's
still worth checking the PSU output voltages for stability, though - it
will help to know whether you can rule that section out or not.

The number of horizontal lines suggests the unwanted oscillation has a
350 - 400Hz component, which might be a helpful clue.

If investigating the PSU doesn't reveal anything, here's how I'd look at
it. You've got a scope and a schematic annotated with expected voltages
and waveforms. The problem shows up on the display, therefore it must be
present among the signals and voltages that go to the CRT and deflection
yoke. If you can figure out which one(s), you can then trace it back to
the origin from there.

It seems highly unlikely to be anything to do with the EHT or the focus
grid voltage, which is good because those are the ones I'd least like to
be messing with.

It doesn't look much like a deflection problem either, as from the
pictures on the edaboard thread I can't see any actual distortion to the
image on screen.

As far as I can see, that mostly just leaves the:

+12V line
+125V line
-20V line
RGB signal lines

going to the CRT neck board as the likely candidates.

The safety guidelines at
https://www.repairfaq.org/sam/monfaq.htm#montrbs include some advice for
staying safe when investigating a running monitor; I particularly
recommend the bit about only ever connecting/disconnecting test leads
when the monitor is unpowered and unplugged.

Let us know if you manage to narrow it down.

Rayner

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