Re: 475 With a bowed display. . . .Ideas?


Chuck Harris
 

Hi Michael,

The spot we see is a picture of the active cathode
area, as is collimated and distorted by the grids,
anodes and lenses in the CRT.

Because of slight manufacturing variations that can
make for beam motions that would cause the beam to
completely miss the concentric holes in the shields
and anodes, there is a need for one or two steering
magnets judiciously placed on the neck of the CRT.

If one is missing, your spot could on a worst case
disappear completely. In a less drastic case, it would
chop off part of the circular spot leaving semi circular.

Tektronix used cellophane tape to hold the magnets (if any)
in place on the neck. One steers the beam horizontally,
and one steers it vertically. I have seen the cellophane
tape with empty pockets where the magnet used to go. Usually
the magnet is stuck on the inside of the mumetal shield.

The various elements after the gun that deflect, magnify,
and distort the spherically projected image into a
flat image for display on the screen all affect the
circularity of the spot by making it look like the
wind smeared its shape... they make its shape look more
comet like in appearance.

To be as distorted as you describe (I wish I read that
earlier), you have to have one of your astigmatism,
geometry, or focus elements that is electrically in the
wrong place in the tube. If it is truly like a fish eye
lens, centered on the center of the screen, you probably
have a bad power supply going to one of the pots that
drives the astigmatism, geometry or focus grids.

This is just a guess, but in any case, it is time for you
to verify that all adjustment pots in that area are properly
fed, good, and are somewhere in the center of their range.

Pay particular attention to any circuit complexity that
exists around these grids. Sometimes there are transistorized
distortion circuits that are supposed to warp things into
flat for the screen.

-Chuck Harris

Michael W. Lynch via groups.io wrote:

On Sat, Aug 8, 2020 at 08:22 AM, Chuck Harris wrote:


Typically, when a proper application of the Geometry,
Focus, Astigmatism, Trace Rotation, and other affiliated
adjustments results in a screen that just doesn't look
as pretty as we would like, it means that there has been
an outside physical influence on the CRT (READ: OOPS! BANG!
$%^! & #!!*^! Why Me???)
No signs of major trauma, but this is still a possibility. What confuses me in this regard is the fact the the unfocused spot on the CRT is beautiful and round, one of the best I have ever seen.

The Mumetal shield can get bent, or magnetized.
Not bent, but magnetized? VERY POSSIBLE. I am curious how such magnetization can occur as this issue has come up in other threads as well?

One of the steering magnets (held with Scotch tape) can fall off
of the CRT neck, causing the beam to partially strike one
many internal shield electrodes that looks like a hole in
a plate of metal.
I can look at this when I pull the CRT. Would this not distort the unfocused spot (mentioned above) somehow?

Or, it can be that we are much more nit-picky than tektronix
was originally.
I'm just not that picky. I do not think I am asking too much for the markers to be vertical and consistent across the entire viewing area (or at least within one minor division). If it was just one minor division of deviation, I could live with this, but the outermost vertical markers are about 6 minor divisions "leaned in" at the top of the marker. It is almost like looking through a "fisheye" lens.

If you carefully read the specifications in the manual, you
will often find that we have been spoiled rotten by all of
the CRTs we get that are much better than the specifications
say they could be...
I totally agree. The CRT is one of those items that is a source of constant amazement for me. How they work and withstand decades of use and many times abuse. They are a marvel.

Bottom Line, sounds like I need to pull the CRT and inspect it for obvious damage. I can try another "known good" CRT, as I have one or two that produce very good and well aligned traces in other machines.
if a "Good CRT" shows the same symptoms in this chassis, then I need to look elsewhere.

I was thinking that I had missed some other related adjustment? I guess not?

Thanks for the valuable input.

Join TekScopes@groups.io to automatically receive all group messages.