Re: 475 With a bowed display. . . .Ideas?
Hi Michael,toggle quoted messageShow quoted text
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.
Michael W. Lynch via groups.io wrote:
On Sat, Aug 8, 2020 at 08:22 AM, Chuck Harris wrote: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.