Astigmatism and uneven focus

[dtvmcdonald]

I hope this is the right place for this. Its sure an early TV: a 1938 MArconi 702. This is British. Its been semi restored by an earlier owner back
in England. I repaired and aligned it. All non-mica caps are replaced
(it has mica caps in the RF that are not potted or molded ... the mica
is just clamped in steel plates. They are reputed to be quite reliable and that seems to be true. Bad resistors are replaced.

Anyway this works fine and makes a good picture, reasonably bright with no sign of ion burn, but a truly bad turn off burn in the center. Given
serial numbers it is likely the original CRT. Its a 12 inch 405 line 50 Hz set, TRF design but for British Ch. 1 which is in fact essentially at our own
usual IF of 45 MHz. Its also split sound, which is AM and superhet. The
oscillator is remarkably stable (but the audio IF is about 100 kHz wide.)

Except for one problem: focus. The focus control is in range but the focus
uniformity is abysmal and even at the center there is astigmatism easily visible with a dot or crosshatch pattern. Oh yes, the scan linearity is quite good. (Its magnetic scan, electrostatic focus.)

With the focus set so that the spot at the center is as round as it gets (with the astigmatism you can make a dot be a line either vertical or horizontal at different focus settings) the focus gets worse as you go out. The lower right corner is a bit worse than center, and the upper left corner is truly terrible, like a 3/16 inch spot. (That's as viewed in the mirror so the image is right side up.) Changing the focus voltage can make the lower right and upper left get in more or less focus, but then the center and other two corners are worse, and vice versa.

I discussed this on the British forums and got various suggestions.
All the DC CRT voltages seem pretty much as per specs, and I find no
hum on them. So far nobody has suggested a fix that works or even an
idea of what the cause is. except a loose and tilted electrode in the very complicated gun. I should add that the grid drive only needs to be 12 volts, and there is no video amp at all ... the video detector feeds direct
to the grid.

And suggestions or discussion?

[old_tv_nut]

If you rotate the yoke, does the astigmatism axis rotate with it?

Describe the centering controls - combo of yoke movement and electrical controls, or just one of these?

Is there an ion trap on this set?

[dtvmcdonald]

I have not tried rotating the yoke. Its apparently never been touched since new, has rusted, and has not yet
responded to PBBlaster. I'm not about to try rotating it until its nicely freed up. The CRT is the ultimate in
unobtainium. And worse, its rather odd. You see, the neck diameter where the gun is is
quite ordinary, but where the yoke is is a constriction. One of the questions
I'm looking for an answer to has always been ... "has anybody seen this
extreme defocus caused by a yoke defect?." Its a somewhat oddball looking yoke, but then again, this
is a narrow angle deflection set. I've considered disconnecting the yoke and seeing what the undeflected
spot looks like ... but I'm pretty sure it will lie right at where the phosphor is seriously burned. (I can of
course add a bias box to the CRT grid lead and get a very low beam current.) No one in England has seen
the same symptom and remembers .. even people who have seen this very set decades ago have no idea.
There is a picture of it displaying an image decades ago on the web, but its too small to tell.

There's no yoke adjustment except rotation ... longitudinally its constrained by the neck constriction.

Centering is by an optional (and uncommon) "walk-about coil" which is a separate thing mounting just
ahead of the yoke. It is an electromagnet with iron pole pieces that has to be rotated to adjust the
direction. My set actually has one but when I got the set it was turned off because it was unneeded,
so I removed it. The scan design guarantees there is zero DC onthe yoke proper.

There's no ion trap and many of the CRTs show bad ion burns, but not mine, which probably was retired
because of the extreme turn-off burn. The picture is still perfectly watchable at ordinary viewing
distance. Remember that its only 376 visible lines so only in the "bad" corners is the defocus a killer defect.

[peter scott]

Astigmatism is probably the result of gas in the CRT. I think you will find it gets better after more extended use. That was my experience anyway.

As for edge focus I have that problem too and have tried various things to no avail although the "push about coil" centring tends to worsen the edge focus but if you have the rubber screen mask rather than the earlier more rigid one then you can move the CRT withing the mask for centring. I did wonder if the edge focus might be caused by the surrounding steelwork being magnetised but no amount of degaussing changes it and I couldn't sense any magnetism with a compass anyway.

You can see my edge defocus particularly in the corners.



Peter

[old_tv_nut]

Note how the diagonal line patches get narrower in the corners as well as blurry. This looks like a yoke design limitation to me. (But it says nothing about center-screen astigmatism.)

[Pio1980]

It looks pretty good for the state of the art and it's age.

[peter scott]

The astigmatism is such that you can adjust focus to display the line structure clearly or as in the photo above adjust it to resolve vertical lines but not both at once.

Peter

[Penthode]

Originally Posted by old_tv_nut

Note how the diagonal line patches get narrower in the corners as well as blurry. This looks like a yoke design limitation to me. (But it says nothing about center-screen astigmatism.)

I may be wrong but the prewar British tube faces are very bulb like and the compressing corners to me appear to be the rounding back of the tube face. In other words, the corners are closer to the electron gun hence the narrowing of the image.

[old_coot88]

Lookin' at that test pattern, I would never know there were any defects without them being specifically pointed out. Doubtful a casual viewer watching regular content would ever notice anything amiss except the ion burn. I would call it a near miracle for a set of that vintage to display the image that it does. There's an old Colonial saying, "perfectionism is the manic form of fuddy-duddyism."

[old_tv_nut]

This was mentioned again during last night's on-line ETF meeting. It was revealed that this tube uses a wire grid rather than a cylindrical one, apparently to get enough sensitivity to dispense with a video amplifier stage.

Is it possible to post a drawing or picture of the gun structure?
From all that has been mentioned (including the unusual grid structure) I would guess that electrode misalignment is the problem, as has been suggested. It could be that sensitivity to alignment is much greater than usual in this gun design.

[peter scott]

See: http://www.nostalgiatech.co.uk/new_page_8.htm

As an aside, I did find that I could increase the brightness by applying an ion trap magnet to the tube. My guess is that I am deflecting electrons from an area of the cathode that normally wouldn't pass through the hole.

Peter