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?