Odd focus problem in 1938 British set

[dtvmcdonald]

I'm looking for suggestion, or at least guesses. My oldest set has a focus problem. Other than that, it now works quite well and is just plain watchable.

Ita a Marconi 702, essentially the same as the HMV 901.

The set uses a very long 12 inch CRT, with a very complicated gun, and is called a "hexode". Its magnetic deflection and electric focus. Its got a very long neck with a smaller diameter neck just before the "bell" so it needs less deflection power. Its grid grive, interestingly it needs only about 12-16 volts drive, supplied by the detector itself with no video amp.

The scanning works fine for an old set ... a test pattern has an almost round circle and a crosshatch looks very "even" in both directions. Its well centered without any DC on the deflection coils.

But the focus is horrendous. It uses about 5000 HV for the final anode and 900 volts for the focus electrode. When the focus is correct in the center,
the upper left and lower right are somewhat out of focus and the other two corners are worse. I can turn the focus so the upper left and lower right are fine, the center worse, and other two corners getting a bit better, but not
actually focused at the end of travel. The lower left is worse than upper right by a lot.

The CRT is magnetically shielded around most of the bell, and on three sides of the top 3/4 of the neck. There are stray AC magnetic fields that make the picture wobble (its British, so 50 Hz vertical) but I've bucked those out with a 60Hz coil that generates 5 Gauss RMS at the position of the CRT. This reduces the apparent out of focus just a little bit, and more in the lower left than upper right. The CRT is vertical so the earth's field is perpendicular to the CRT and goes from bottom to top.

The focus voltage was checked on a scope is is well filtered. I've ascertained that the CRT shield is magnetized bottom (S) to top (N) to the tune of a few Gauss. I've been able to reduce this quite a bit with a degauss coil, and it made only a tiny difference in focus problem.

Other owners of such sets have not had such a serious focus problem, and all that I've heard of were symmetic in all cour corners.

Any suggestions? Is your guess that the problem is electrical or magnetic?
Has anybody seen something like this in a "conventional" set?

Doug McDonald