Astigmatism and uneven focus

[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.