Hmm, I still can't get it to focus right
Removing the yoke and focus coil, I found the ion trap had the following effects:
1) Rotating it moved the spot left and right.
2) Sliding it forward moved it up and down.
Not exactly, but on a slight angle. The spot was bounded by a circle - the cutoff happens when you go outside it.
Without the focus coil, it's a round spot, very large even when dim.
With the coil, it's not a spot but an ovalish line.
I can't tell if the focus coil isn't strong enough, or isn't far forward enough, or what. The focus control has minimal effect on the spot, though the coil's resistance checks out.
ugh