Ken, I'm having a similar issue, and please pardon my ignorance... but you talk about adjusting the "rings" and adjusting the "yoke" and I thought they were the same thing...
I have a Wells-Gardner D9100, which is actually an SVGA monitor on an open chassis. When I first got it at my office it was razor-sharp, but after I got it home I had convergence issues on the right side, more in the upper right, so I figured I must have jarred it as I moved it or something. But I'm afraid to toch anything in there for fear of getting zapped and dying, or worse, knocking the picture even farther out of adjustment...