[Rob]

Chuck,

I worked for many years on Schmidt optical systems setting up and servicing big events projectors for closed circuit boxing, etc. On the larger commercial projectors that utilize this optical system to create pictures up to 15x20 feet and larger there are adjustments to converge the entire relationship between CRT, its depth towrds the main spherical mirror (focus), the centering of the CRT in the main mnirror, and the centering and plane of the corrector plate lens. I don't know how many of these adjustments are available in the RCA as I haven't dug into either of my 9PC41's yet.

If focus is all you are worried about you could get pretty close by shining a powerful flashlight down through the corrector plate to illuminate the CRT face from a bounce off the spherical mirror. You'd want to stretch a couple of black sewing threads in a cross against the CRT face with tape or a rubber band around the bell of the tube. In a darkened room you will see the image of the cross on the screen.

In actual operation the focus setting will change a small amount because the image actually occurs on the inside of the glass faceplate, not the outside. When you are at this point and have optimized the mechanical focus on a working projector you will also see a color shift as you rock through best electrical focus from yellowish image to bluish as you play with the focus control. This is caused by varying the focus plane of the scanned raster so it falls on the backside of the phosphor, in the middle of the thickness of the phosphor or deeper in towards the faceplate of the tube. The opacity of the phosphor causes this yellowing. I find the best picture when the e-beam is focussed towards the blue.

There is a procedure for correctly setting the geometry of the corrector plate lens with the CRT removed. If the corrector is adjustable laterally (side to side) this needs to be done. With one eye about a foot from the front of the optical barrel looking into and coaxially centered on the CRT hole in the corrector plate you will see some circle images which represent the hole in the corrector plate and its reflection in the spherical mirror. One of these is a virtual image and it takes a bit of staring sometimes to see it properly. By moving the corrector plate lens laterally you should see this virtual circle shift in the opposite direction you moved the corrector lens. You want to lock the corrector plate in place so that its virtual image is precisely concentric with the hole in the corrector plate. Inotherwords all these circle images will be aligned perfectly with each other.

Some Scmidt assemblies move the primary spherical mirror towards or away from the CRT face for mechanical focus. Adjustment procedure for the corrector lens will be the same.

Rob