Starrett HB350 optical comparator

I just got this Starrett HB350, serial 1108. It is basically working but the stage needs some TLC and probably the optics need cleaning. It makes a sharp image and all three readouts work. Does anyone know when these were made? Were they a Starrett design or a rebranding of someone else's design? I have the 20x lens but would like to also have a 10x. Do I have to get a Starrett lens or could I adapt one from another 14" comparator? I have asked Starrett if they have a manual and lenses.

I have this comparator cleaned up now and mostly working. Starrett kindly sent a copy of the manual. The problem I have now is that the screen won't rotate when you turn the knob that is supposed to make it go around. All of the rollers are turning freely but it seems there is not enough friction between the drive wheel and the perimeter of the screen. The "tire" on the drive wheel was a pair of old torn o-rings, so I replaced them with new o-rings of about the same size. Maybe the tire is supposed to be something else with more friction, like a piece of latex tubing?

A couple of ideas that may help (or may not!). Many of the Starrett optical comparators have one of the rollers on a cam. You can adjust the tension against the screen by slightly rotating the screw in the center of the roller. This model may not have this feature, I'm not sure.

A mistake I have made in the past was replacing the o-rings with viton o-rings. Seemed like a good idea due to the increased chemical resistance of viton in shop-floor environments, but the viton proved to be too firm and slick and never got a good "bite" on the screen perimeter.

Most Starrett models originally used 3 o-rings, and the manufacturer's part number is 5K18 in case you want to try the ones recommended by Starrett. Otherwise, I believe the equivalent generic size is AS568A-017.

Hope this helps!

Thanks, all of that is very helpful. Mine only had scraps of o-rings when I got it, so I could not see what was supposed to be there. I will try 3 of the size you suggest and make sure to use buna rather than viton. I did notice the cam and have played with it. The upper roller turns freely when unloaded but once I apply pressure with the cam it seems to bind. I need to look at it more closely. Maybe there is a washer missing.

Using 3 o-rings instead of 2 did the trick. Maybe making sure the o-rings and the screen perimeter are free of any oils is also important.