[Ron Ramirez]

Have you given your 91 a good alignment per factory specifications?

The older shadowmeters are basically not adjustable; you can only bend the lamp bracket in back to center the shadow on the screen. Usually when the shadowmeter does not respond well to changes in signal strength, it is an alignment issue. Occasionally it is caused by a sticking vane. Some of the older shadowmeters do have a (very tiny) screw adjustment inside the shadow box to loosen/tighten the vane.

Be extremely careful if you attempt this. It would be very easy to screw the shadowmeter up while you have it open, especially if you get overzealous with the screw adjustment. Also, be aware the metal tabs on the bottom may very well break off; the thin metal is very old, very thin, and very fragile.