Buhl overhead projector focus adjustments

[Lenny]

I have gotten in a number of Buhl model 80 and 80/14 overheads that
will not focus properly for the entire screen. Either they're good
three quarters of the way down and then the focus starts dropping off
or the image is good three quarters up or its one side good and the
last 10% on the side perhaps is bad, or its one or more corners, etc.
I have been told that any misalignment in the optics will cause this
but looking at these units it is difficult to see where anything has
changed. The lenses don't appear to be warped and all look slightly
concave as they should. It is possible to shift the lightbox from side
to side or tilt it, ( I haven't done this yet) but I'm not certain how
to draw a relationship between what is out of focus and as a result
what to adjust. I have a transparency with symetrical lines of one
letter on it. I'm told that this focus process can be a real pain so I
don't want to make it worse than it already is. Does anyone have any
experience with this? Thanks, Lenny Stein, Barlen Electronics

[jakdedert]

Overheads are notorious for focus problems. Most can be resolved by proper
placement of the unit relative to the screen. They were never meant to be
hi-res. Make sure your screen is perpendicular to the projector and
perfectly flat. Tilting the screen can alleviate 'keystone' issues. The
projector needs to be set exactly in the center of the screen horizontally.
Move it back and forth, or measure the diagonal from each corner of the
screen to the center of the projector lense.

jak