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Locate the stop screws located on the opposite side of the motor. They might be behind a rubber plug inside of the housing.
Once you locate the two screws, use a hex wrench to adjust the screws. Clockwise will raise it, and counterclockwise will lower the stop.
As you adjust the screws, you should see the screen adjust itself in real-time. If the screen does not adjust in real-time, it should be about 1-2 inches per full turn of the screw. Continue to adjust the screws until the screen is where you want it.
Finally, raise and drop the screen a few times with the projector on to be sure the screws are set correctly.
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