Proper care of an overhead projector will keep it functioning.

http://www.mech.port.ac.uk/av/OHP.htm This website also has useful information to enhance your teaching environment and to explain how visual-aids affect learning. I have used their information to improve your satisfaction with the function of your overhead projector (OHP).
We have Standard Power overhead projectors with the Economy Lamp setting. We do not have twin lamp function OHPs.
OHP: Troubleshooting
The following are problems which can be identified and resolved by the Presenter, during a Presentation:
General Practice
Anything on the outside of the OHP belongs to the Presenter. This includes all controls, switches or adjustments.
Anything on the inside of the OHP (this means anything ‘inside’ the projector's casing) belongs to the ‘Audio-Visual Technician’. Send a note to the library and I will come as soon as it is ‘possible’ to look at your machine, or you can wheel the OHP to me on its cart so that I may more quickly serve you.
Lamp and Fan do not Work
Check Main Power.
Check that the plug is in the electrical socket in the classroom wall because often the cords on carts that have electric cords attached do not function. Also, last year some wall plugs did not function, so please test a lamp or radio which you know works in that wall socket to see if the socket is ‘live’. If you have a ‘dead’ wall socket, inform the lady in the front office to send a work order to maintenance to repair the wall socket.
Press the OHP's Stage.
If the OHP has been lifted by its stage, the stage often becomes loose fitting. Even the slightest nudge will lift the stage off the micro-switch and cut off the power to the projector. Pressing down on the edges of the stage (not the glass, - it might break!) should restore the power to the OHP. Please note that on some types of OHP, the fan may still operate during this condition.
If the fan does not operate, turn off the projector and notify me and I will issue another OHP to you and send that one in for repair. Without the fan, the very hot bulb will expire and this could result in an exploded light bulb.
Lamp Blown
Change the lamp: The lamp is red hot if you had it on for even a moment. It must thoroughly cool. It is best if I change the bulb for you. These OHPs can severely burn a person and the bulbs can explode and send glass shards at velocity.Caution: Isolate the OHP from the Main power before opening/removing the stage. Do not count on the on / off switch. (I learned that lesson the hard way.)
Caution: The projection lamps are the Halogen type: if touched with bare hands - they tend to blow. Also, nothing used to handle the bulb can stay in the machine. When I tested the OHPs before issuing them to teachers, I discovered several OHPs had the instruction liner from the bulb packaging left inside them and this could easily have started a fire. Also there were bits of paper, sunflower seed shells, bits of broken pencil and small pieces from board games inside the machines which could easily have burned. Students must not be positioned so close to the machine that they are able to stuff things into the casing.
Damage Limitation
Overhead Projectors are designed to be switched on and off repeatedly during a presentation. Leaving the OHP switched on constantly throughout a presentation can cause it to overheat and blow the lamp prematurely. It can also burn out the Lamp Contacts.
Focusing
To minimize focus loss, focus on the center of the image. If the small plastic reflector under the bulb has come loose or if the lens shield has not been closed, you will not have a good image being projected.

Pre-Presentation Checks = make certain the OHP is operating correctly in advance of when you need to use the machine. I need to have time to service the machine for you.