I would say, for $100, a commercial door controller with life-saving anti-entrapment functions should be replaced with a new one - not one with internet forum suggestionsLiftmaster CPS-EI
A photo of the back of the board would be useful. None of the components on that are complicated or expensive, so repair should be possible.The faulty CPS-EI unit causes door to reverse and go all the way up. With the unit disconnected, all the functions of the door work perfectly except for there is no sensor reversing function. I purchased and installed new interface and everything works perfectly. Being that this is the 2nd time around for replacing this in a few years, it would be nice to know what is actually causing these to fail, and maybe try to address the issue.
Unfortunately I do not have an oscilloscope.I think Driver300 is thinking along the right lines. I suspect the IR beam unit sends a string of pulses to the board when the beam is NOT broken. (This is the way my Chamberlain door works.) As the IC is a dual retriggerable monostabe I think it is configured as a missing pulse detector. If you trace out the schematic of the board we will probably be able to suggest how to trace the fault on the board. It is just possible that the IR beam unit is sending out pulses at a frequency that is just on the threshold of the missing pulse detector. (Slightly too low a frequency.) Do you have an oscilloscope to check the pulses though the board ?
Disregard the left-over soldier globs from removing the screw terminal connectors in order to access the board.A photo of the back of the board would be useful. None of the components on that are complicated or expensive, so repair should be possible.
If the fault is time-related it could be the capacitor, but that is just based on the fact that electrolytic capacitors degrade with age.