I have an older Craftsman model 139.18415 door opener, that worked fine until recently. Found that the main gear was worn, which I successfully replaced (got one very cheaply and quickly from Ebay). But after reinstalling, I could not get the door to open or close properly. Messed around with the up/down limit controls as well as the spindle that runs in between the two - just made matters worse. Now the trolley jams against the front with the door open while the worm gear continues to turn and slip/grind. How do I reset the contraption and get it working again?