I have a Sears model 139.53615SR GDO. Quit working while I was on business trip (?). Anyway, I noticed the worm/drive gear had worn considerably. Replaced it. When I hit the button, it only travels three feet down and then back. I've adjusted the force knobs all the way around with no reaction. Motor, gears, etc appear to be OK. Any ideas before I purchase another one? HELP!