I have an older Craftsman opener. The chain broke and jammed. I took it apart and replaced the chain. I reassembled everything but now when I try to activate the opener it just hums briefly, twice, and does nothing more. I have taken the chain off the sprocket and it still does the same thing. The sprocket turns about 1/8 or 22 degrees of a turn, stops, goes again and stops, and that's it ! I have tried adjusting the up & down power but .... I am stumped. Any help would be appreciated.