I have a classic garage door opener problem: motor moves chain about a foot, then reverses it about a foot and stops. Happens just the same with the door engaged or disengaged.

I've checked the archives which tell me I should probably be looking at the interrupter cup and maybe the rpm sensor.

My question is: how difficult are these parts to access generally on an older Craftsman 1/2 horse chain unit. Are they fairly accessible with the unit in place after removing the cover, or will I have to take it down? Anyone ever done this or familiar with it?

(The problem opener is on my folks' garage in another town. I'll be visiting soon so I'm trying my best to anticipate what I'm up against!)

Thanks as usual for any help!!