Garage door opener won't run even with door off

I've got garage door opener trouble. I have a Craftsman 1/2hp. The symptoms are:
--will only open 6 inches
--on closing, it will hit near the bottom (but not actual touch the ground) and go back up
--The problem gradually got worse until we gave up, unplugged it and opened in manually for a while before I got around to fixing it.

Here's what I've tried:
--I noticed there was only ~0.8V across the sensor terminals and the lights on the sensors weren't on so I replaced them. Right now I have them configured in "always work mode" (ie, taped together on top of the opener) until I get the rest of the problem solved. The lights stay on, they seem good.
--I've found dozens of forum questions just like this one and everyone can always solve it by replacing the springs, adjusting the tracks, or adjusting the open force. These things all point to a problem with the door, so I disconnected the door and tried running the opener with no load. Same problem.

This makes me think there is some sort of controls problem? Anyone have any advice?

Thanks!

Garage door opener won't run even with door off