Don't dig into repairing the motor. Just replace the whole thing. Far less trouble.

The motor is shot and your opener system (the whole door, tracks, drive, etc.) is likely set up wrong or has worn to the point of complete disfunction.

A garage opener doesn't shear off the drive shaft and sprocket unless the opener isn't centered on the load or the wheels/tracks are binding up. Most garage door openers just stop working like your motor currently is when they wear out.

You should reassess the entire system and just replace the opener.

New openers are cheap, quieter, and won't waste hours of your time trying to troubleshoot. Unless you just want to tear into it for fun, I wouldn't consider trying to fix it.

What you need to do is figure out what caused it to fail in the first place. These things do wear out with time, and if it's been a long time perhaps there's no issue, but usually there's a cause.

When was the last time the spring was replaced? They wear out with time, putting the door out of balance and making more work for the motor. Tightening the spring (a very dangerous operation if you don't know what you're doing) is not a real fix. It can lessen the most severe strain on the motor, but it will not balance the door. It will just cause too much return force when the door is raised. Also check that the tracks are straight and parallel, clean and lubricated.

Basically, you don't want to put a new motor on only to have it break in a year or two. Make sure the door is in good shape. You should be able to disconnect the motor / drive chain and move the door with your arms. If it's very hard to open or close, that's a problem that needs to be addressed.

Thanks for your thoughts. The opener installation is about 18 years old, so I expect it is time for a new motor head. The spring is that age, too. The opener had recently stopped responding to any remote control, too (tried to re-program to two remotes).

The door is not particularly "heavy", so I think the balance is good. IE, it opens and closes by hand with reasonable effort.

All signs say new motor head.

Thanks for responding!

If your door is properly balanced you should be able to bring it up to shoulder height, let go, and the door should not drop or raise on it own.

If you have extension springs and need to replace them note the color on the loop, that indicates the weight it is rated for.

Thanks, will double check that way.

Don't trust the color. They can mean nothing. Colors can be used for right or left wound, inventory, or even installation purposes. Source: me, in the door business since the 90s.

Replace it, have the springs checked. Don't mess with the springs yourself and don't mess with a worn out opener.