Liftmaster 5580 opens halfway and get stuck

Hi,
 
Couple days ago my one of my tension springs broke in half, I've contacted my dealer and he managed to replace the spring with no issues, after that when we tried to operate the motor the door goes up halfway only and gets stuck, vice versa the same issue when closing as well, but when I push the door up with my hands while its opening it will open all the way (it feels like the motor don't have enough power to pull up the garage by itself),  the technician didn't take time to inspect the motor, straight away he jumped to conclusion that the motor is damaged and needs to be replaced with a new one as they don't have the parts for the one I'm using.
 
My question is, from your experience based on what I have explained what do you think the issue is?  How do I figure out which part of the motor needs to be replaced? after all I don't trust my dealer's technician.
 
Appreciate your time and response.

When you pull the emergency release on the opener and lift the door manually does it go up fairly easily or does it take some effort to lift the door? You should be able to the lift the door 1/2 way and let go of the door and it should stay there in the 1/2 way position.