Thank you all for your suggestions.
I called the manufacturer of the opener (a Chamberlain Liftmaster
2220) for their input. While the technician was on the phone, I
disconnected the door from the trolley and rolled it up and down. The
door rolls smoothly, then gets difficult to roll, then rolls smoothly
again at various points in the opening process. It gets difficult to
roll when each door segment reaches a 45-degree angle during the
transition from horizontal to vertical.
The tech's acid test was asking whether a ten-year old boy could open
the garage manually. Absolutely not. His opinion was that it is not
the unit that is the problem. BTW, the unit's force is already set to
its highest level to make the door close, something the tech said
would wear the opener out in no time.
Sooo, if the springs have not actually broken, does it sound like
something that a spring *adjustment* would correct? If the springs do
need to be replaced, does $400 for springs for a metal roll-top door