Help Troubleshooting a Chamberlain Garage Door Opener

I have a model 1055-6, half-horesepower, Lift-Master / Chamberlain garage door opener. The device was installed in 1987 but has worked reliably since. It has needed some repairs, a broken lift spring, gears, and a lifting cable, but it is presently all good but that it isn't running right. 

We went to put the door down and dropped about 3/4 of the way down and stopped. Further attempts to raise or lower it only resulted in a humming sound. I poked around the web and quickly found that this issue is commonly associated with a failed capacitor. Before I ordered a capacitor, I removed the case of the unit to examine the gears and make sure the unit wasn't jammed. It wasn't. The gears are fine. I then turned the motor a bit using a pair of pliers on the shaft, ahead of the worm gear. This did not change the issue. I then loosened the chain and removed it from the sprocket. I turned the motor by hand and saw that it rotates OK and the top bearing seems fine. Again, I tried to run the motor (I just use a box end wrench to short the terminals on the back panel to fire the motor.) and this time it ran fine. I ran it back and forth half a dozen times without any further issues. 

Thinking that the motor may run OK without the door attached but having trouble with the door and trolly, I reconnected the chain and tested again. The door worked perfectly other than that the stops were off as I had moved the chain relative to the motor. I re-set the stops and was about to just assume the problem was a fluke and that the opener was OK when it failed again, locking in a half-down position and only humming when I tried to run the motor. 

Since the common repair for a humming motor is replacing the capacitor, which only costs $13, I decided to buy one and try it even though I suspected that the capacitor was not the issue. I got the new capacitor installed this morning and it did not fix the issue. As before, the motor only hums. 

I disconneded the door, but leaving the trolly mechanism attached, I rotated the motor by hand about 90 degrees. Sometimes this will get it to operate. Sometimes not. If it does operate, it goes to a stop position and stops. When you start it again, sometimes it starts but then stalls partway in the run. Further attemps to make it run only result in the humming noise. 

So, the problem does not appear to be the capacitor. I'm suspicious that I might have a bad winding in the motor, but that would not explain the mid way stops. Before I start throwing expensive parts at the problem, how can I test the motor, controller board and power board to further determine where the real issue is?

OK, I've not gotten any answers and a call to Chamberlain informed me that the 1055-6 has not been supported for years. No diagnostic help, no parts. What to do?

In my searching around for better answers I came to realize that garage door openers are not as expensive as I thought. A complete, new, Chamberlain D2101 is only $138. Not much more than a system board in most appliances. I picked one up at Homedepot. I was wondering if the chain and track might interchange with what I have in place, which would simplify installation if I only needed to replace the head unit. After dropping the old head I compared it to the new head - they are near identical as far as the mounting and track go. Swapping out the head was easy. I kept the old chain and track, which are in good condition. I did need to add the optical safety sensors that were not in use when the 1055-6 unit was installed. I also had to change the door bell switch as it was not compatible and threw a code on the up/down arrows of the D2101. Neither change was difficult. 

What was a bit complicated was getting my two vehicles to synch up with Homelink to the new head. The Chamberlain unit now uses Security+ 2.0 radio code and this is not supported in my 2010 Highlander. I will need to add a $36 "bridge", which is a code converter, if I want to use the Homelink feature in that car. Of course, I could just use the remote provided with the system. My other car, a 2013 Acura was difficult to program but eventually I got it to work.

The bottom line is that in over thirty years, the mechanical side of these door openers has not changed much. Many of the mechanical parts would be interchangable and I'll keep the old unit for spares. The electronics, however, are totally different and this includes the radio signal. So be aware that with older vehicles that have early versions of Homelink, you will probably incur the additional cost of the radio bridge.  

Mounting Side of Units