(Message repeated because I posted it in the wrong place yesterday)

Can someone help me with a problem I have on my Sears Model 139.53664SRT1 door opener.

The first symptom was the door opened about 4 feet and then stopping. No sound of a stalled motor. Since then, if I try to move the door with the hard-wired control or the remotes, all I hear is a light motor hum for 1 sec, silence for 1 sec, and then a louder hum for 1 sec. The same noises occur after disconnecting the door from the drive trolley. I have the casing off the motor unit, and when the humming occurs, there is no sign of the motor trying to move even with no door load on it.

The door hinges and rollers are regularly oiled, and the door moves freely by hand, and will stay put in all the mid-positions when disconnected from the drive trolley.

The opener is 10 years old, but I replaced the nylon gear train two years ago, and they still seem to be in excellent condition. The rubber belt looks good too.

I have tried increasing the up drive force. The only change I notice is that each hum seems to be longer-about 1 1/2 or 2 secs rather than 1 sec.

Other than this change, which is now back in the original setting, there have been no changes to the travel or load settings. The trolley is still in the location where the door first stopped.

There is an SRT button on the drive unit. The indicator light beside it flashes 5 times ( about 1 per second), then stops for about 4 seconds, and then repeats the pattern.

Can anyone help with a diagnosis and a solution, please?