Problem with Sears 1/2 HP garage door opener going down

Hi,
I've got a Sears 1/2 HP garage door opener (model 139.53535SRT)that I've installed back in March 1996. The double car wooden garage door by itself (i.e. without garage door opener)works. Also, the garage door opener when opening the garage door works. The problem is that the trolley itself (without the garage door attached) goes down only goes a few inches, then backs up and the white lights flash. There are no obstacles in the way at the base of the garage door. The little green light on the side panel also blinks 4 times although I can't find what that means in neither the manual nor the inside cover. I've lubricated the tracks for the garage door, the hinges, as well as the chain. I also greased the T-rail as well as the gears (which look fine) inside the unit and it still won't close. I've tried increasing the down force to where it is nearly at maximum setting. I've also checked the safety reversing sensor and both sides have their green lights on to indicate that a connection from the beam is good. Can anyone help with what the problem may be? Thanks in advance. Jay
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Jay wrote:

If you pull the safety release on the trolley, will it go up and down with out the trolley connected? Perhaps you have a bad gear inside the unit that binds in one direction. Make sure the chain tension is ok and not too loose or too tight. And that the chain is not twisted where it connects to the trolley.
Most of the problems i have had with my sears is when one of the tension springs break on the door.
Bob
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snipped-for-privacy@aol.com (Jay) wrote in message

If the opener moves exactly 4 inches then reverses it is most likely the revolution counter board has gone bad or the revolution counter "cup" as come unseated from the motor shaft (although it is possible the main logic board is bad). The revolution counter board is the small board that's beside the motor shaft. The cup w/ notches goes on the end of the motor shaft & the logic board reads the timing pulses that the counter board sends thru the notches in the cup. So when the door hits something the timing of the pulses change & thus the opener reverses. Therefore if they are not working properly the opener acts as if the door has hit something.
If it is moving a different amount then 4" inches the suspect would be the bearing where the chain sprocket shaft comes thru the motor head. Once the bushing gets worn enough the shaft will bind in the bearing and cause the opener to reverse. The bushing is an assembly that includes the bushing, the mounting plate, the chain sprocket & shaft, & the plastic worm gear. Remove the small limit sprocket on the end of the shaft & the three screws that hold the plate to the motor head. Remove the chain (make not of the chain & door position & re-install in the same position) & lift the whole assembly out thru the top of the motor head (may have to lower unit from ceiling for clearance).
From your description my 1st guess would be the revolution counter board system.
Doordoc www.DoorsAndOpeners.com
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