Over-adjusted Limit switch problem?

I recently purchased an older home with a Liftmaster 1/3 hp operator.  I can find no model number on the operator.  The operator would not close the door without repeatedly pressing the wall-mounted button, so I “adjusted” the DOWN travel limit in an attempt to correct the problem.  Repeated adjustments failed to correct the problem; they actually worsened it.  Having failed to notice the caution to NOT turn the limit screw more than ¾ of a turn, I’m certain I turned it much more.
Eventually the operator refused to operate.  It just hums when the button is pressed.  The hum stops when the button is pressed again.  I’ve disconnected the operator from the door and unplugged it.

Have I destroyed this old operator?  Is there any hope I can resurrect it?
   
Any advice or pointers would be most appreciated.