Sharp SG-500H phono problem

Hello:
My phono harm is automatically only going up and down, in the rest position, times and times again. Nothing more. If I force it moving it until the end of a record, he will light the cut led and comes back to start position, then tuns the play led on, goes to the beginning of the record, starts playing for one second and goes back again to the rest position, starting the up and down dance again. Also thee is no sound getting of but if I connect the headphones it is playing good. Any ideia of what can it be wrong with him. Thank you so much.