My Panasonic PV-8661 will load a tape but will not run. It stops immediately after pressing Play, Fast Forward or Rewind. The large gear wheel just rotates back and forth.

There is a possibility of some damage to the mechanism. You will have too look around for plastic parts that are damaged.

Another possibility is the mode switch is playing up. This tells the electronics what position the mechanism is in. Sometimes a spray of Servisol Video 40 on it will cure it. But if it is worn out, then it won't. You can get them, but to fit it in you will need the service manual, as they are very tricky to set up right. For example they are rarely set up in the eject mode!