The rewind/ffwd heads spin just fine, but when I put a tape in it either spins for a while and then stops before reaching the end or it doesn't at all spin the reel, it just moves on its own. If it works it only does so to one direction. If I change sides, the effect is reversed; if it rewinded only on one side it ffwds in the other and vice versa.

How can this be happening?

Here is the Service Manual. http://freeservicemanuals.info...anual/Philips/D6641/

I hope posting three times in a row isn't against the rules.

Someone suggested lubing up the gear that changes between rew and ffwd. I hope it works.