I replaced the belt in my WM-F65 and for some reason it still won't play tapes.  The reels 'attempt' to move but won't turn.  When I listen closely I can hear the motor turning but that's it.  Any ideas?  I've spent a few hours on this now and I'm almost ready to give up.

Sounds like the belt is too big. Are you sure it's the right size? Have you tried playing the unit with the back removed. This may give you a clearer idea of what is causing the problem.

The belt I have is actually 2mm smaller in diameter than specifications and 0.2mm thinner.  That shouldn't cause the problem, should it? 

I tried to play it with the back off but I don't have a power supply and I couldn't get something rigged up with batteries, so I was unable to do that.

The only thing I can think of is that a metal part fell out when I first disassembled it, maybe it was supposed to act as a spacer and now things are binding.  It's the rectangular piece with the two holes (three, counting a screw hole) pictured here:

Unfortunately I also broke two wires and don't know where they reconnect to, so I think this Walkman is done for.