Locked motor

I've just received a 2nd hand lego motor (peeron #71427), and to my 
surprise and (... well let's leave it at that) it's jammed.

I remember seeing a website that showed the internals and possibly a way 
to solve this (except for complaining to the person I bought it from). 
Can anyone help me with a link (or other good advice) to see if anything 
can be done.

Tanks in advance

Linmix