How can I repair a red 12v train motor from Set# 7730? I have a LEGO 7865 Red 12 volt train motor from a LEGO 7730 train set.  The motor has stopped working as it has been dropped on one of the wheels.

How can I open the motor case to effect a repair?
According to Bricklink,  Set # 7730 has this motor :

bb12va Electric, Train Motor 12V Modern Type I with 2 round contact holes

For disassembly and repair you can look at this large post with pictures on Eurobricks by Andromeda