First thing I'd do is disconnect the motor leads from the board (make sure you make a note or take photo to show where they go!) and then see if the fuse still blows - it will help determine if the board is bad or the motor is bad. Unfortunately, it won't tell you much if both are bad, but it will give you a starting point. Not quite sure what you mean when you say that the spindle moves forward or rearward when spun by hand - does it turn freely? You can disconnect the gears by setting the speed selector between the two speeds - when you do that do you feel less resistance when you turn the spindle by hand? If so, put it in high gear and short the motor leads together (unplug the power cord for this) while turning the spindle by hand - you should feel more resistance to turning the spindle when the leads are shorted together than when they are are not shorted together.