Well, that's right across the full-wave bridge that powers the board, so it's just a filter cap. Interesting that the original board doesn't have that cap. I'd stick it on and see if it helps.
FWIW, I just connected the one I have to a transformer and a motor. It starts up in less than a second and reverses properly through the standard F-N-R-N sequence. It has no added capacitor.
