At what point in the travel are you tripping? If it is immediate it is prob a bad SCR or a bad SCR firing board. A bad 3ea board cand also do it. If it trips during accel or decel, it may be mis- adjustment of the Acel/Decl board. Also check out the Tach and wiring. These things are hard to trouble shoot without spare boards. You can check out the SCR's with a good multimeter.