difinately scr. usually a directional problem is not speed controller related as it has no idea which direction its going (about like our congress). i would go back and check the rectifiers (big ones) look very closely at the contactors sometimes when people change tips they loose the insulators behind them and that makes a frame short. but my bet is on the rec's. test them for reverse bias shorts with a high voltage continuity tester (like a handyman)