When I replaced my O-T control, I replaced the servos with modern hardware (DMM Tech), and powered the spindle motor (6S, 7.5kW) with a VFD. I gave up trying to interface with the original Fanuc spindle drive. You do lose any indexing with that combination, but the spindle worked fine, including rigid tapping, for about 3 years until I replaced the spindle motor with a servo for a full C axis.