The BPT will run in rapid or jog with the spindle off. To do a feed rate it needs to see the spindle on. This is a safety feature to prevent bashing end mills into the side of a part. There are small switches on the side of the spindle motor contactors to tell the control that the spindle is on. There is another switch or each contactor that locks out the other direction contactor preventing both from turning on at the ame time.
Put the machine in jog, setup, z axis and step. press the minus button repeatedly. Each time you do watch the small shaft and dial benath the Z axis motor. made sure that it does not step in the same direction for three steps and then reverse direction for one. This would indicate a shorted final drive transistor (2N6547). There are 3 groups of these on black aliminum heat sinks above the ACC board.
It is also possible that you voltage in is wrong, you have a bad bridge rectifier, your current is set wrong, you have a chip or washer between two diodes on the small board beneath the ACC card, a chip or washer in the motor terminal strip beneath the ACC card, or the wrong transistors, or a high resistance in the Z axis DC fuse (screw in type, tan in color), or your quill needs to be claened with kerosene and use Mobil Vactra number 2 for way oil. I also once had a bad opto coupler on the ZDI to the SMD.
That gives you a bit to check.