It is also possible that one of the 12 diodes on the board beneath the ACC card is bad. When on of these goes, it does what you have described. By putting a meter on the base of the transistor, I notice that they do not turn on and off crisply. Instaed they turn off half way then finally off fully. By the way, the ACC is symetrical and by turning it around, you switch the X and Z current circuit but not the Y. In twenty years of BPT service I have only found one blown SMD board.
I cut and pated another answer in part below. Just substitute Y for Z.
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 your 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 cleaned 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. Drove me crazy.
That gives you a bit to check.  |