I have not worked on one of these in years but had a set of wiring diagrams scanned into my laptop. I see where all 3 drives plus the 4th if so equipped interfaces with the FSSB. I see V ref going to each drive but the 24 V does not come through the FSSB but is shown below it. It may be that the FSSB supplies 5VDC for the encoders and such. Is it possible you have a encoder cable shorting out? Check them for kinks, cracks in the outside, chips, etc.
I have one of these machines not too far from my house and will try and get a look at it.
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)