CNC3040 erratic stepper motor operation (fixed)

I recently had a problem with my CNC3040 machine, with the Z stepper motor suddenly running erratically (wrong direction or not at all). Wasn't difficult to diagnose and fix it, but I thought others striking a similar problem might like some assistance.

I've documented my repair for other owners of CNC3040 or CNC3020 machines [CNC3040_fault_(cable).pdf]. But since this forum refuses to upload my file (maybe there's a size limit?), I've had to store it at MediaFire. Here's the link:
CNC3040_fault_(cable).pdf

I've uploaded some of my older CNC3040 files and my G-code quick-reference there too, for those who might have missed them on this forum:
My Files

Daniel
Gerroa, Australia