one motor not turning

Hi

I have a CNC that is well over 10 years old. It's running Mach3 on a Windows XT machine. The breakout board is a CNC4PC that hasn't been supported in years, so I have found in the past asking them for any information is useless. Drivers are Gecko 202. Motors are a big set of pacific sciences. The machine has been running flawlessly for well over 10 years.

Turned it on the other day and the z axis motor isn't moving. It buzzes and try's to move, but doesn't get beyond a twitch. It also doesn't lock up solid as it should when powered up. All my other axis are moving without any issues.

I tried swapping out the motor to see if that would help and get exactly the same behavior.

My next guess is the Gecko, especially with that one motor not locking when powered up.

All motors are wired off of the same buss connected to the same power supply, so it doesn't seem to be an issue with power going to the stepper. I don't think it's the breakout board since the motor tries to turn and buzzes, so I'm assuming that indicates it's getting the step/direction signal properly.

Any ideas what to check next or have I done enough to isolate it to the Gecko?

Thanks
Steven