Simon, Trying to dust off my 400 knowledge here, I can picture the 500 and 1500 better in my head. I think the 400 also has a Z axis amplifier box in the back of the machine. Remove the rear lower door and pull out the big square vacuum box. If it does have this amplifier I believe there is a reset button. If it has an error light or missing a voltage the amplifier could be at fault, but it can be very difficult to determine if the error is the amp or the motor, or possible the crate card. You can also check the Actuator (Z motor) brushes and wiring.
Hope this helps
Aaran
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