Z axis stepper motors lose lock

I had been frustrated by the Z axis appearing to reference at different heights when attempting to print. Sometimes the nozzle would be digging into the hotbed, other times clearing the bed by random amounts. I have now discovered that the stepper motors are losing lock; one more than the other.

When testing the axes, the behaviour appears correct. It's when attempting print that things go awry.

They are wired according to the instructions and run in the right direction. When the system is powered up, it is very easy, too easy, to turn the Z motors by hand with very little mechanical resistance. The x, y and e motors lock solidly and strongly resist turning by hand.

The x, y, and e motors are set to 0.4V +/- 0.1V as per the instructions. The instructions do not address the Z motors specifically; I reasoned that two motors running in series require twice the voltage of a single motor, so I set it to 0.8V +/- 0.1V. The motors are stone cold at all times.

So, I have tried turning up the voltage in order to increase motor current. So far I have increased the pot voltage to 1.5V. Even at this voltage the motors do not hold lock as the others do.

Do I have a motor problem, or is it the control board that is faulty. Or, do I need to turn the pot voltage even higher?

I would really appreciate some guidance with this.

I forgot to say that the Z carriage runs freely.

I have been looking at what people do when wanting to run 2 stepper motors from one driver. Nophead has a device running 4 motors in parallel from one driver. Could it be possible to wire the Huxley with the Sanguino Melzi controller with parallel motors.

I don't want to try doing this without having confidence that the contoller will cope with this

I have now conducted further tests. I first measured the voltage on all 4 Z axis motor connections, at the terminal block. All 4 measure 19.74 Volts: clearly this is wrong. It is the rail voltage.

So, I then tried swapping the Z axis wiring temporarily into the Y axis terminal block. Measuring Voltages now I See similar values as on the Y and E axis terminals. The motors are now torque locked as they should be.

This I think, can only mean that the Z axis driver is faulty. It's odd though that the motors can be controlled for position in both directions.

I am now wondering if it is possible to patch in a separate driver board. Does anyone have experience of this?

I have found the RAMPS Sanguino driver board:

[www.ebay.co.uk]

Would these be compatible by hacking into the Sanguinololu Melzi controller board?