Motor Stepper diagnostic issue on PrintrBoard

I ordered a brand new Printrboard Rev. E from reprap.me earlier this month.

Upon arrival I set about testing it using a minimal config (thermistor and motors only). Tests on the X and Y axis went beautifully, but when I ran a test on the Z axis, I ran into an issue where the motor just vibrates. I thought it might be a pin order issue, so I physically remapped the pins every way I could think to no avail. I then thought it might be a trimpot issue, so I adjusted that, which only appears to result in making the vibration of the motor more or less violent. I contacted reprap.me for a return, and was told that they will accept the board for testing, but that they test all of their boards before shipping (implying I was at fault). When I went to fedex to ship the board back, I was quoted about time and a half the cost of the board for shipping. I tried flashing the bios to the latest repetier with the correct motherboard setting, which was successful, but did not fix the problem in any way.

I would really like to get the board working rather than to have to hunt down some bargain shipping rate at UPS or the post office to send it back to Denmark, as I am guessing they will then want to charge me to ship it back, or for shipping for a replacement board.

I went ahead and ordered another board from a distributor more local to me, but now I have this board which I could put to use running a micro CNC of some kind, or perhaps a grounded delta (I'm looking at the geared up simpson when Mr Seward releases the files) :]

I was without a multimeter, so I tried testing the voltage levels with an LED, just to make certain all pins were conducting current and not bridged. Using the LED on the problematic stepper results in solid on/off. Using the LED on the working stepper pins results in the LED cycling through different levels of brightness, and eventually burning out the LED (heh.. what I get for not grabbing a multimeter).

Any ideas of anything else I can try?

How much current are your motors rated for? What happens if you plug one (and only one) "known good" motor from the X or Y into the Z axis (unplug both Z's and just plug in the one X or Y)?

The motors I have are rated for 5.3v .75A, they are bipolar steppers, but I'm using them as unipolar.
No matter the motor config, the Z motor always vibrates. The X and Y always work fine.
They are Vexta 2 phase C4041-9012 motors.

I think we can rule out the motors as all of them work when plugged into X or Y.

If you have a normal Z axis, it runs two motors in paralell. They each get half the current the driver is set to. You may simply need to bump up the Z current a little bit. It's also possible you have a blown Z driver.

If the Z driver is smoked, would it be possible to add an external motor driver off the expansion pins so that I can still run a printer off of this board?

I was able to find some reference to add a second stepper using the expansion pins. I may have to add two. I have purchased two stepper drivers from a local shop (MatterHackers) and am awaiting their delivery, most likely later today.
Has anyone done this before?

If you hook up motor drivers to odd places - make sure the firmware can be configured to recognize those locations. There are a fairly finite number of places the firmware expects to find things. I'd also wonder what else might be smoked on that board. A brand new board with issues is not a good thing.

Yeah, it is not ideal by any means. Going to use it for experiments and get a new board for my delta-pi. The expansion pins on the printrboard can run at least one extra motor (for dual extruders) so assuming the extruder driver is only disabled because of temperature, it should be OK for me to use the secondary extruder pins for the Z axis and update the firmware accordingly. Will post here once I get the stepper drivers with my progress.