Strange Z axis behaviour Prusa Mendel i3

I started to build a Prusa I3 and to get the plastic parts I borrowed a friends Prusa i2. He had built this from a kit but not commissioned it. I did the comissioning and it printed pretty good after a few teething problems. The electronics was a sanguinolulu and I rebuilt the software a couple of times to adjust the steps/mm, the directions of axis polarity of endstops etc. I have had to give back the i2 printer now but managed to print all the parts for my I3. After the experience with the I2 I felt reasonably confident with my build as that had identical electronics. I also knew what the motors sound like when they are working comfortably.

I loaded the working firmware from the i2 onto my board and used this on the i3. (in fact I did this whilst comissoning the i2 to test out another problem and used my board on his printer) I expected that directions of travel and endstops may be different but everything else should be the same. X and Y axis were easy enough, just needed to adjust direction. End stops were a bit odd, they are mechanical on circuit boards. They have leds to show when they are on/off. However connecting a 5V supply to them seemed to cause feedback on the other axis so I have disconnected that now. Stepper boards adjusted to 0.48v.

The Z axis though is behaving very strange. When I try to move them using pronterface the motors just sit there buzzing. The z axis motors are wired in series. If I turn up the current a little I can get them to rotate a bit though this is clearly not the answer. I disconnected the rods to eliminate binding to remove this from the equation.

If I plug the z axis motors onto the y plug I can set them going and they rotate cleanly and for around the correct amount of time. Hit Y home and they rotate (nice happy noise) until I press the y end stop. I would say the motors work OK.

I plugged the extruder motor on the z axis plug and it also sits there buzzing and not rotating. Swapped the z stepper board with the extruder one and the same behaviour is observed. Extruder motor runs fine on the z axis stepper board (use M302 and extrude 20mm).

From this I am pretty sure that the stepper boards and motors are fine and that the problem is the way the Z stepper board is driven. I would reckon it is unlikey that the main chip is damaged and strongly suspect this is a firmware issue of some description. I saw a previous post from someone called Carl that suggested reducing the Z axis accelleration and incrementing it slowly until it stalled again. I will probably try this but I would think it is unlikely because the same firmware worked on the i2. I suppose really it this aspect that I can't quite fathom, why did the i2 work and my i3 not, It has the same motors and electronics.

Whilst I am pretty sure I will crack it I would prefer to do this sooner rather than eventually so any suggestions however outlandish wopuld be gratefully received.

Well I have cracked it. It wasn't firmware, I was was wrong about that. It turned out to be a solder bridge under the long strip of sockets that the stepper driver boards plug into. When you plug the board in it makes a connection between circuits 2a and 2b when you ease it out it breaks the connection. A tiny bit of pressure on the connector was enough. I had to take out the pins from the board and pare away the plastic of the connector to slip a scalpel between the two pins. I don't have a decent camera other wise I would have posted photos.

This is the second problem I have had with this board which was bought off Amazon preassembled. Came from HK I think. My wife bought it for me for Christmas. The other problem was that the ATX socket, a vertical one, was soldered in the wrong way round. A full 180 wrong. I replaced it with a right angle version. If I hadn't already had the experience with the other printer I would have just plugged in the ATX PSU and more than likely fried the board.

Anyway Z motors happy now.