only 1 Z motor turns when running gcode

Hi,

about 3 month ago I moved, and with me my mendel90. I took very good care it, drove it seperately in the back of my car and made shure it could not move around or anything. Everything went well, but since then my mendel is acting in the most weird ways.
The first few prints after the move went absolutely fine. But then suddenly the z-height dropped on the right hand side. So I losend all the clamps made shure everything is at it’s right place, tightend everything again and recalibrated. But when I printed something the z-height of the right hand side seemed to drop with every print. So I figured the couplings must be the problem. But at this point the misalignment was at it's worst and I couldn't get one print out of the machine, so I ordered cnc milled aluminum couplings from amazon and now the motor and the threaded rod are bonded together like never before. And I was able to do a few calibration prints (3d benchy) and they got better each time, so I figured it might be the time to print some backup parts, just in case the printer might act up again. And there I was clicking on the start button on my octoprint setup. The homeing procedure of all the axis starts and after that the extruder should move down towards the bed to start the print, but it doestn't. At least not as it should. Only the left z-motor moves, wich leads to an horrible tilt in x-axis, and the motors make a horrible sound. I immediately pressed my power switch and tunred the threaded rods by hand to get the x-axis in level again. I thought something might have blocked the motor so I checked: nothing. I started the printer again and was able to move the extruder in all axis via my octoprint server. I leveled everything again and hit that printing button: The axis home in all directions and at the point where the extruder goes towards the bed, again, only the left part of the x-axis went down. It’s frustrating.
In the next step I decoupled the motor from the threaded rods and did a test run: the motors are turning fine when I control them via octoprint and home correctly, but after the homing, the right motor maybe does one turn and the left one turns quite a bit, but does stop at some point. The terminal interface from octoprint shows no error.
Does anybody have a clue what could be wrong with the printer? Is the melzi board the point of failure? I think wiring is out of the question since the motors turn as they should when they are controlled via octoprint. I even thought about tuning the stepper driver, but again, when controlled directly from octoprint the motors turn fine.
I also did these kind of test with old gcodes I already printed successfull, to the problem must be in the hardware I guess. But for now I am out of ideas. Any help would be appreciated.

Mathias

Check that the power and ground connections on the Melzi are tight. The bed is turned on before the Z axis descends so a bad connection causes the 12V rail to drop. Since the Z motors are wired in series they are the first to be affected by a low 12V rail.

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[www.hydraraptor.blogspot.com]

another thing to look at would be to check if the nut is binding anywhere on the threaded rod, does it happen in the same z-height or is it at random heights?

Hi,

it seems that nophead was right. The screws from the power and ground connections were a little loose. After I fastend them I tried a run without the threaded rods attached and everything went fine. I'll try to print something when I come home from work to check if this really did the trick.

Thank you for your help!
Mathias

Fastening the cables really did the trick. Thank you!!