Z-Axis sometimes not moving

[Lonzo]

I have a Geeetech Prusa I3X Acrylic
The problem is, the Z-Axis is sometimes not moving. With very little force it is easily possible to stop the Z-Stepmotors with two fingres. So i searched for this prroblem in different Forums. There i found the tip, that there are 4 driverboards (A4988) on the mainboard which have a potentiometer to adjust the Voltage for the motors. This voltage should be between 800 and 860 mV.
My Multimeter shows this on all of the driverboards, except the one for the Z-Axix. on this board i get 210 mV. So i tried to turn the Potentiometer clockwise, like it is shown in the video of Pololu for their A4988 Driverboard. But the Voltage does not change. All other 3 boards react like shown in the video, but this does not.
So i think, this driverboard is defect. Is it possible to change this single driverboard against an intact one, or is the whole mainboard not usable now?
And how do i get aonother Driverboard or mainboard now?
What can i do now?
( sorry for my english, it is not my motherlanguage.. )

[_kaktus_]

Hello.


You can replace the stepstick modules.

But first replace the Z-axis module with an axis module that works properly. This way you will have a comparison.
Remember that this should be done when the power supply is disconnected.
Be careful.