I bought a self-made Reprap Prusa Mendel 3 printer, modified to be built from cheaper materials. And immediately after the deal I got various problems. I fixed the majority of them, but don’t know what the reason of strange extruder behavior: stepper motor it is not rotating while extruding filament, it’s moving back and forth with small steps instead. I made a footage:

hoping someone met similar problem before.

If so, please tell me what to do to make it work as it should.

The obvious option while encountering this problem was to slacken the bolt which holds this parts together (in the left bottom corner of the video), but that did not help.

Any ideas are very welcomed. Thanks for your time.

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Replace the cable to the motor. Also if you can the stepper driver could be suspect.