In article , Andrew May
writes

As far as I can see the initial reference position is found by moving
the carriage to the back where there is possibly some sensor, optical or
otherwise.

This is known as 'homing'.

Then step back a known number of steps to achieve the
reference position. This all works fine. It is when it tries to move
back to this position after focussing that it fails to get there and the
problems ensue.

Would you say it can drive in one direction OK but not the other? If
so, the drive transistors to the stepper may be failing.

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