Thanks mate - you've clearly been there before
![Smile :) :)](styles/default/xenforo/smilies/xsmile.png.pagespeed.ic.Ag8h2FS5_U.png)
. That's exactly what I did. In fact my test rig is completely disassembled because I too was convinced that it was magnetism from screws or base block etc. And like you I won't demagnetise the stepper motor itself because, I think, it has permenant magnets on the rotor (hence my comment about taking that apart too). It's a real mystery to me what else could cause it.
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