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OMTech Laser: It Was The Focus Pen Wire
Because the focus pen worked on the bench, I was certain this had to be true:

OMTech focus pen – failed 24V wire There is a break somewhere along the blue wire carrying 24 V to the focus pen. The signal and 0 V wires are fine.
I updated the original post, because I’m going to use that picture a lot whenever the subject of laser machine wiring comes up.