hi,
i think when you get a printing not centered, its a problem of the polygon mirror of the laser unit, the mirror was not calibrated properly, when the exposition step start, the laser motor must be in constant speed, if the speed change printing will not be correct,so you must disassemble the LSU and check if there is a foreign body. but the best solution is changing ths LSU.
here the picture of LASER SOURCE UNIT:
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