Canon MP638 Service Manual page 28

(7) Carriage unit removal
1) At first, on the main chassis, mark the positions of the screws that fix the carriage rail to the main chassis (3 points for each
screw: the left, right, and center).
2) Remove the timing slit film. Be cautious to keep the film free from any grease or scratches.
3) Using a longnose pliers, release the left end of the pulley holder spring, and remove the carriage belt.
Be cautious to keep the belt free from any grease.
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