Canon D646U Scanner stops in mid-scan

[Robert Akins]

Hi:
I have this scanner which won't make a full pass of the reading head; it
always stops somewhere in midstream. I have even taken the drive motor off
the belt and powered on the scanner, and the motor runs for the same amount
of time as if it was driving the head. Cycling power to the scanner will
allow it to return to the start position. No mechanical binding or
obstruction is evident. Any ideas?
Power supply?
Robert