Repair notes
Unplug the scanner before opening it. These notes are reminders for ordinary cleaning and inspection, not a service manual. High-voltage lamp supplies deserve respect.
Glass cleaning
Clean both sides if the machine has been open before. A fingerprint on the underside can look like software blur forever. Let the glass dry fully before closing the lid.
Calibration strip
The white strip is easy to forget. If it is yellowed, dusty, or marked, the scanner will calibrate against bad information. Some units have the strip under the lid edge instead of beside the glass.
Belts and carriage noise
A thump at the same point in every scan suggests carriage motion. A bright or dark line at the same point suggests optics or calibration. Listen before you disassemble.
Document feeders
Old ADF rollers polish smooth and drag pages sideways. Do not test an unknown feeder with a unique original. Use ten sheets of scrap and watch for edge chew, hesitation, and double feeds.