Brother HL-L3230CDW Online User's Manual page 292

To avoid print quality problems, DO NOT touch any surface of the drum unit except the drum unit gear.
11. Tilt your left hand up to lift the frame so the numbered grid is visible, as shown. Use your right hand to rotate
the drum while you look for marks or debris on the drum surface.
Use the column numbers next to the drum roller to find the mark. For example, a dot in column 2 on the
Drum Dot Check Sheet would mean that there is a mark in the "2" region of the drum.
12. Wipe the surface of the drum gently with a dry cotton swab until the mark on the surface comes off.
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