Brother MFC-J3930DW Online User's Manual page 639

In addition to cleaning the scanner glass and glass strip with a non-flammable glass cleaner, run your
fingertip over the glass to see if you can feel anything on it. If you feel dirt or debris, clean the glass again
concentrating on that area. You may need to repeat the cleaning process three or four times. To test, make
a copy after each cleaning attempt.
When there is a spot of dirt or correction fluid on the glass strip, the printed sample shows a vertical line.
After the glass strip is cleaned, the vertical line is gone.
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• Clean Your Brother Machine
• Telephone and Fax problems
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