|You can look for dust with a bright flashlight. |
It may be that the lamp is failing or how the system determines how the white or black level is.
Some cameras on scanners use a single black pixel. That is what determines the black level.
The best way is to get the factory service manual. I doubt you can get it but you can look. It would help to see how the white or black level is set or how to test the lamp.
Why did it take me over a year to phone in a problem to ATT?