The optics need to be cleaned under the scanner glass. This is best done by a tech. This has to be done with the machine unplugged. The glass has to be removed then clean both sides then clean the small glass to the left on both sides, this is the glass where the document scanning is done. Also clean the lens and full and half rate mirrors. Using an air cleaner is not a good idea because it blows the dust all over the optics. Usually lens and mirror cleaner or alcohol is used. After it is cleaned I would use Kimzi's calibration links.
Try performing a copy calibration and see if that helps, have a look HERE.
And also take a look on the color balance options, just to see that the sliders hasn't been changed, HERE.
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Our Xerox 7556 is producing light copies, with even some light fading when copying from the document feeder. When printing to the device, the output is excellent wtih no issues.
I tried cleaning the scanned glass, without much impact. The client scans "tickets" from the warehouse that have white dust on them. They sometimes blow the debris off with an air cleaner, but it seems as though this is likely effecting some sort of optic within the scanning process.
Any ideas on how to work on correcting this issue?