I have a scanjet 6300c at home, and for quite a while now, the glass on the innards side of the scanner has had a thin film or fog like quality to it. My wife was scanning pictures in and was complaining that the little 'all-in-one' scanner/printer/copier scans much better than the 6300. True enough the scans on the 6300c have a fog or haze to them (like when you have the contrast too high and the colors get less defined) .
Well, I'd been adverse to actually doing something about it since the manual and HP's online documentation stated you cannot clean the inside of the glass on the 6000 series. However, after a few web searches I found some third party evidence that you can open it up safely and clean the innards.
Sure enough, it was _extremely_ easy. Just pop the scan lid off (the manual shows you how so you can attach the page feeder), and then pop two screw covers on the front end of the glass top portion of the scanner. After that just take a torqx-10 head and undo 4 screws holding the lid on.
With the lid off, it's obvious you do not want to remove the glass since it's packed tightly with spacers that look like they would break easily if you tried. However, since it's just a hunk of plastic and glass, it's fairly easy to clean in place.
Cleaning it, according to the common sense instructions here:
I discovered there was a thin sheen of oil residue on the bottom side of the glass. Apparently, I deduced, the manufacturing oil/grease used on the head, plastic belt, and gears evaporates from the heat of the lamp and gets plastered on the glass. This happens slowly over time, but will always affect picture quality.
After several thorough cleanings with some photo lens cleaner and cloth, both my wife and I could not see any residue or streaks on either side of the glass when held up to the light from several angles.
I then used some mini-vac attachments to suck up a very tiny amount of dust that had collected on the bottom (the chassis on this model is probably not as air tight as it could be) and then I used compressed air to blow off the mirrors and light. I then popped the scan glass back on, re-tightened the screws and replaced the scan lid.
Some test scans proved a world of difference in quality. I'll probably be maintaining a clean schedule with it now every three months.