Have you tried to clean the Laser Window of the Yellow Imaging Unit with the Laser Window cleaning wand? The wand is attached to the inside of the front door and you have to remove the Waste Cartridge to gain access to the Laser Windows. The directions, along with color pictures, for how to perform this cleaning can be found in the Phaser 7760 Knowledge Base. Here is the link to that Knowledge Base.
Just type in, "Clean laser window" in the search field and follow the steps to clean the Yellow Imaging Unit Laser Window. Hopefully this helps to resolve your problem. If not, at the top of the knowledge base window you will find the 800 number for contacting the Xerox Customer Support Center (800-835-6100).
I've gone through many of the problems regarding "smudges" and print quality on my Phaser 7760GX and none of them seem to work. I get Yellow streaks at the top of the page every time I print. The print quality is excellent, but there are always these streaks at the top of the page. As I said, the quality is good, nothing is wrong with the actual printing, it looks like I dipped the top portion of a perfectly printed page into some yellow water or something. What should I do? Do I need to simply replace the ink? My yellow seems to be at like 90% capacity while the others are at 20% or so, if that matters. Please help! Thanks