gtcrshr,
Charge Corotrons are a user replaceable item, just like Toner, Waste Toner, Fuser Web and Fuser oil. Go onto Xerox website and download the Corotron test sheet and run it off just for fun. If you see banding in the individual colors you more than likely need to change the corotrons. If you don't have any on the shelf you can switch them around. In other words if you see banding in Magenta but not in Cyan, switch those around and see if the banding shows up in Cyan, and goes away in Magenta you had a bad Corotron.
When you call Xerox for supplies, please make sure you order Charge Corotrons, I keep 8 on hand all the time. They are part of you maintenance contract just like Toner.
One of the other parts that I have had banding show up in is the Drum Module, this is only replaceable by the tech. and you should have 4 of these on hand as well. According to my tech, there should be no car parts for him to haul around. I have every likely part required to make my 8000AP operational on any given day.
Again, if you are not getting the required service keep going higher up the food chain until you get results. Each time you talk to someone make a point to ask who their boss/supervisor is and how to reach them before you hang up.
Xerox service in my experience has been second to none, they don't play around when it comes to the production boxes. The only time I have been down longer than a few hours was when I had 2 of 4 brand new drum modules fail. (I now have 6 on the shelf) On day 2 my service guy brought in a specialist and after several hours on the phone with Rochester NY with the lead xerographic engineer they were able to track it down. The engineer at that time was in the process of clearing her calendar of to fly in the next day to get it resolved. 2 1/2 days is the longest I have been down, most of the time I am only waiting for 2 hours on a bad day..
Keep up the pressure!