HP LaserJet 8000 horizontal grey bands - why?

[RobertFE]

Can anyone tell me what is causing these horizontal grey bands on my HP LaserJet 8000 DN? The printer was working perfectly for years before this began. It started with a single narrow horizontal band near the middle of the page, and progressed to more and wider bands over time. Print is visible through the bands, although it has a lower density than in the normal areas. The bands are horizontal (parallel to direction of paper travel.)

I tried the following experiments to narrow down the cause:
1. Replaced cartridge with a new one. (I'm using HP C3909X cartridges.) The first page or two were okay, but then the problem came back.
2. Opened door while page was printing. Removed cartridge and examined partially-printed page and drum. Grey bands were already visible on drum, and were on paper before entering fuser.

This suggests the problem is not in the fuser or transfer to paper, since the bands are already visible on the drum. It also suggests anything in the cartridge is not at fault (drum, charge rollers, etc.) since changing the cartridge didn't help? A fault in the high voltage power supply could maybe cause vertical bands, but not horizontal ones, right? So what's left? Some strange kind of light leak? The laser scanner or the electronics that drive it? It seems like unwanted light must somehow be hitting the drum on those areas where the bands are happening?

I'd be grateful for any ideas!
Regards, Robert

[cs_steve]

Sounds like 2 bad toner cartridges in a row. Old stock HP cartridges are prone to failure just sitting on the shelf ; some of them are 15+ years old.
It's difficult to find a good quality cartridge ...