It could be a number of things. There are several rollers the page goes through when printing (including the drum it self). They each have a different diameter. You see that repeating horizontal line/zone across the page? That is telling you which roller it is. In service manuals it would tell you the circumference of each roller so you could narrow down your troubleshooting. See if you can get the service manual and if HP still publishes these numbers for your printer.

This is most likely an issue with the drum. The way most laserprinters (including this one) work is by applying an electric charge to a plastic drum using a so called corona wire. The laser then hits the drum, causing it to discharge where it’s hit. The drum then passes over the toner, which is tiny little grains of solid ink. The static charge picks up the grains, but only where the drum is still charged. Then the paper is squeezed against the drum by a roller and heat is applied. This will cause the ink to turn into liquid and stick to the paper.

The repeating nature of the fault clearly indicates ink is getting deposited where it should not be, probably indicating damage to the drum. Each time that part of the drum rolls around, the same issue is present.

The drum is a wear item and can be somewhat easily replaced. Although it can be more tricky than replacing the toner. The drum is also super fragile, so handle with care. Do not bump it, or it will get damaged. Also be aware you will get into parts of the printer that likely are contaminated with toner. So do this somewhere you can clean easily and wear a mask. If it gets onto anything, clean with cold water! If you use warm water the ink will turn liquid and you’ll have a terrible time cleaning it.

If you can get your hands on a drum for an old machine such as this it will be fixable. However I would recommend against it. Old printers like these often have many wear issues. The paper pickup will be worn out, all of the rollers will be worn etc. It will fight you every step of the way, even though it is technically fixable.

In some cases the drum is included in the toner set, especially when there are multiple toners that usually get replaced at the same time. For this printer that’s not the case, replacing the toner won’t help, you are looking for an imaging drum.

I still have an old Brother laser I keep on life support for the every once in a while I need to print something. The amount of hours I’ve put into that thing is insane. Replacement parts, several total rebuilds etc. And every time I want to use it, it refuses to work properly. But it still works and I hate throwing stuff out, so it’s still here living out it’s retirement years. But I feel this is bordering on mental illness on my part, so do as I say not as I do on this.

Could be the transfer belt which tends to be a very expensive item.

Personally had a brother printer with issues similar but not nearly as severe. The toner cartridge had a rubber roller attached that had some gunk buildup that i carefully wiped away and then it came out printing fine

The toner is a very finely powdered abs plastic so you probably don’t want to be using anything you plan on keping to wipe it.

Still using the original sample toner cartridge and drum like 10 years in

so a new toner cartridge fixed everything, got it off some local business that was selling off its surplus for $5 apiece. the thing is these models have the toner and drum in one cartridge; that used to be two things way back when I was using those things last.

Seems like the wrong instance for this community as evident by the printer in question