My designjet z 3100 is banding.

SOURCE: HP Laserjet 1100, band of unprinted area after new toner install

Hi Friend,
You have mentioned this is a 2nd hand printer,
you have to service the printer,(fuser unit)
if the problem exists after fuser service check Laser Scanner also,
Thanks
Lux5

SOURCE: HP Designjet 500ps 24" Plotter- screen says to replace printheads

Printheads are available everywhere, you replace them your self. Machine will usually indicate just one bad print head at a time.

SOURCE: ink "banding" on hp laserjet 2600n

Viscosity of the ink is likely your problem. Sure the inks are not leaking at the same time, but that is likely because the different colors use different tints which can be slightly thicker or thinner. Many printer manufacturers (HP most notably) makes many patents on their ink, and not solely on the colors, but also the viscosity. The ink jets could slowly be leaking and might need to be replaced, finding these parts yourself might be easy, but replacing them is not.
I'm going to assume you've replaced the blue and suggest you remove the roller and clean it with glass cleaner. You may find it EXTREMELY difficult to remove the roller, so I would probably find a printer repair/ sales shop. Usually these places are not easy to find, so you'll probably want to find an office equipment sales business. These are the companies that usually sell the larger copier/fax/printers for large offices, or you could call HP and ask where a certified HP printer technician is in your area. I will tell you though, the cheaper ink may have just caught up to, having a copier technician repair this printer could cost quite a lot, but doing the job yourself, could be disastrous. The amount of small screws, easy to break plastic tabs, and interesting clips, etc make the job nearly impossible for a novice.
Good luck -Skyler

SOURCE: k550 air in ink feedlines

I have had this happen a couple of times and have cured it easily. It has only happened with the black with me. It seems to be caused by air in the ink feed line. With the printer switched on open the top cover and hold the resume button for 5 seconds. The printheads should move to the left and you may be able to see air in the pipe(s). Remove the printhead of the offending colour(s) then insert a syringe with the plunger fully down into the rubber that the printhead connects to. Draw up the syringe plunger to draw ink/air through until you can see no more air in the pipe. Replace the printhead and close the cover. The printer should now do a calibration with the offending colour hopefully now working.

SOURCE: black ink problem with printout on hp7780

Always try replacing the ink cartridges first.

It sounds like the print heads in the 7780 are bad. Don't confuse the ink cartridges with print heads. The cartridges fit inside the print heads. You will need to change the heads. Here is a link to HP's how-to.
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00841890&tmp_track_link=ot_faqs/top_issues/en_us/c00841890/loc:1&lc=en&dlc=en&cc=us&lang=en&product=2512010

Could have the same problem with the Dell.