Hi,
You will need a tech to replace the black developer housing and black developer material. Diagonal lines indicate a lack of developer material into the developer housing.
Gabi
Our older Phaser 7500 has an issue with the black only. I have attached an scan of two prints I did. So far we have installed a new fuser and new black toner cartridge. I pulled the cleaning rods on all 4 cartridges. Not sure what my next step should be.