Pull the drum drawer out. Take the black drum out being careful not to touch the drum cylinder and cover it so light doesn't hit the drum. Cover the other 3 drums with paper. Look into the area where the black drum cylinder was touching, that is the black developer housing. see if the beads look even across and that there is no void from front to back. Don't try to clean or touch it. If you see a void near the area your having the problem with then that is the cause of it. If not then shine a strong flashlight into the area above where the black drum/ developer housing would be if you pushed the drum drawer back in. Look at the laser glass window and see if there is black toner on it. This could also cause a deletion line in black.
