I have a Dell C1760nw printer and it has started to print text only in grey instead of black. I have tried all of the solutions suggested in the manual - changing black toner cartridge, cleaning everything etc to no avail. I have noticed that in the printer preferences dialogue box, the "halftone" selection button is set to on and it won't let me change it ! The printer is only two years old and I have lots of cartridges so any help would be gratefully received!!