hey guys. Just swopped a black dev tank to try to stop random toner marks on copies, now we get almost blank copies in mono (OK IN COLOUR) We put the original tank back in , still blank. Tried firmware,reboot,calibration. Any ideas?

Try service mode 410 " manual halftone adjustment"

Check the grounding springs on the back of the machine that connect to transfer belt assy. They come out of place very easy and will cause fits.