Brother HL 4570CDWT - Manual (Page 127)

Routine Maintenance
121
5
Cleaning the corona wires
5
If you have print quality problems, clean the corona wires as follows:
a Press the front cover release button and then open the front cover.
b Hold the green handle of the drum unit. Pull the drum unit out until it stops.
c Hold the handle of the toner cartridge and slightly push it toward the machine to unlock it. Then pull it out
of the drum unit. Repeat this for all the toner cartridges.