Brother MFC-8220 Service Manual page 160

<Initialization of the Developing Bias>
If running the machine at a constant developing bias voltage,
the print density is light at the beginning and gradually increased as the printing is performed.
To correct this problem and
pages after the toner cartridge is replaced and changes the bias voltage according to the
number of the printed pages.
The steps to change bias voltage are as following.
(1) When it is detected that the toner cartridge (which is empty or near empty) is replaced with a
new one (which is full of toner), the developing bias is set up to 400V(initialized).
(2) The bias voltage is gradually decreased as the printing is performed.
(3) The bias voltage will change to 300V when 7000 sheets are printed and it will get fixed once
reached to 300V until the message, either "Toner Empty" or "Toner Low", appears.
The reason the toner cartridge needs to be replaced from the one (which is empty or near
empty) to the new one (which is full of toner) is that the machine is initialized to set developing
bias at 400V only when
the toner sensor when the user replace the toner cartridge.
If the toner cartridge (which
(which is full of toner), the machine cannot
the toner sensor and fails to initialize the developing bias.
In this case, the toner cannot get enough bias voltage (400V) to print properly and the print
density gets lighter than usual.
maintain the steady print density, the machine counts the printed
the machine detects the output from both
remains enough amount of
recognize that the toner cartridge is replaced by
5-7
because of the toner property
the front cover sensor and
toner) is replaced with a new one
MFC-8220
SERVICE MANUAL