Canon NP6012 Service Manual page 219

MB INSTALLATION
Grip the toner cartridge
handle. Turn the cartridge
counter-clockwise until it
locks into place.
Close the main unit by Note:
4. The drum unit has been
installed, and the drum unit
lever is at the LOCK position.
Drum unit fever
2. The toner cartridge has been
turned and positioned as
specified.
3. The fixing lever is at the
LOCK position.
Fixing lever
Unless the above three points
are satisfied, the lock
mechanism will activate,
preventing the top unit from
closing.