Marks on drum won't come off

The 9840 uses a WT100CL cartridge for collection of Waste Toner.

If you remove the Drum Unit then the Plastic Sheet (Transfer Belt) like thing the Waste Toner sits underneath this. Lift out the waste toner and replace it or put one in if it is missing.

Put the Transfer Belt and Drum Unit back in. Close the machine and away it goes again.

Hi,I think this guys can have more helpful information about your problem.See this link:http://www.quikshiptoner.com/catalog/brother-mfc-9840cdw-toner-color-set.html#resett_drum_manualview More Info button to see how to install the Drum unit. Pay attentionthat after replacing a Drum unit in Brother MFC-9840CDW you have to reset thecounter. See steps 9, 10 and 11 for doing so.9) Close thefront cover, plug the power cable back in and turn on the printer.10) Press the Menu button,and then enter the number 84 to access the Reset Menu.11) Press the scroll keys until Drum is displayed and press OK.
I hope this can be helpful for you.

Just the belt

Remove drum & cartridge and clean opc drum with dry soft cloth

go to LINK and try to find the best answer.

Brother MFC 9180 change drum soon

Wipe the paper feed rollers with a clean rag wetted with plain water. You can also replace the feed rollers, search for "paper feeding kit" for this printer. Should be $20 to $30.

press the 3 and 9 button at the same time in ready mode.

then the reset menu will appear, press the up or down arrow until you get to drum.

press ok,
and select 1 to reset or 2 to exit.
it will return the ready state and your done.

Have you tried cleaning the corona wire?  You can also use a cotton swab and clean the drum.  Just be sure to unplug the unit from the wall and allow everything to cool down.

look at the drum, that's going to be where you will find the dots on...