The resolution on the dp 3510 is not good been

go to(*+a3original+3) will appeare f1 then write 6 and press copy button then write 21 till u have (f6-21) then choose input from lcd an put large value like 35+ after that go to f6-c65 and put 0 value after that make it on f3 mode and choose an empty casssete and press copy button and leave it 3 minute till the toner bottle go around and press stop to have make a copy then the darkness will be very good after it will be good go to f6-c21 and make the value (35-) and go off from the function by clicking (*+cancel ) good luck.