I want to darken copys on my canon pc850

Question

with my cannon pc 850 i recently needed to get a new toner cartridge, last time that i did so i had a professional refill it and adjust the darkness of the copies.. since i have replaced it this time the copier dose not copy as dark, we use a lot of red pencil sketches in the copier machine and now even at max darkness they are still to light of copies for what we need here.. i know there is some sort of way to adjust it using the key pad on it i just don't know it. any help would be appreciated