Instructions for changing the printer paper can be found here.
If the print quality is unsatisfactory/too light, perform the following steps:
Check the cleanliness of the print head. Clean it if necessary.
Confirm that you have been using thermal paper, NOT plain paper. Using the wrong kind of paper would cause the printer to print lightly or not at all, even thought the paper scrolls.
Verify that the paper was purchased from HunterLab. High-quality thermal paper must be used. The HunterLab Part Number used for ordering this paper through HunterLab's Order Processing Department (703-471-6870) is A13-1000-746.
Adjust the trimpot on the printer board that can be used to increase the current on the thermal printer head. First, turn off and unplug the DP-9000. Then, loosen the two thumbscrews at the back of the processor and slide the top cover back. Remove the paper roll from the printer. There is a hole on the bottom of the paper roll holder than leads straight down to the printer current trimpot (VR01). Turn this trimpot counter-clockwise to darken the printed characters. Reassemble the processor and restore power.
If the printer will still not print satisfactorily after following these instructions, instrument service may be required.
If additional assistance is required, you may contact HunterLab.