If your Hewlett-Packard DesignJet 500 printer is smudging, smearing or otherwise not accurately printing your graphics, chances are you need to clean the plotter heads (also known as the "printheads"). The electrical contacts between the plotter heads and printer cartridge can accumulate dust and dirt, breaking the connection and leading to printing errors. Because this is a large-form graphics printer, cleaning the plotter heads must be done manually.
Turn your printer off, then back on. Wait for the initialisation process to complete.
Scroll down to the "Ink" menu on the menu screen located on the front of the printer. Select "Replace Printheads" and hit "Enter."
Lift the printer cover and open the carriage cover containing the plotter heads. The printer will have moved the carriage (the moving rod that puts the ink on the paper) to the centre, and the cover should pop right up.
Remove the plotter heads contained beneath the cover.
Unplug the power cable from the back of the printer to avoid getting shocked.
Clean the electrical contacts both on the carriage (still in the printer) and the plotter heads (removed) using a damp, lint-free cloth or coffee filter.
Let the contacts air dry (it shouldn't take more than a few minutes) and then reinsert the plotter heads into the printer.
Close the printer, turn it back on, and print a test page to confirm the fix.