Homebrew PCBs

Hello all,

I have a question : I recently bought a secondhand Laserjet 4P to make 
my own artwork.
Besides the fact that the printer doesn't print that black (with a new 
tonerkit) it also prints irregular traces (small bits missing).
On my former job we also used a laserjet 4P. Printing always went great 
on heavy tracing paper).

I have tried several types of transparants. (overhead and tracing 
paper) but I can't get any good results no mather what I do in the 
driver menu or printermenu.

Is it possible that I have to clean the printer? Or is there a 
possibility (potentiometer or simular) on the printer to increase the 
amount of toner? 

Thanks in advance for your time,

Pieter