Have you tried a HP7475 driver.Originally Posted by vacpress
I played some with this software and had the same problem. The older the plotter the more basic the HPGL will probably be.
This is a guess but the drivers may be expecting ght HPGL to understand fonts in text and outputing ascii tather than HPGL for text.
Download Mach three and feed it your hpgl and see what it draws in simulation. It was originally concived to do HPGL and is supposed to be strong on HPGL imports.
Also Dancam was originally designed to spport HPGL. It is free and will drive your motors.