The reason you are getting a "negative" image is that you are probably setting too high a screen frequency (Lines Per Inch) for your document. The higher the lpi, the lower the number of grays. When selecting a Quark PDF or Apple PPD file for the LaserWriter IINTX, the default lpi (lines per inch or screen frequency) setting is 150. With 150 lpi, you can only get about five levels of grays. What you need to do is to click the checkbox "Use PDF screen values" in Page Setup which sets the lpi to a lesser value (60), and allows for a greater number of grays.
Something you might want to remember is that Page Setup settings are stored with the document, so if the printer was working with a previous document, it probably had a "correct" lpi setting.
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