Laserwriter Pro 630 Prints as a negative

There was a thread about this last year but no one answered it. I'll try it as a new one:

My Laserwriter Pro 630 just started printing negatives after many years of no problems. Black where it should be white. It's definitely not a driver or software setting. I even tried to reverse the image in the driver to trick it into printing correctly- but that didn't work. I just bought a new toner cartridge with 1 page on it- hate to ditch my favorite printer, which I'll have to do if it's the formatter. I was told some Laserwriters had a button that reversed print but mine has no such button. Does anyone have any idea if there's an easy fix?