How can I fix my HP Laserjet IIIp?

I have a HP Laserjet IIIp laser printer and have a problem with print quality. The actual printed text is fine however the whole page looks like toner is smudged all over it. I have tried a different cartridge which was ok at first but after a little use has given the same results. Can anyone give me some advise please. I printer is relativeiy old now, and for the price of another toner cartridge, I may as well buy something new if the fault is serious.