HP printer smears

[IdOp]

I'm having a non-software problem with a HP LaserJet 4ML printer, a picture of which is here. It was purchased in 1996 (for a whack of money because it has an Adobe PostScript processor in it). While it's old, it hasn't been used very much, I think I'm only on about the 3rd or 4th 500-page ream or so.

The problem started about a year ago, when a light gray vertical band began to appear along the left edge of pages. Over time the gray area got darker and spread out to the right. Now it covers the whole page. A scan of a test page showing the problem is here. The manual is no help.

I'm wondering if anyone here has encountered this kind of problem or has advice. Some obvious questions are:

* Is this likely to be a problem with the toner cartridge? I.e., easy to fix.

* If it's not the cartridge, is there any hope of fixing it oneself by cleaning something inside the printer?

* Is there a good, more targeted, forum to ask about this on?

I'd hate to throw the printer out, but suspect having it serviced, if even possible, would cost a lot more than buying a new one (i.e., $100 or less). If I buy a new cartridge to test that out, and it doesn't fix it, I'd be stuck with one-and-a-half good cartridges and a bad printer.

Thanks for any suggestions!