I recently got my hands on an IBM ProPrinter II. It's a dot-matrix parallel printer, but has the flexibility of taking both continuous papers as well as individual papers without holes on the side.

The thing was abandoned for quite a while, so it's dusty, but otherwise has no major damages. I turned it with the test routine, and it printed the page you see in the attachment.

The two issues I noticed are the missing thin lines and the fading out of the left side of printing.
I tried repeating the test several times and the ink started to come back stronger, but those empty streaks are still there. Would this be a symptom of the ink ribbon or the printing head?

I have no idea how to maintain those old printers (nor new ones for that matter), so I don't know what would be a good procedure to fix those issues.

Looking for suggestions