I have an oldish Canon printer which, when printing colour documents is fine - including the black and white text. Same same if I print a page as an image.

 

However, when I print a mono document, the text is broken and faint - for all the world as if the jet was partly blocked. It doesn't matter what format it is, pdf, Word, whatever. I have tried the cleaning routine in the printer software, without effect.

 

Any ideas what the problem might be?

Sounds like the Black print head is up the Swannee.

If the print head is removable - Canon print heads usually are - try soaking it in Windex or some similar glass cleaning fluid for a few hours.  You don't need to immerse the whole thing, just the part with the tiny little jets.  A bit of agitation probably helps. You may need to repeat a couple of times.  Then take the head out of the fluid bath and carefully remove the excess liquid, using a tissue.

 

Now reinstall in the printer and run the head deep cleaning cycle - you may need to do this more than once.  Hopefully, the printer should then work normally.  No guarantees but I have used this technique on blocked print heads and it has always worked for me.  Worth a try anyway.

 

You may need to re-align the head after this - there will be a procedure for this in the print driver.

if it is not expensive

"Wreck it buy a new one"

sounds like printer head is blocked
can try cleaning by soaking as suggested above etc
you can also buy a new printer head from China to fix
(buying locally will be more expensive and usually not worth it)

you might be better to just save time and go buy a new cheap printer
as it doesnt sound like you use it much
remember to print a page at least once a week just to keep the heads clear

Thank you all for your replies. I think Patman is right - the problem has come from not using it much, and one cheap printer to replace another may be the way to go. It is, after all, over 6 years old.

Does it have print heads or are they part of the ink cartridge (just get a new b/w ink in that case)?