The HP DeskJet 1120C Saga
The problem - an HP DeskJet 1120C inkjet printer which was printing colour with no problems but which was printing black unevenly.
The printer was giving white lines across a solid black box initially, it was missing out every third line in printed documents after a while and when the problem was at its worst, most of what should have been a black band on a test print just wasn't there!
Going through the full print-head-cleaning ritual given in the help file made no difference and an inspection of the printer's black cartridge showed that vast amounts of black gunge had collected on it. Removing the gunge cured the problem briefly but the gunge soon returned to sabotage the printer again.
Hewlett-Packard's solution? Send the printer to them for repair - along with a cheque for £138 plus VAT!
The Guru's Solution? Use the cotton buds on long sticks, as supplied in a PC cleaning kit, to wipe the muck of ages off the mechanism which seals the print-heads when they are parked and prevents ink evaporation. Once that reservoir of jet-clogging gunge was gone, the problem went away.
The Result? A printer that was working again and £138 plus VAT saved.
So it looks like it's always a good idea to think about a problem and work out what could be going wrong as an alternative to throwing cash at PC peripheral manufacturers.