Canon i550 doesn't print correctly

---Is the incrustation of the heads or the ducts such that I need to throw the i550 away

This one...

I had 3 Canon i550 and i560 printer, unfortunately if you use non original inks and leave the print head with no ink cartridge for some hours the ink can dry inside the print head = dead.

I have try several methods of cleaning print head and waste a lot of time on this, finally here some suggestions:
- use pure alchool (at least 90 °) to clean the print head, you can also use clear water but after you had to rinse with alchool because alchool removes the left water that can cause shortcircuit when you power on the print head.

- the electrical connections of the print head (brown ribbon with integrated circuit path) are very fragile if you decide to unmount the print head removing the 2 phlilips screws you must be VERY careful

- for black ,one safe trick is to remove the round (8 mm) glued upper metal sheet filter on the upper part of the print head where the ink cartridge sets. Once removed you can use a syringe needle filled with alchool to remove the dry ink and spray alchool into the small duct hole to remove all the bad dry ink. Be patien it will take some time. You can also leave the syringe filled wyth alchool and without the syringe piston for a night inside the duct hole.

Take a look to the service manual also for other inspirations:
http://rapidshare.com/files/91114670/Canon_i550_i850_i950_Service_Manual.pdf.html