Canon Pixma MP640

The Pixma MP640 is an A4 inkjet all-in-one printer. It was a sensible buy, 5 separate inks so cartridges can be replaced individually. The printhead was intended to be "semi-permanent" - it is replaceable but it handles all five cartridges and prints at a fairly impressive photo quality so its an expensive piece of work. A new printhead is something like a third of the price of a printer of the same quality.

Separate cartridges with a single printhead unit is a compromise. Separate printheads would be better because only one would need replacing when nozzles block. Separate printheads tends to be for a different price class of printer - one meant for heavy workloads.

Inkjet printheads will innevitably block. To get that photo quality using one picolitre drops the printhead nozzles are just a couple of microns across. Each nozzle fires thousands of times per second, raising the ink droplet above boiling point momentarily so that a bubble bursts out and onto the page. The slightest impurity in the ink will block a nozzle. Other things can block nozzles, a tendency for salts from the ink composition to cling to the heater surfaces for instance, called "kogation". The ink itself has to withstand heat and pressure cycles without denaturing, or congealing. Canon Chromalife 100+ is intended to have a life of 100 years or more without noticeable fading on the page. In other words, when you bought the printer, you got a miracle of modern engineering and chemistry.

Printer manufacturers like Canon emphasise that you must use their proprietary cartridges. They would say that; they make their profit selling ink. It is generally reckoned that the profit to the manufacturer on selling ink is something like 80 percent; which is quite high for any goods and exceptionally high for IT (where there is lots of competition).

Printer users naturally get tempted by ink refills and clones. (Both as printer users, and ink resellers, we are ourselves). The problem is that ink chemistry is unlikely to be quite right so colour balance and gamut could be poor. The probability of printhead failure goes up. There are times when this might be acceptable, when you have thought of buying another printer anyway, or if you are merely trying to print at the lowest cost regardless of potential trouble. After all, new printheads for the MP640 aren't that expensive compared with professional print.

The inkjet nozzles themselves are just about invisible to the naked eye. Nothing commonly available could be used to unblock them, they are much finer than a human hair. (If you have a micromanipulator and a supply of carbon nanotubes it would still be difficult). If several cleaning cycles don't work and the print remains streaky or gets worse then the chances are you are dealing with a blocked head.

Some printers have electronic or optical methods to detect and report whether the printhead is producing droplets correctly. The MP640 doesn't have this ability.

Nobody can promise you that a replacement printhead will definitely cure a problem. Here are problems to look out for.

Other problems likely to produce streaky print are bad connections between the printhead and the carriage and splits in the trailing cable connecting the carriage. In principle, it is possible to design printer electronics to report breaks in the communication with the printhead but we don't know whether the MP640 can do that; many low cost inkjets either can't do it or don't report it.

If the printhead connectors look clean then they are probably working properly. It is worth reseating the printhead to see if that resolves the problem. Bad connections are a common problem.

If the printhead connectors have ink or other contamination on them they need cleaning. Normally the advice is not to touch the contacts because of the potential for contamination or static damage.

If the connectors have marks on them then they need cleaning. Damp tissue is often the material available in a household or small office; it isn't ideal, static damage and tarnish are possible. The printhead is probably going to be replaced if the printer isn't scrapped so damaging one that isn't working anyway is a risk we might discount. The connectors in the carriage need some care; it is difficult to replace.

It is also worth taking a close look at the carriage cable itself. This is a "Flat Flexible Cable" (FFC) with multiple flat copper paths covered in plastic - usually polyester. The cable itself will ultimately wear out, splits can develop in the plastic or metal fatigue points in the copper conductor. Faults in the plastic may be visible, those in the metal are difficult to spot and can be intermittent. However the FFC on the MP640 is unlikely to have bee worked to the point of failure and anyway it requires the whole carriage assembly to be replaced - not an easy job. If the cable looks damaged a new printhead could be a waste of money.

Another possibility is the purge unit, which is intended to remove day to day soft blockages from nozzles. The printer puts the head on the purge unit and pumps ink as it applies a bit of suction to remove blockages. The printheads sit on rubber cups on the purge unit and there is a rubber wiper to wipe wet ink on the head, if either of those are damaged things won't work properly. the soft rubber parts are a bit difficult to examine; they tend to have a build-up of congealed ink. There is often a congealed ink hill which shouldn't be tall enough to reach the printheads (it will persistently contaminate them) and the cups and wiper should look wet not dry - if they are dry then so is part of the printhead.

If the connectors, cable and purge unit look OK then the fault really does seem to be the printhead.

An old printhead unclips and lifts out. When you purchased the printer installing the printhead was part of the setup procedure, taking it out is just the reverse. The pictures here derive from various Canon documents and give an idea what to look for.

Note that the process of setting up a new head uses quite a bit of ink, and an alignment procedure is required. This follows the procedure in the user manual for setting the printer up in the first place.

Most of the people who try a new printhead get their printer going again. However before buying a new head more frugal souls might like to try the ideas in the box to try to get the old one going.