Printer Faults - Colour Registration

Colour printers need to keep the four spot colours: cyan, magenta, yellow and black aligned to better than a 50th of a millimetre to achieve 1200dpi printing. (Probably to as little as 5 microns to make sure the four spot colours all line up reasonably well). The cartridges have to slide into their respective slots easily enough - which suggests a little bit of play in the fitting. The cartridges are moulded plastic so their mechanical alignment is not likely to be perfect.

Most recent colour laser printers have a registration sensor (sometimes it's called a misregistration sensor). The printer runs calibration when it is first used and periodically thereafter. Colour calibration can be started at other times. (see below)

Calibration prints patterns on the transfer belt, then rotates the belt under the registration sensor which looks for their exact position. By measuring the position of the patterns the printer can then set the timing of the laser scans and the movement of the belt so that anything printed will be registered. The sensor is essentially a camera and it is likely to be used to make colourimetric measurements as well, so that the printer can set the voltages that control print density. Calibration is the process that can be heard when a colour printer is powered up - it takes some time and does use a bit of toner - for instance, it clears oxidised and exposed toner off the drums.

After some operation the temperature will have changed and the belt may have stretched a bit so the registration will drift out. If the cartridges are removed and replaced their position will be different and registration will be out. Every time an event like this happens and every couple of days otherwise the printer will re-run calibration.

Colour misregistration is easily spotted in a line diagram. Photographs just look poor but that could be resolution or an issue with the computer settings. Rather than try to puzzle it out get a standard page that definitely will show the fault - most laser printers provide a configuration page that has a test pattern on it.

The test for registration is usually something like fine lines in the corners - the red, green and blue lines should look solid, blue should not have cyan and magenta fringes. If the printer can produce a good configuration page then any problem with apparent misregistration and quality is elsewhere - in the computer print drivers or software.

Use the configuration page or whatever has block and line test patterns - not the demo page. Demo pages tend to have photos or suchlike and don't give enough diagnostic information.

If the colour registration seems to be out the user might need to force calibration. That is usually done via the control panel or some computer utility like "HP ToolboxFX" and tends to be a bit different for each printer.

Color LaserJet 1600/2600n/2605dn

Select Button
Right Arrow to System Setup, Select
Right Arrow twice to Print Quality Select Twice
Right Arrow to Calibrate Now, Select

Color LaserJet 300/3600/3800 3500/3700 4600/4650 5500/5550

Menu
Down to Configure Device, Select
Down to Print Quality, Select
Down to Calibrate Now (or Fully Calibrate Now), Select

See the user manual for details. There may be different levels of calibration and some of them might only be documented in the service manual.

Calibration sensors work next to loose toner and will get contaminated. The printer ought to detect this and give an error. (Ideally it might detect it and clean the sensor or at least prompt the user to do it). In practice printers sometimes just don't calibrate properly.

Adjusting Colours

Most colour laser printers use a sensor to test the density of the colour print and set the precharge and developer voltages accordingly.

Printer drivers often have manual settings to allow the user to change the density of colours. Not all printer drivers offer this; that might be because of doubt as to whether it is wise to change any one colour.

Other balances that might be adjusted include the treatment of greyscale in text and in graphics.