Laser Printer Faults - Grey Background

Grey backgrounding can be one of those annoying faults - it is most likely due to a bad cartridge, but could involve any of half the parts in the printer.

First note that:

• Very dry conditions change the static field and can increase the background shading.
• Paper that has previously passed through the printer will be rather dry and may be inclined to give a grey background.
• Heavy media can cause speckling and greying.

Background toner is likely to be excessive DC bias on the developer, but reducing the DC bias too far is likely to cause mottling on the print

What Happened Last?

As a rule of thumb the last thing that happened is the cause of a current fault. If you just changed the print cartridge then try changing it back (assuming the old cartridge has a page or two of remaining life). If the fault changes with the cartridge that is half the diagnosis done - the issue seems to be the cartridge, not the machine. It doesn't necessarily mean the cartridge is bad - see below.

Print Density, Mono Printers.

Most likely the print density setting is too high and pushing background shading from the odd specs to visible.

Note the current density and media settings in your print driver. Make sure the media settings do correspond with what you are printing.

Try changing the toner density to lower and if that doesn't work try higher - the settings to get best contrast and lowest backgrounding can be counter-intuitive.

At one time it was common to change the darkness setting a bit on installing a new cartridge. These days cartridges are more consistent but it is possible to have good cartridges that need an adjustment in the settings.

If the toner density is turned up high enough to give backgrounding then areas of print should be very black. Toner density is normally a user setting that will override the printer setting. Some older printers had adjustable potentiometers to change the print density - modern printers may have them as well but they are normally factory preset so shouldn't need adjusting.

If there is backgrounding but actually the text doesn't look all that good, that suggests a bad photoconductor drum, transfer roller or voltage.

Print Density, Colour Printers.

Setting the print density on colour printers would be a fiddly job with four different adjustments to make. Rather than ask the user to do this manufacturers have opted to put a colour density sensor on the printer - essentially a camera pointing at the transfer belt. During its calibration procedure the printer "reads" part of the image placed on the belt and analyses it, then makes appropriate adjustments.

Cartridges.

There is no real test for cartridges except to try another.

With a new cartridge the drum's photoconductive surface (the OPC) should look shiny and new - they do vary in colour from shiny green to dull blue-green but they tend to get scuffed particularly around the edges as they are used. There is usually a shutter over the drum to keep light out but they usually push out of the way with finger pressure. Don't touch the OPC, that will de-nature it.

Don't leave a cartridge in the light even with the shutter closed for a long period. Exposure to light can cause the problem outlined here. If a cartridge has to be stored put it in one of the opaque bags that come inside the toner box. If there is no opaque bag use a heavy polythene bag like a bin liner and a box. (Yes there is a concern about polythene zapping the chip, if you have an anti-static bag it would be better).

A cartridge will give some backgrounding when the OPC is past its life, so refilled or recycled cartridges could do this. However it is worth experimenting with the toner density adjustment. Obviously refilled cartridges are less likely to be calibrated exactly like manufacturers originals.

Contact Faults.

Many problems of shading in a laser printer lead to a question of contact cleanliness.

On the backplane of a computer are sockets with gold-plated pins to stop them tarnishing, self-cleaning mating actions and protection from finger-prints and dust. On the side of a cartridge electrical contacts tend to be bits of tin plate and sticking-out axle ends mating with bendy metal springs.

It's usually possible to spot the contacts by looking at the inside of the printer - there are usually about four or five recesses with spring contacts in them. The contacts shouldn't be carrying high voltage with the lid open, a safety cutout should have operated. If you have any doubt unplug the printer.

The best way to clean a contact is usually with a cotton bud dipped in isopropyl alcohol. Slightly damp tissue can also be used. Wiping with a finger will also work but if your hands are sweaty you have then contaminated the contact with corrosive salt mixture so it will tarnish more quickly in future.

If a contact does look tarnished then scratching with a screwdriver may get it going - if it's part of the printer it's probably going to need replacing in the medium run.

Minor faults like a bit of greying can also be caused by toner collecting on contacts and in the printer generally so the printer may need cleaning.

Transfer Roller.

A dirty transfer roller is more likely to cause fading than background problems but it's certainly worth checking both the roller and its housing. There is often a little tool in the bottom of the printer for hooking the roller out - you aren't supposed to touch the roller surface. There are normally instructions on how to remove and replace the roller in the user guide.

Rollers are normally dark-grey to black in colour but tend to turn white-grey when they are contaminated by paper dust. If the roller is dusty it is possible to clean it up a bit with light brushing - sufficient to see if it improves the print. Make sure the transfer roller housing and contacts are clean as well. The contacts on a transfer roller are sometimes nothing more than conductive plastic. The roller itself is a conductive rubber.

Fuser.

Some recent fusers have their own voltage feeds to the top and bottom rollers intended to keep the toner perfectly placed as the page passes through the fuser. The fuser can also become contaminated with toner and this may give speckling or greying of a page. Blurred characters might suggest a fuser voltage issue.

The fuser is not always easy to examine but parts of the rollers can be seen through the rear of the printer. There are two types of fuser heated roller - metal coated with teflon and teflon film. The rollers should look clean - if they are speckled all over then there is a problem.

Don't poke at the fuser with anything sharp - both kinds of roller are easily damaged.

There is more on cleaning a fuser elsewhere; fusers aren't very likely to be the cause of an all-over even greyness.

Stop Test.

If the printer is clean and still giving the fault then try a stop test (sometimes called a halfway test).

A stop test deliberately creates a paper jam for diagnostic purposes. The stop test can be done by setting any page to print - a configuration page generated by the printer might be best. Wait until the page is actually printing and then open the lid. If you listen to what the printer is doing it is usually possible to gauge when the paper is actually being printed. The idea is to stop the machine with a page half printed and then examine the result.

The parts of the page that have been printed but not passed into the fuser will show the image in loose toner. They should be examined to see if they show the fault - it is likely that they will but if not that suggests the fuser really is at fault.

Next examine the photoconductive drum. Since the drum was in use when it was stopped about a quarter of it will be visible and should show the print - again in loose toner. If the image looks clean and fresh that suggests the fault is the transfer roller. If the image shows the same grey background then the fault will be further back in the print process.

If everything else has been tried the fault may be with the High Voltage PSU. It is not easy to test an HVPSU. The maximum input for electronic meters tends to 600 volts. Several voltages used by laser printers are at or over that point. Most recent printers work under 3,000 volts but they may generate more and it could be voltage-limiting circuits that have failed. Metering what is happening on the HT side of a laser printer needs a high-voltage probe and they are not usually part of a field service kit.

HVPSU's are expensive and quite difficult to change so try a new cartridge first.