Classic Computer Magazine Archive COMPUTE! ISSUE 137 / JANUARY 1992 / PAGE 68

You look marvelous. (laser printouts) (Column)
by Mark Minasi

I just finished developing my new course on fixing laser printers, so I've got printers on the brain.

Lasers are no longer something owned only by the rich and powerful. An HP LaserJet series IIP can be had for about $800--not much more than I paid for an Okidata 93 dot-matrix printer back in 1983. And the IIP/IIIP line is well worth considering. The IIIP actually prints graphics faster than the III. The HP printers are reliable and produce high-quality output. But sometimes problems can creep in, and sometimes we do things that invite problems.

How a Laser Printer Works

Laser printing is a multistep process. Understanding the process is more than just a techie exercise; it's essential to understanding what can go wrong and how to fix it.

I'll explain this in detail latter, but here's basically how it works. First, the printing drum is cleaned, and an image is painted onto it with electrostatic charges. Then print toner moves to the charged areas, and the toner is transferred to a piece of paper. Finally, the toner is permanently fixed onto the paper with a heated metal roller.

The heart of the print process is the photosensitive drum, an aluminum cylinder coated with a photosensitive material. The drum's job is to pick up laser printer toner--a fine black dust that's the ink of the laser printing process--and deposit it on the paper. The drum turns during the printing process. As it touches the paper, it transfers the toner (and therefore the desired image) onto the paper.

Before that can happen, however, the drum has to be physically and electrostatically cleaned. There's a metal blade called the cleaning blade that gently scrapes across the drum, removing any stray toner particles. Then a bright light called an erase lamp shines on the drum, essentially blanking the drum and erasing any prior images. A uniform negative charge of -600 volts is then applied to the drum, preparing it for the new image to come.

That -600-volt charge is applied by a very important thin wire called the primary coronal located in the disposable laser cartridge. The corona must actually emit a -6000-volt charge in order to get the -600 volts applied to the drum. The drum is now clean and ready to receive the image. The image is drawn on the drum with a mirror that directs a narrow laser beam across the drum. Anywhere the laser touches the drum changes in voltage from -600 volts to -100 volts. The drum then rotates past a fine layer of toner particles. The toner particles are attracted to the -100-volt areas; they prefer more positive voltages, and -100 is more positive than -600. By the way, the voltage can be adjusted at this point with the toner density control inside your laser. More or less voltage makes for a darker or lighter image.

The desired image now exists on the drum in the form of fine toner particles. Toner is about 50 percent iron oxide and 50 percent plastic. You can actually get toner out of materials by rubbing a powerful magnet across the material. Next, the laser printer transfers the toner to paper by giving the paper a strong positive charge (+600 volts). That charge is applied by the transfer corona, another important thin wire permanently mounted in the printer. The toner then jumps from the drum to the paper. Once the toner is on the paper, the paper run past the static charge eliminator, which reduces the paper's charge.

Now the image is on the paper. But it's only rendered as dusty black toner on a page--touch it, and it'll smear. That's why there's one more step: fusing. The paper moves past a heated metal roller called the fusing roller that melts the toner onto the papaer, fixing it in place. The fuser is kept at 180 degrees Celsius (356 degrees Fahrenheit), explaining why the paper is so hot when it comes out of the laser printer. By the way, it's the fusing foller that makes you wait when you turn you laser printer on. The system doesn't call itself ready until the fuser has reached at least 160 degrees Celsius.

Symptoms and Solutions

Now that you know how a laser printer works, you can see what can go wrong. There are lots of potential problems, but I only have room to cover the most common ones.

* Symptom: vertical white streaks on the page. Since the paper is transported from top to bottom through the laser printer, the paper also passes the coronas from top to bottom. If a part of the corona is covered with toner, it can't transmit all of its charge, leaving either the drum (if it's the main corona) or the paper (if it's the transfer corona) with insufficient charge. That leads to a vertical stripe with little or no charge, thereby leaving no toner--or a white stripe on the paper. The solution is to clean the coronas.

As I mentioned earlier, the main corona is in the toner cartridge, so if you're lazy, you can simply change the cartridge. Otherwise, you can use the brush that's located inside your laser printer (at least, HPs have them) to clean your corona. Take a look inside your HP manual for details.

The transfer corona sits in a metal trench inside the laser printer. To see it, open your laser printer (I'm talking II, IID, III, or IIID here--you can't get directly at the transfer corona on a IIP or IIIP). Fairly close to the front of the printer, there's a metal trench that runs the width of the printer. It's protected with a webbing of monofilament threads. Shine a flashlight into the trench, and you'll see a hair-thin wire. That's the transfer corona. Dip a Q-Tip into some rubbing alcohol and carefully clean it end to end. (As my friend Brock Meeks says, "Once you clean the Corona, it's Miller time.")

* Symptom: smearing on the page. What keeps the toner from smearing? The fusing roller. It's covered with a Teflong-like coating to keep stuff from sticking to it, but it can become scratched, or junk can become baked onto it. In either case, the heat doesn't get transferred to the page. Try cleaning the roller with a soft cloth and some alcohol, but let the thing cool down before you mess with it!

You can also get smears when you try to print double-sided on lasers that are designed only to print single-sidded. It's tempting to create double-sided documents by running paper through the laser twice, but it's not a good idea. For one thing, there are rubber rollers that grip the paper in order to pull it through the printer. Ordinarily, they grip the underside of the paper and cause no trouble. But if you're printing on both sides, they end up gripping the underside of the paper--even though the underside of the paper has printing on it. The rubber rollers smear the already-printed side.

* Symptom: horizontal streaks on the page. If you see a regular horizontal line on your output, it's probably caused by an irregularity in one of the many rollers that the paper must pass by on its journey from the paper cartridge to the output bin.

To identify the roller, you'll need to measure the distance between the lines. If the horizontal lines are always spaced the same distance apart, then that distance is the circumference of the bad roller.

Use the following numbers as a handy-dandy key. Just measure the distance between the regular horizontal lines with a ruler, and then read off the name of the bad roller. Whether or not you want to try to replace the problem child is up to you; getting to some of those rollers is a bit hairy. In my experience, however, the most common distance is 3.75 inches: the circumference of the photosensitive drum.

* Symptom: black line down the side of the page. I don't know why this happens, but you see it when the toner is low. Replace the cartridge.

* Symptom: paper jams. Trying to print double-sided can cause problems. The first time you run the paper through the printer, the paper is given a slight curl. Turn it upside down and run it through the printer again, and that slight curl can become a paper per jam. Another cause of paper jams is printing on the wrong side of the paper. There are, believe it or not, two different sides to a sheet of paper, called the wax and the wire. Paper will have a "print this side up" indication on the wrapper--pay attention to it. Paper can acquire a curl in humid environments, but the wrapper keeps the paper dry, so don't take paper out of the ream until you're ready to use it. Using cheap paper can also lead to paper jams. In addition, old laser printers can have rollers that are no longer perfectly round, leading to jams.

Looking Your Best

Now you know what to do when something goes wrong. How can you make sure that everything goes right? First, clean the coronas. And use a fresh ream of paper, not one that's been sitting in your laser printer's cartridge for the last two weeks soaking up moisture and developing a curl. Distribute the toner in the cartridge. Take the cartridge out of the printer and rotate it 15 times. Then shake it side to side 15 times. To help the laser's toner-transfer process, you can clear its throat by printing three to five totally black pages.

You can do that with a short LaserJet program. I'll be discussing laser language in an upcoming column, but for those who know how to use it, keep this command sequence handy: &10E &10L * p0x0Y *c2400a3300B*c0P E. That will print a black page.

Next time we'll learn how to speak the mystical PCL5.