Articles and Technotes
Printheads Basics and Troubleshooting
Dot-matrix printers are commonly used in applications that utilize multipart paper. A receipt copy from a store is a good example. The store clerk gives a customer a receipt and he keep the other copy for store records.
The printhead is the part of the dot-matrix printer that actually does the printing. It uses a series of vertically aligned pins to form each character. Each pin is responsible for a specific part of a character. For example. A nine pin print head has a broken top pin (#1) and a broken bottom pin (#9). If you print the letter "E" , the top line of the "E" would be missing. If you print the letter "g" the bottom line of the "g" would be missing . The more pins a printhead has, the better print quality it can achieve. These pins impact on an inked ribbon to form each character.
Printhead repair is a good alternative for the money conscious consumer. Having your old printhead rebuilt is in most cases, cheaper than buying a new one. The rebuilt printhead is repaired to meet the manufactures specifications. This means that it should function in your printer, just as well as a new one. In most cases the printhead will come with a limited time warranty. The warranty will cover any problems that may occur during normal working conditions.
Faulty printheads will normally cause poor print quality or incomplete characters. When pins are missing or do not fire, a character appears to have a blank line going through it. This is because the pin that is responsible for that part of the character, is not firing. Printouts can also appear with a line going through each character. This can be caused by a sticky or bent pin. The pin keeps the ribbon pressed on the paper which causes a line. There are a few checks that can be performed, if you suspect your print head of being faulty. The first thing I recommend is removing the printhead from the printer so the pins can be visually inspected. All pins should be present and even. This is not a full proof way to determine that the printhead is totally functional. The best way is to swap the printhead with a known good one. (I know. We all have extra printheads just laying around at our disposal). If you don't have the convenience of having an extra printhead, try checking the following areas to help identify where the problem could be coming from. Who wants to pay for having a printhead rebuilt, then find out that wasn't the problem in the first place.
It is important to isolate what is causing the printer to have poor print quality. Faulty print can be caused from different parts of the printer. First make sure that the platen gap adjustment is set correctly. Most printers have a lever to move the printhead further or closer to the platen to accommodate different paper thickness. This is a common problem users tend to over look. Inspect the carriage assembly. Everything should move smoothly. Measure the printhead gap to the platen. It should be checked at the left, right and middle side of the carriage assembly. Make sure the platen is in good shape. The platen can become pitted if the printhead has been printing on it for long periods without paper loaded. The pins need a flat surface to impact on, to properly form a character. A faulty printhead cable can cause dots to be missing intermittently. Checking a printhead cable for continuity with a meter is not the best way to be sure the cable is good. I have seen technicians make this assumption many times. Replace it. The last thing to check that can cause pins to be missing or sticking is the driver board. If you are not sure which board has the pin drivers, follow the printhead cable. The driver board must be replaced with another one to be sure it is not the cause.
This information should give a basic understanding of how printheads work and how to troubleshoot print quality problems.