Epson dot matrix print head troubleshooting

Epson dot matrix, print head technical information.

The basics, that apply to all dot matrix heads. (FX, LX, LQ and DFX models.)

The most common print quality problem with dot matrix printers is a, "missing horizontal line of dots." For business applications, such as invoicing, the pins responsible for underlining, or crossing letters like A, E, H, etc., will usually fail first because they’re doing more work than the others are. The pins simply break off and the remaining portion of the dot wire is too short to impact the paper. If nothing else is damaged, repairing the problem is as simple as replacing the head. There are, however, other reasons a pin may fail to strike the paper!

While a broken pin is the most common reason a row of dots does not appear, it’s also possible that the signal to fire the pin is not reaching the print head. If the ribbon cable(s) connecting the print head to the main board is damaged, one or more of the signals to fire a pin will not reach the head. This can happen if the ribbon cable snags something while the carriage unit is moving back and forth. We've also seen head cables simply wear through from rubbing against the bottom of the ribbon cartridge after years of use. Therefore, the head cables should always be examined for any obvious flaws before concluding that the head has failed. Better yet, unplug both ends of the cable and test continuity with a meter.

Aside from examining the head cables, the second basic step in diagnosing a dysfunctional dot head is measuring the resistance through the pin solenoids. A 9-pin head (FX or LX model) has 9 individual solenoids. A 24-pin head (LQ model) has 24. Checking solenoids requires Volt-Ohm meter and, in most cases, a technical manual. Compass Micro is not allowed to distribute technical information of any kind. 

Checking solenoids may sound like overkill, but this is where you can get in serious trouble replacing a print head! Right after "broken pins" and "damaged head cables" on the list of why print heads don’t print correctly is, "failed driver circuitry." This is the reason Compass Micro will not accept a returned print head – damaged driver circuitry is bad news! Measuring resistance through the head solenoids will help reveal a problem with the driver circuitry on the main/driver/logic board. A solenoid that measures very high or very low resistance – relative to the norm given in the tech. manual – indicates there is a problem with the transistor responsible for firing it. If a new head is installed prior to addressing the defective driver circuitry, it will be rapidly damaged in the same manner as the old head. (Sometimes in dramatic fashion!)

If reading schematics and using a meter aren’t part of your job description, there's an easy way to find somebody better qualified. Call Epson’s automated referral system at 800-442-2110 to locate an authorized service provider based on your zip code.

Specific information regarding DFX series printers.
(DFX-5000, 5000+, 8000 & 8500)

The print head cables are permanently attached to ALL DFX series printers. If one is damaged, the head must be replaced.

With the exception of the DFX-5000, adjustments must be made after replacing a DFX series print head. This requires a technical manual and wide format (14 inch) paper. These adjustments write the new head’s "alpha" value to memory on the logic board. This value is related to the printer's automatic head gapping routine – setting a correct gap for thick and thin paper – and important to the life of the new print head. The adjustment procedures for the DFX-5000+ and DFX-8000 arerelatively easy to get through but the DFX-8500 is difficult. (Fair warning!) We strongly advise ordering the technical manual first, so you can read about ohming the print head and making the necessary adjustments before purchasing a print head.

Ohming each head solenoid is very important when troubleshooting a DFX printer. This is especially true if the printer is giving an error even remotely related to a, "driver abnormality." We have talked to many a customer that has replaced a DFX print head only to discover (the hard way) that there is something more wrong with the printer.

And, again, if you prefer to "play it safe," call Epson’s automated referral system at 800-442-2110 to locate an authorized service provider based on your zip code.



Technical Tips are informational and supplied as-is.  Compass Micro assumes no responsibility for their correctness or their  suitability to your particular purpose.