Epson FX-870 Service Manual page 117

Principles of Operation
4. As soon as the current through the coil is switched off, the force induced in the coil rod stops.
The plate then returns to its original position (the position assumed before the coil was
energized) through the action of the plate spring. After the dot wire hits the platen, the
rebounding force of hitting the platen works with the wire resetting spring to pull the wire back
to its original position.
Figure 6-1 shows the action of the printer mechanism when a single dot is printed.
Wire Resetting Spring
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Actuating Plate 7
Actuating Plate Spring
The printhead tends to heat up after a period of continuous printing. To minimize the dot wire drive
coil's overheating, the head is equipped with a thermistor that can detect the head temperature.
When the thermistor detects changes in the printhead temperature, the voltage signal changes.
The CPU detects the voltage change and stops printing until the head temperature cools.
The printhead is also used as a buzzer. Head driving coils move all the dot wires back and forth
without impacting the platen, so that the wires vibrate. The vibrating dot wires sound like a buzzer.
6-2
Dot Wire
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Figure 6-1. How the Printhead Works
Epson FX-870/1170