Mechanical Operation; The Printhead Mechanism And Its Operation (See Figure 2) - OKIDATA ML1120 Maintenance Manual

2.3

Mechanical Operation

2.3.1 The Printhead Mechanism and Its Operation (see figure 2)

The print head is spring-loaded, utilizing a permanent magnet, and can be easily removed or
installed. The print head is mounted on a carriage that runs parallel to the platen and is
connected with the control circuit via the head board.
The print head consists of:
(a) Wire guide
(b) Print wires
(c) Armature assembly
(d) Yoke
(e) Springs
(f) Spacer
(g) Magnet assembly
(h) Thermistor
(i) Printed-circuit board
(1) Print head operation
When the print head is in the non-printing state, each armature is attracted by the
permanent magnet, and the springs holding the armatures are compressed by the thick-
ness of the spacer, The print wires, which are fastened to the individual armatures, are
therefore held retracted within the wire guide.
When signals corresponding to a character to be printed are detected by the control circuit,
currents flow through the corresponding coils to nullify the magnetic flux generated by the
permanent magnet between the armatures corresponding to those coils and the permanent
magnet pole. As a result, those armatures are driven toward the platen by the force of
the armature springs, and the print wires fastened to those armatures eject from the tip
of the wire guide and strike the paper through the ribbon to print dots on the paper.
After the character is printed, the magnetic flux of the permanent magnet attracts the
armatures again so that the print wires retract into the wire guide.
The print head has a built-in thermistor to prevent the coils from overheating and burning
due to continuous bi-directional printing over a long period. If the coil temperature exceeds
the limit (approximately 100 degrees C), the control circuit detects the thermistor signal
and stops the printing operation until the coil temperature drops below the limit.
43471801TH Rev.1
Paper
Ribbon
Wire guide
Figure 2
Oki Data CONFIDENTIAL
Amature assembly
Thermistor
Print wire
Core
Yoke Magnet coil
12 /