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Service Guide ML172/ML182 Series
Chapter 2 Principles of Operation
2.1.2 Control PCB
The Control PCB contains the microprocessor and related circuits, DC power circuit, motor drive circuits,
paper end sensor and interface connector(s). If a serial interface is installed, it will be connected to the
Control PCB. The DC power circuit converts the AC input from the power supply board into +8 VDC, +5
VDC and +30 VDC to be used throughout the printer.
The Control PCB also contains the following two protective circuits:
a) Driver Fault - Alarm Circuit
This circuit causes the AC fuse (on the power supply board) to open when a fault occurs in the printhead
drive circuit, linefeed motor drive circuit or the spacemotor drive circuit. If the drive time of any drive circuit
exceeds a pre-determined time, an alarm condition will exist as follows:
HDALM - Printhead drive circuit alarm
SPALM - Spacemotor drive circuit alarm
LFALM - Linefeed motor drive circuit alarm
When any of the above signals become active, the Driver Fault - Alarm Circuit will output the ALM signal.
This signal then turns ON an SCR (which is located on the Control Board). When the SCR turns ON, the
secondary winding (30V) of the transformer is short-circuited. This induces an over-current condition
through the primary winding of the transformer causing the AC fuse (F1) to open.
b) Head Overheat - Alarm Circuit
In order to protect the printhead coils, this circuit monitors the head temperature using the printheads
built-in thermistor. When the temperature of the printhead exceeds 100 degrees Celsius, a head overheat
alarm (HEAD TEMP) is generated. This alarm causes the printing operation to stop until the printhead
cools. Once the printhead temperature returns below the alarm detection threshold, the printing operation
is restarted.
Copyright 1997, Okidata, Division of OKI America, Inc. All rights reserved. See the OKIDATA Business
Partner Exchange (BPX) for any updates to this material. (http://bpx.okidata.com)