OKIDATA ML380, Service Manual

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Service Guide ML380
Chapter 2 Principles of Operation
2.1.08 Alarm Circuits
Fault Alarm Circuit (in Power Supply Unit)
This protective circuit causes the AC fuse to open when a fault occurs in the printhead drive circuit or line
feed drive circuit, thus preventing component failure.
To accomplish this, the circuit monitors the overdrive signal of each drive circuit. If the duration of any
drive circuit exceeds a predetermined time, the appropriate signal (HEAD ALM, LF ALM1 or LF ALM2) will
be sent to the fault alarm circuit. The fault alarm circuit generates the ALM signal (high), which switches
on an SCR.
This causes the secondary coil (40 volts) of the main transformer to short-circuit, causing an overcurrent
to flow through the primary coil and open the AC fuse.
Printhead Overheat Alarm Circuit
In order to protect the printhead coils, this circuit monitors the printhead temperature by using a thermistor
contained in the printhead. The CPU senses this circuit every time a line is printed.
Whenever printing, the printhead temperature increases. If the head temperature reaches approximately
90 degrees - 129 degrees Celsius, a head overheat ALARM 1 is generated. When ALARM 1 is detected,
bi-directional printing stops. Uni-directional printing begins.
If the printhead temperature continues to increase (130 degrees Celsius), ALARM 2 is generated. Printing
stops until the head temperature decreases. When the head temperature drops below the ALARM 1
threshold, printing resumes.
Low DC Voltage Detection Circuit
The + 35 vdc drive voltage is monitored for a low voltage condition during each print operation. If the + 35
vdc drops, the negative terminal input voltage of the comparator (Q8) decreases, the comparator reverses
state, and the DC LOW-P signal is switched ON.
When the signal DC LOW-P is active, printing is accomplished by using twelve of the twenty-four
printwires in a dual-pass print.
Copyright 1997, Okidata, Division of OKI America, Inc. All rights reserved. See the OKIDATA Business
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