Toshiba 4580, Service Manual

Page: 253 / 379

6570/5570 FUSER UNIT 14 - 6 Mar. 1999 © TOSHIBA TEC
R165
+5
R170
R174
C125
HTRTH-
Thermistor-1
Main
Processor
14.4.2 Detection of thermistor burnout
* Since thermistor burnout detection circuits of the thermistor-1 and -2 are common, it is explained with
the thermistor-1.
•
The input voltage is obtained by voltage dividing
R165, thermistor, R170 and R174.
•
The surface temperature of the heat roller
changes.
→
The thermistor resistance value changes
→
The input voltage also changes
•
The main processor detects an abnormality
→
thermistor broken
14.4.3 Control for abnormal heater condition
When something abnormal occurs in the heater turn-ON circuit, such as a shorted triac, it may become
incapable of ON/OFF control. In this case, the microcomputer detects abnormal temperatures through
the first and second thermistors, determines the corresponding error codes and counter values, and
causes the main power to switch OFF to protect the fuser unit.
(1) Temperature detection
The following abnormal temperatures are detected by the respective thermistors:
First thermistor: 230°C
Second thermistor: 270°C
(2) Error codes
“C44” : Displayed when keys C + 8 are pressed simultaneously at the time of Call
Service.
Counter value “9”: Displayed when you input “400” in the setting mode.
(3) Machine condition
After de-energizing all outputs (heater lamp, exposure lamp, control-panel displays, motors, etc.),
the microcomputer causes the main power to switch OFF.
(4) Corrective action
After the error code and counter value in (2) have been determined, abnormal temperature detection
in (1) continues. So, if the main switch is turned ON immediately, it will be turned OFF again as long
as the temperature on the heater surface remains above the abnormal detection temperatures of
thermistors. Thereafter, this will be repeated. So, after waiting for the heater surface temperature to
drop, turn ON the main switch and you can check the counter value before the main switch is turned
OFF again. After confirming that an abnormal heater condition has occurred, repair the problem and
then clear code 400 of the setting mode with “0”. After that, you can bring the copier into its normal
operating condition.