OKIDATA DOC-IT3000, Service Manual

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Service Guide DOC-IT3000/4000
Chapter 2 Principles of Operation
2.2.03 Fuser Unit
The fuser unit heater is controlled by a thermistor, a comparator, an LSI, and a CPU to keep the
heat roller surface temperature within a predetermined range (about 180 degrees centigrade).
The comparator output signals, TEMPH0 and TEMPH1, resulting from the comparison of the
voltage level associated with changes in the thermistor resistance with the partial voltage of the
reference resistance enter the LSI ports TEMP0 and TEMP1. The CPU read the status of these
ports at fixed time intervals and turns on or off the HEAT signal according to the port status to
control the temperature. The CPU also switches its port P13 output signal PREHEAT at power-on
time to check for a blown or shorted wire in the thermistor according to the status of the TEMPH0
and TEMPH1 signals then in effect.
A thermostat within the fuser unit prevents abnormal temperature rises in the fuser unit if the
thermistor fails.