CHAPTER 6 FIXING SYSTEM6–6 COPYRIGHT © 1998 CANON INC. CANON NP7161/NP7160 REV.0 AUG. 1998 PRINTED IN JAPAN (IMPRIME AU JAPON)2. Surface Temperature of the Fixing Upper RollerThe main thermistor (TH1) and the sub thermistor are arranged nearby at the rear of thefixing upper roller, and are used to monitor the difference in voltage between two thermistorsignals (TH1, TH2).If either thermistor suffers an open circuit, the CW input terminal of the differential amplifiercircuit will be about 5 V. At this time, the voltage of the other thermistor will decrease as thesurface temperature of the fixing upper roller increases, gradually increasing the output of thedifferential amplifier circuit.When the output of the differential amplifier circuit exceeds the value set on the comparator,the comparator will send a signal to the gate array. When the gate array detects the signal fromthe comparator for a specific period of time, the DC controller PCB will cut off 24 V and ACpower supply and, at the same time, will indicate ‘E000’, ‘E002’, or ‘E003’.If the connector of the main thermistor is not connected or both thermistors suffer an opencircuit, the microprocessor will not be able to detect any excess increases in the surfacetemperature of the fixing upper roller, causing it to indicate ‘E000’, ‘E002’, or ‘E003’ on thecontrol panel.3. Activation of the Fixing Heater (H1)The fixing heater error signal (HEAT_ERR) from the AC drier is monitored by IC113 on theDC controller PCB, and its reading is compared against the fixing heater drive signal(HEAT_DR) to find out whether the power to the fixing heater is normal or not.If the microprocessor on the DC controller PCB identifies the state of the fixing heater to befaulty, it will cut the power supplied to the fixing heater and, at the same time, will indicate‘E001’ on the control panel.