Canon PC720 Service Manual - Page 91
detected temperature
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Ik!;,,, ,",-i./ 011111111111M1,MII IM0PERATI0NS AND TIMING 5. Checking for Overheating at the End of the Fixing Heater The rear end of the copier's fixing heater is equipped with a sub thermistor (TH2) to check for overheating. The thermistor serves to prevent damage to the fixing film otherwise caused by overcurrent occurring in areas where paper does not come into contact while copies are made continuously on paper of A4 or small- er. In multifeeder manual mode, if the detected temperature of the main thermistor (TH1) is 190°C or less for the black developing assembly or 195°C or less for the color developing assembly, the distance between sheets of paper is increased (12 to 8 cpm), thereby decreasing the supply power to the fixing heater to prevent overheating at the end of the fixing heater. Once changed, the distance between sheets of paper is maintained until copying is over. Should the temperature at the end of the fixing heater still keep rising after increasing the distance between sheets and the sub thermistor (TH2) detects 255°C or more, copying operation is stopped after discharging the ongoing copy; at the same time, 'H' is indicated in the display of the control panel, disabling the Copy Start key. The 'H' indication turns off automatically when the detection temperature of the sub thermistor (TH2) falls below 240°C, and a press on the Copy Start key then will make the remaining number of copies. 6. Protection Mechanisms The copier is equipped with the following protection mechanisms to prevent malfunction of the fixing heater: a. Thermistor (TH1, TH2) The microprocessor on the DC controller PCB monitors the voltage of the thermistor (TH1, TH2); if it detects a high temperature error or a low temperature error, it sends the fixing heater error signal (HTNG) to the timer circuit and, at the same time, indicates an error code (E00 through E003) on the control panel. In response, the timer circuit sends the relay off signal (RLOFF*) to the auxiliary power supply about 1.5 after it has received the fixing heater error signal (HTNG). The auxiliary power supply in turn turns off the relay (RL601) to turn off the power supply. The data for problems associated with `E0', i.e., related to the heater, is backed up and `E0' will be indicated on the control panel next time the power is turned on. The data, however, is retained only about 5 min; when the power is turned off while 'E000' through `E003' is indicated on the control panel, 'EU will be indicated on the control panel if the power switch is turned on within 5 min, and the Copy Start key will not be enabled. If you have turned on the power switch about 5 min or more after you turned off the power switch while 'E000' through `E003' is indicated on the control panel, the copier will respond to a press on the Copy Start key. If an error associated with the heater is detected during copying operation, the copier once again will indicate `E000' through 'E003' on the control panel and will turn itself off. b. Thermal Fuse (FU1) If the area around the thermal fuse exceeds 226°C and remains so for a specific period of time, the thermal fuse melts to cut off the power to the fixing heater. COPYRIGHT 0 1994 CANON INC. CANON PC720/740/7501770 REV.() AUG. 1994 PRINTED IN JAPAN piPoit AU JAPAN) 3 - 51