Service manual

Chapter 8
8-4
8.3 Protective Functions
8.3.1 Outline
0006-4147
The fixing heater safety circuit is part of the DC controller PCB, and is used to monitor the fixing temperature for an
error at all times. If the output voltage for the main thermistor or the sub thermistor reaches about 0.37 V or lower
(about 310 deg C/590 deg F), the relay will be turned off regardless of the state of the relay drive signal (RLYD) from
the CPU to shut the power to the fixing heater.
If the temperature of the fixing heater increases abnormally, on the other hand, to exceed about 230 deg C/446 deg
F, the thermal fuse will melt to cut the power to the fixing heater.
8.3.2 Detecting a Fault in the Fixing Assembly
0006-4151
The CPU on the DC controller PCB will identify a fault in the fixing assembly if any of the following conditions (a
through f) occurs; as a result,
- it will cut the power to the fixing heater.
- it will cause the relay drive signal (RYLD) to go '1' to turn off the relay and, at the same time, will communicate
the presence of a fault to the image processor PCB.
a. Overheating 1 (all conditions)
The CPU will identify overheating 1 if the main thermistor detects 230 deg C/446 deg F for 1 sec or more
continuously.
b. Start-Up Error 1 (start-up)
The CPU will identify start-up error 1 if a level of temperature lower than 120 deg C/248 deg F is detected for 1 sec
or more continuously 10 sec after the heater is supplied with power.
c. Start-Up Error 2 (startup/paper passage)
The CPU will identify start-up error 2 if a level of temperature 15 deg C/59 deg F lower than the paper passage
temperature control target is not detected at all 75 sec after the heater is supplied with power.
d. Temperature Control Low Temperature Error (paper passage/sheet-to-sheet/rest heating/temperature
control)
The CPU will identify a temperature control low condition error if a level of temperature lower than 120 deg C/248
deg F is detected for 1 sec or more continuously in paper passage, sheet-to-sheet, rest heating, or temperature control
state.
e. Main thermistor Open Circuit (all conditions)
The CPU will identify main thermistor open circuit if the A/D conversion value of the output voltage from the main
thermistor is in excess of a specific value for 1 sec or more.
f. Overheating 2 (all conditions)
The CPU will identify overheating 2 if the sub thermistor detects 300 deg C/572 deg F or higher for 0.2 sec over