Temperature Detection Section; Fuser Unit Error Status Counter Control; Temperature Detection Circuit - Toshiba e-STUDIO 5540C Service Manual

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Temperature detection section

6. Fuser unit error status counter control

• To enhance the safety of the fuser unit section, the engine CPU provides the following protection:
When a third [C411] error has occurred after two consecutive [C411] errors, the heater lamps are not
turned ON and an error [C412] is displayed immediately even if the operator turns the power OFF
and then back ON. However, if the equipment goes into the ready state normally with the fuser unit
error status counter value "1" or below, the counter is cleared to "0".
• If any of the error codes [C411] to [C490] is displayed and still not cleared even though the power is
turned ON after the thermistor, thermopile, thermostat, heater lamps and other parts were repaired,
check the fuser unit error status counter value in the Setting Mode (08-2002) to clear the value to
"0".
Remarks:
The fuser unit error status counter (Setting Mode (08-2002)) never has any values other than 0 to
71.
• If the does not turn ON and the service call [C411] or [C412] is displayed immediately after the
power is ON, ensure the fuser unit error status counter is "2" or over. If it is "2" or over, be sure to
check the thermistor, thermopile, thermostat and heater lamp. Reset the counter to "0" after
repairing them, then turn ON the power.
• If the fuser unit error status counter is "71" or over (e.g., 80), the data in SRAM itself may possibly
have been ruined due to causes such as leakage from the chargers.
Check the bias, high-voltage transformers and needle electrodes to see if any of them is defective,
and also look through all the data.
• When the thermistors or thermopiles detect overheating, the engine CPU decides the error code
and counter value of the fuser unit error status. After turning OFF each output (the heater lamp,
exposure lamp, control panel display, motors and so on) to protect the fuser unit, the engine CPU
shuts off the power supply over all the parts, except for the control panel.
Error code: C449, C468 ([C] and [8])
Counter value of the fuser unit error status: 8, 9, 18 to 23, 25 to 29 (08-2002)
Thermistors and thermopiles continue detecting the abnormal temperature even after the error
codes and counter values are decided. Even if the main power switch is turned ON immediately, the
power supply over all the parts, except for the control panel, is shut off when the surface
temperature of the fuser belt is still higher than the abnormal temperature detected.
Wait until the surface temperature of the fuser belt is lowered enough, and turn ON the power to
check the counter value while it is turned OFF again. After confirming that it is the fuser unit
abnormality, correct the abnormality and reset the counter value (08-2002) to "0" to start up the
equipment normally.

7. Temperature detection circuit

The thermistor is a device whose resistance varies according to the detected temperature, and the
thermopile is a device whose output voltage varies according to the detected temperature. The ASIC
detects voltages output from these devices, and judges whether the operation of the fuser unit is
normal or abnormal from the changes in voltages.
If one of the thermistors and thermopiles is broken, the control circuit judges that the temperature of the
fuser belt or pressure roller is extremely low and keeps turning the heater lamps ON. As a result, the
temperature of the fuser belt or pressure roller rises, and possibly activates the thermostat which is a
safety protection device. To prevent this in advance, the ASIC works to detect whether each thermistor
and thermopile is broken or not.
Also, the control circuit constantly checks the temperature of the heat roller and the pressure roller to
prevent them from excessive heating by circuit abnormality or thermistor abnormality, and automatically
shuts OFF the power when one of these temperatures exceeds the specified temperature.
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