Canon LBP1760P, Service Manual

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2. Fixing heater safety circuit
This circuit is located on the engine controller PCB, and constantly monitors the abnormal rise
of fixing temperature.
In case of abnormality, the output from this circuit turns OFF the relay (RL101) and inter-
rupts the power to the fixing heater. The operations of this circuit are shown below.
If the output voltage from the thermistor becomes more than about 1.2V (240
°
C) due to tem-
perature rise of the fixing heater, the output of No.13 pin of the comparator (IC301) becomes “L.”
When No.14 pin becomes “L” and Q302 is turned OFF, the relay (RL101) is turned OFF.
3. Protective functions
To prevent the excursion of the fixing heater, the printer has the following three protective func-
tions:
•
The CPU monitors the voltage of TH901. If it is abnormal, the CPU assesses a fixing heater
failure and turns OFF the relay (RL101). At the same time, it reports the fixing heater fail-
ure to the video controller.
• If the temperature of the fixing heater rises abnormally and the voltage of TH901 falls below
about 1.2V (240
°
C), the fixing heater safety circuit interrupts the power to the fixing heater,
regardless of CPU outputs.
• If the temperature of the fixing heater rises abnormally and the temperature of the thermal
fuse exceeds about 230
°
C, the temperature fuse melts, which interrupts the power to the fix-
ing heater.
4. Fixing unit failure detection
The CPU assesses a fixing unit failure in case of the conditions (a~d) shown below, and does the
following:
1) Interrupts the power to the fixing heater.
2) Sets the RELAY DRIVE signal (RLD) to “L” and turns OFF the relay (RL101), as well as report-
ing the failure to the video controller.
a. Detection circuit failure
The printer detects the power supply frequency and voltage when the fixing heater is ON.
If the power supply cycle (about 30Hz ~ about 80Hz) is not detected within one second after
power supply frequency detection starts, the CPU assesses a detection circuit failure.
b. Thermistor broken wire detection
The CPU detects thermistor broken wire in the following cases, and assesses warm-up error or
abnormally low temperature.
1) The thermistor, in the low-temperature detection mode, does not exceed about 20
°
C within
about 0.7 seconds after the heater is turned ON.
2) The thermistor, in the high-temperature detection mode, does not exceed about 50
°
C within
about 5.0 seconds after the heater is turned ON.
3) The thermistor, in the high-temperature detection mode, detects over about 50
°
C after the
heater is turned ON. Then the output voltage from the thermistor is more than about 3.4V
for more than about 100ms continuously.
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CHAPTER 2