Canon LBP-1000 Service Manual page 44

CHAPTER 2
signal detection accuracy.
When the fixing heater temperature reaches the point of 5 C lower than the target print tem-
perature, the CPU starts the print temperature control.
The CPU controls the fixing temperature in the following four modes.
1) Initial temperature control: After a print instruction command is input from the video con-
troller, the CPU brings the fixing heater temperature to the point of 5 C lower than the tar-
get print temperature.
2) Print temperature control: During printing, the CPU controls the fixing heater temperature
to be the target print temperature.
3) Between-page temperature control: The CPU controls the fixing heater temperature to be low
compared to the target print temperature between pages in order to prevent the elevation of
the temperature.
4) Postprint temperature control: Except for the paper with less than 246mm in length, the
CPU controls the fixing heater temperature to be 200 C for 5 seconds after printing.
2. Fixing heater safety circuit
This circuit is located on the engine controller PCB and constantly monitors the abnormal rise
of fixing temperature.
If the output voltage from the thermistor becomes less than about 0.7V (250 C or more) due
to temperature rise of the fixing heater, the output of No.13 pin of the comparator (IC503)
becomes "L" and Q304 is turned OFF. This turns OFF the relay (RL301) to interrupt the power
supply to the fixing heater regardless of the RELAY DRIVE signal (RLYD) output from the CPU.
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 TH701. If it is abnormal, the CPU assesses a fixing heater
failure, cuts off the power to the heater, and turns OFF the relay (RL301). At the same time,
it reports the fixing heater failure to the video controller.
• If the temperature of the fixing heater rises abnormally and the temperature of the thermal
fuse exceeds about 230 C, the thermal fuse melts and the power to the fixing heater is cut
off.
• If the temperature of the fixing heater rises abnormally and the output voltage of TH701 falls
below about 0.7V (250 C equivalent), the fixing heater safety circuit cuts off the power to the
fixing heater regardless of CPU outputs.
4. Fixing unit failure detection
The CPU assesses a fixing unit failure under the conditions (a ~ e) described below and conducts
the followings:
1) Interrupts the power to the fixing heater.
2) Sets the RELAY DRIVE signal (RLYD) to "L" and turns OFF the relay (RL301), as well as
reporting the failure to the video controller.
a. Abnormally high temperature (all modes)
The CPU assesses an abnormally high temperature when it detects the temperature of 220 C or
more for 1 second or more continuously.
b. Abnormal warm-up 1 (Initial temperature control mode)
The CPU assesses an abnormal warm-up 1 if the detected temperature is lower than 120 C for
1 second or more continuously after the heater has been energized for 20 seconds.
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