Canon LBP-3000, Service Manual

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Chapter 2
2-23
F-2-25
The surface temperature of the fixing heater is monitored by the thermistor (TH1) that remains in contact with the fixing heater. The resistance of the
thermistor decreases in response to increases in the surface temperature of the fixing heater, causing the voltage of the fixing heater temperature detection
single (FSRTH) to change.
The CPU (IC902) found inside the engine controller monitors the signal (FSRTH), and generates the fixing heater drive signal (FSRD) according to the
voltage of the signal operating in conjunction with the CPU. The fixing heater drive circuit, on the other hand, uses the signal to control the fixing heater
so that the temperature of the fixing heater remains at a specific level.
The machine executes the following 4 control mechanisms according to the temperature of the fixing heater and the size of the selected paper:
1. start-up temperature control; from when the print command arrives at the engine control to when the fixing heater reaches the target temperature.
2. paper passage temperature control; during printing so that the temperature of the fixing heater is identical the target temperature.
The target temperature may be any of 5 settings, used according to the selected fixing mode. Any of these modes is selected according to the settings of
the driver and such parameters as paper size and the number of printouts.
The following shows the relationship between the paper type setting and the fixing mode in relation to the fixing target temperature:
T-2-5
3. sheet-to-sheet temperature control; between sheets during continuous printing so that the temperature of the fixing heater is lower than the target
temperature, thus preventing overheating of the fixing film.
4. down sequence; during continuous printing so as to prevent overheating of the edges of the heater.
In this control, the CPU of the engine controller checks the length and width of paper; if the size of the paper matches a specific set of conditions, the
machine is shifted to long narrow mode (Note) or small paper mode regardless of the settings of the driver.
In this mode, the CPU forces the printing speed to decrease conversely increasing the distance between sheets, thereby increasing the sensitivity of the
thermistor located in the middle of the fixing heater and, consequently, preventing the edges of the heater from overheating.
The following shows the relationship between the printing speed and the conditions used to make a shift to this mode:
T-2-6
Paper type selected for
driver Fixing mode Fixing target temperature
Plain paper Normal 165 to 175 deg C
Heavy paper Rough 180 to 190 deg C
Heavy paper H Extra 180 deg C
Transparency, plain paper L Local 150 deg C
Envelope Envelope 165 to 175 deg C
Down-sequence mode Paper length Power width Printing speed (ppm)
No down sequence (normal
state) 267.4 mm or more 197 mm or more Number of printouts according to
paper size
Small size paper mode 267.4 mm or less 197 mm or less 8-->6-->4
Long narrow paper mode 267.4 mm or more 197 mm or less 3
+
-
Thermal fuse (TP1)
J302-4
J302-3
J303-1
Fixing film unit
J303-3
+3.3V
SSR101
Q102
Fuse
(FU101)
Power supply PCB
Power
switch
(SW101)
IC902
CPU
Engine controller PCB
Thermistor (TH1)
Q103
Relay
(RL101)
+24V
FSRD
+24V
IC501
J201-19 J902-5
RLYD
J201-21 J902-3
FSRTH
J201-20 J902-4
Fixing control circuit
Fixing heater safety circuit
Fixing heater (H1)
Q104
Pressure roller
Fixing heater drive circuit