Triac Malfunction - Canon LBP-CX Series Service Manual

IC208 sends a TRUE FSERL to the buffer IC
(IC203) to turn on transistor Q204 and charge
capacitor C235. When C235 is charged, a TRUE
FUSER ERROR signal (FSER) is applied to IC208.
Microcomputer IC208 never generates a TRUE
FSRD command as long as FSER is TRUE. FSER
does not go FALSE unless a certain period has
passed (which is determined by a timing circuit
comprised of C235 and R252) after the printer is
switched OFF.
If FSER is input to microcomputer IC208,
IC208 does not actuate fixing roller heater Hl even if
the power is switched OFF and immediately there-
after switched ON. Fixing roller heater Hl cools
down until FSER becomes FALSE.
If fixing roller heater Hl warms up before micro-
computer IC208 detects an open circuit in the ther-
mistor, which takes about 70 seconds after the power
is turned ON, the printer will not be damaged.
( 4) Triac malfunction
Line voltage
H1
both terminal
voltages
08101
output
0105
switching
0104
base voltage
0104
collector
voltage
0106
collector
voltage
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Figure 3-47
Fuse resistor
R117 off
If line voltage is applied to fixing roller heater
H1 even if a FALSE FIXING ROLLER HEATER
DRIVE command (FSRD)
is
transmitted from micro-
computer IC208, the line voltage is turned OFF by
the fixing roller heater safety circuit to prevent
damage to the printer.
When microcomputer IC208 applies a TRUE
FSRD command to photocoupler PC103, PC103
turns transistor Q 102 ON through the differentiation
circuit comprising capacitor C105 and resistors Rl08
and R 109. Trigger signals are applied by Q 102 to the
latch circuit comprising Ql03 and QlOS. Ql03 and
QlOS are switched ON. QlOS then discharges C106
via Rl16, QlOS and R112. Transistor Q106 is turned
OFF and DC voltage is applied to the coil of relay
RLl 01 through fuse resistor R 11 7. RLl 01 is turned
ON and line voltage is applied to fixing roller heater
HI.
If line voltage is applied to fixing roller heater H 1
when FSRD output from microcomputer IC208 is
FALSE, the line voltage is full-wave rectified into a
lower AC voltage, which is then applied to PC102 by
rectifier DB 101. PC 102 transmits a positive pulse to
the base of the transistor Q 104 when the AC voltage
from DBlOl approaches OV. Ql04 is turned ON
when a positive pulse is applied to its base.
When FSRD becomes FALSE, PC103 is turned
OFF, and Ql02 turns OFF the trigger signals to the
latch circuit comprising Ql03 and QlOS. Ql03 and
QlOS, therefore, turn OFF when Ql04 turns ON.
The output of Ql04 is applied to capacitor Cl06
via resistor Rl16. Since Ql04 stays OFF longer than
the period when it is ON, the voltage in Cl06 in-
creases gradually. When the voltage of Cl06 exceeds
that of zener diode ZD102, transistor Ql06 turns
ON. The current in fuse resistor Rl 17 increases until
Rl 17 fuses, which turns RLlOl OFF and the current
flow to fixing roller heater Hl is cut off.
3-38 COPYRIGHT
©
BY CAllON INC. CANON LBP-CX SERIES REY.2 JULY 1985 PRINTED IN JAPAN (IMPRIME AU JAPONJ