TM 5-6675-324-14
a. Drum clock. The microcomputer times the various operations by counting the
clock pulses generated when a rotating, toothed wheel passes through a photo-inter-
rupter. The toothed wheel is driven by a gear on the shaft of the photosensitive
drum and generates 175 clock pulses per drum rotation.
b. Fuser heater control. Switching the power on causes the fixing roller
heater to switch on and WAIT/STANDBY indicator to flash. This condition is called
the wait period. The surface temperature of the fixing rollers is measured by
thermistor TH1, which has a negative temperature coefficient. Its resistance is
measured by comparator Q302, which controls solid-state relay Q51 which switches
power to heater H2 on and off. If the surface temperature of the fixing roller is
below 356F (180C), the following events occur: Q302-1 = 1 (high) and LED301,
Q306, Q51, and H2 are on. In addition, LED301 turns on Q305, which causes Q308
output to be high, thus causing Q309, Q250-23, Q205-4, and Q203-39 to read zero
(low), preventing copying action from taking place. WAIT/STANDBY indicator LED605
flashes due to Q206-7 output being high and the pulsed 1 Hz output of Q212-3. When
the surface temperature of the fixing roller is above 356F (180C), the exact
opposite action occurs. LED605 no longer flashes, the input to the microcomputer is
high, and copies can be made. At normal room temperature, the resistance of the
thermistor is high. This would activate the Failure Detection circuit and make
Q302-7 output high. The purpose of the 1 Hz timer circuit is to cause Q302-7 output
to be low for 30 sec until the thermistor resistance decreases as the roller heats
up. When power is first applied, C303 starts charging. Q303-7 output is high,
causing Q304 output to be low. After 30 see, Q303-7 output goes low, turning Q304
off, and Q302-7 output is high. If, after this period, the thermistor fails, Q302-7
output is kept high, causing LED302 to light, turning Q307 on, Q306, Q51, and the
heater off, and the input to Q203-39 is low. No copies can be made and the WAIT/
STANDBY indicator flashes.
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