Philips EM7U LCOS, Service Manual

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The voltage represents the ambient temperature of the set from the Sensor Board which is
fed to the System Board via connector 1262. (Figure 76) The voltage is fed to the positive
input of IC 7110. This IC converts the voltage into an 8-bit data signal which is fed to the
System Microprocessor, 7108. See Figure 72 for 7108. The ADC_SEL line from 7108 sig-
nals 7110 to sample the voltage on Pin 2. This line is held Low during the sampling period.
The SHIFT_CLK reads the data out to 7108. The S_DATA_IN line sends configuration data
to 7110.
The System Control Microprocessor provides a PWM drive to the Fan drive circuit. (Figure
77) The PWM_DRIVE signal is amplified by Transistors 7011, 7012, 7013, and 7014. The
Fan is powered by the +12-volt supply which is fed to the Fan via connector 1251. The return
side of the Fan is fed to Transistor 7014, which provides speed control drive. A Hall Effect
sensor on the System Fan feeds drive back to the System Board via connector 1251, Pin 3.
Feedback from the Light Engine Fan is fed back on connector 1251, Pin 2.
The SYS_FAN_TACH for the System Fan and the ENGINE FAN TACH signals are fed to a
Fan Failure detection circuit which signals the System Microprocessor, 7108, if the Fan
should stop. (Figure 78) When the signal line goes High, the Comparator connected to Pin 2
will output a High which will Set the Latch. The Output on Pin 3 will go Low. When the signal
goes Low, Transistor 7015 turns On, discharging Capacitor 2325. If the Fan stops turning,
the signal will cease. Capacitor 2325 will charge via Resistor 3347. The output of
Comparator connected to Pin 6 will go High, resetting the Latch, causing Pin 3 to go High.
This will signal the System Microprocessor that the Fan has failed. The set will then shut
down. IC 7118 monitors the ENGINE FAN TECH in the same manner to signal the System
Microprocesor if the Light Engine fan should fail.
FIGURE 76 - TEMPERATURE SENSING A/D CONVERTER
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