Hitachi C28W40TN, Service Manual

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20
CRYSTAL OSCILLATOR CONNECTIONS
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Pin 11 is the 4MHz crystal oscillator input (OSCIN).
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Pin 12 is the 4MHz crystal oscillator output (OSCOUT). By connecting a x100 scope probe to pin 11, it can be seen if a
4MHz sine wave is present at the oscillator input to the microcontroller.
RESET CONNECTION
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Pin 54 is the active low RESET input of the microcontroller. This input is normally high (approx. +5V) under operating
conditions, but changes state when the s5V power supply is typically below +4.5V. In this circumstance, the reset
IC (I021) pulls pin 54 low until the input of it is above +4.5V. The diode (D001) ensures that the capacitor (C015)
discharges quickly when the standby supply falls, so that the reset operates quickly. The capacitor (C015) charges up
slowly when the s5V supply is restored, ensuring that there is some hysteresis.
INFRA-RED (IR) RECEIVER INPUT
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Pin 25 is the IR receiver’s filtered output. This input from the IR receiver consists of PWM pulses between 0V and +5V
which are decoded by the microcontroller into useful commands from the handset. When a valid command has been
decoded, the Red LED on the front of the TV will briefly flash.
HORIZONTAL AND VERTICAL SYNCHRONISATION CONNECTIONS
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Pin 48 is the vertical synchronisation input from the deflection stage. This input is used to ensure that the OSD is
displayed in a stable vertical position. When the TV is in the standby state, this input is normally low. The vertical input is
triggered on the rising edge (positive polarity).
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Pin 9 is the horizontal synchronisation input from the deflection stage. This input ensures that the OSD is displayed in
stable horizontal position. When the TV is in the standby state, this input is normally low. This input is rising edge
triggered (positive polarity).
GENERAL INPUT CONNECTIONS
HEADPHONE INPUT
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Pin 19 is used to detect if the headphone has been inserted into its socket. This input is normally HIGH (+5V) unless the
headphone has been inserted, in which case it is near 0V. When the headphone is inserted, the headphone mode option
is then available in the "Sound Mode" Menu and the loudspeakers (and internal sub-woofer if available) in the television
are muted.
PROTECTION INPUT
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Pin 26 is used to determine if a protection fault has occurred on the chassis. This line is connected to the comparator
outputs 1, 13 and 14 of I903 in the power supply. This line is normally high (approx. +5V) unless a fault has occurred, in
which case it is near ground potential (0V).
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Comparator output 1 monitors the +16V audio supply rail for over-current. If this rail is shorted, or draws too much current,
then the comparator output (pin 1 of I903) will change state to 0V.
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Comparator output 13 monitors the EHT voltage generated by the FBT in the deflection circuit. If this voltage is too high,
then this output will be low to indicate that there is a problem with the deflection.
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Comparator output 14 is connected to the secondary side of the deflection’s line input transformer. It is primarily used to
indicate whether too much current is being drawn from the line-input transformer and FBT.
POWER-GOOD INPUT
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Pin 27 is used to indicate if the power has been removed from the TV or whether a static discharge has occurred. This
input is interrupt driven to react instantaneously to a falling edge (+5V -> 0V). Under this circumstance, the audio power
amplifiers are muted and the television begins to power-down into standby. This input is derived from the comparator
output (pin 2 of I903) in the power supply. This comparator provides an "early warning" indication that the primary supply
has been removed or a static discharge event has occurred. Under a static-discharge condition, the TV will enter the
standby condition temporarily before powering back up a few seconds later.
DTT FAN FAIL INPUT
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Pin 32 is used only on a D8 chassis fitted with a DTT module. It is used to indicate if a fault with the fan on the DTT unit
has occurred (e.g. fan stuck). This input is normally HIGH (approx. +5V) unless a fault condition has been detected (0V).
FRONT PANEL BUTTONS AND SAV3 SOCKET INPUTS
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Pins 36 and 38 are 2 of the 4 ADC inputs of the microcontroller.
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Pin 36 is connected to the /- buttons and the SAV3 (Hi-8) socket on the front panel of the TV. When the voltage
on this pin is changed to a value in a certain window, the microcontroller will interpret it as either a , volume -,
/- command and/or an SVHS Hi-8 connector was inserted into the Hi-8 socket.
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Pin 38 is connected to the Pro/- and Menu buttons on the front panel of the TV. When the voltage on this pin is
changed to a value in a certain window, the microcontroller will interpret it as either a programme change or the menu
button was pressed.