Hitachi C28W40TN, Service Manual

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RXD CONNECTIONS
•
Pin 11 is the RS232 receive line from the microcontroller (pin 37) which is routed through to pin 10 when pin 12 is HIGH.
Under normal operating conditions (and when the TV is NOT in service or diagnose modes), pin 10 is disconnected from
pin 12 (and pin 12 of scart2). When the TV is in service mode, a PC can be connected to scart 2 to perform diagnostic
functions on the chassis (see separate section on diagnostic protocols for more information).
I2C CONNECTIONS
I2C CLOCK CONNECTION
•
Pin 8 is the I2C master clock input from the microcontroller (pin 24). When pin 6 of I007 is HIGH, this line is connected to
pin 9, which enables the microcontroller to communicate with other I2C devices on the chassis. Pin 6 is normally LOW
when the TV is in the standby state, or when power is first applied to the chassis.
I2C DATA CONNECTION
•
Pin 4 is the I2C data input from the microcontroller (pin 23). When pin 5 of I007 is HIGH, this line is connected to pin 3,
which enables the microcontroller to communicate with other I2C devices on the chassis. Pin 5 is normally LOW when the
TV is in the standby state, or when power is first applied to the chassis.
74HC4066 (I008) D8 CHASSIS ONLY
INTRODUCTION
The 74HC4066 is a 14-pin high speed CMOS quad bilateral switch. It is used primarily for connecting/disconnecting signals when
out of/in the standby state.
DESCRIPTION
SUPPLY/GROUND CONNECTIONS
•
Pins 14 and 7 are the +5V supply and 0V ground connections respectively. This device is always powered up, even when
then television is in the standby state. Capacitor C026 ensures that the supplies are properly de-coupled.
TXD CONNECTIONS
•
Pin 1 is the RS232 Transmit line from the microcontroller (pin 30) which is routed through to pin 2 (DTT Tx) of I007 when
pin 13 is HIGH. Pin 2 is connected with pin 3 of P001, which is in turn connected to the DTT module. When
communicating with the DTT module, pin 13 of I008 is HIGH. However, when communicating with the RS232 on scart 2,
this line must be LOW to prevent simultaneous communications.
RXD CONNECTIONS
•
Pin 11 is the RS232 receive line from the microcontroller (pin 37) which is routed through to pin 10 (DTT Rx) when pin 12
is HIGH. Pin 10 is connected with pin 4 of P001, which is in turn connected to the DTT module. When communicating with
the DTT module, pin 12 of I008 is HIGH. However, when communicating with the RS232 on scart 2, this line must be
LOW to prevent simultaneous communications.
DTT RESET CONNECTION
•
Pin 4 is the DTT reset connection that is routed through to pin 3 when pin 5 is HIGH. Pin 3 is then connected to pin 1 of
P001, which in turn is connected to the DTT module. The DTT reset line is normally HIGH when powering up/down the
DTT module, and is low otherwise.
POWERED CONSOLE
The powered console is used make the A7 and A8/D8 models into full Dolby pro logic sets by amplifying the centre and surround
channels with the addition of a sub woofer.
The amplifiers used for this are three TDA7482 which are mono class D. In this application the Bass output (IC2) is rated at 20W,
and the Centre (IC3) and Surround (IC1) channels are rated at 12W the output being restricted by the supply voltage.
These devices usually require two supplies one positive and one negative but in this case a dummy ground is created for each
amplifier by using a resistive / capacitative divider network. The Bass is supplied from 38V and divided by R11 / R12 and C11+ C16
/ C17 + C18. The Centre and Surround are supplied from 32V which are divided by R56 / R57 and C72 + C77 / C78 + C79 (Centre)
and R50 / R51 and C57 + C64 / C65 + C66(Surround).
The brief principle behind the class D is that the signal is divided in to segments at a rate set by the internal oscillator (the frequency
is set by the R / C components attached to pin 6 in this case approximately 110kHz). This is then amplified and output from pin 1 as
a pulse width modulated square wave form with an amplitude almost equal to the supply voltage. The inductor / capacitor network of
which the speaker is part then filters out most of the fundamental frequency leaving the amplified audio signal.
The class D type amplifiers were chosen because of there high efficiency (low heat losses within the device).
The centre and surround inputs are fed straight forward from the phono sockets to pin 9 of the amplifiers via 470nF capacitors. The
bass uses the left and right outputs from the TV, these signals are passed through an active filter with amplifier(IC9), the output of
which is fed to IC2 pin 9 via R68 and C2.
The only control on the IC is pin 10 which has three level ranges :-
1. 0 to 0.7 V above pin 7 voltage standby (no output at all from pin 1).
2. 1.7 to 2.5V above pin 7 voltage mute (only the 110kHz output can be seen at pin 1).
3. 4 to 5V above pin 7 voltage play range.
As the Bass, centre and surround amplifiers stages are identical, only the bass will be described. On the powered console a fast
falling 5V rail is used to stop pops and squeals when the TV set is turned off or put into standby (removing the 5V), this is applied to