Akai LTA-26C904, Service Manual

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27
Chapter 3 Whole Machine Signal Processing Analysis and
Measured Data at the Key Points
This chapter Mainly introduces the receiving and processing of image signal and sound
signal, whole machine system control process, and whole machine power supply system
(Model of LTA-32C904 is taken as an example, other models is similar and will not be explained
here for each. There are minor differences in power supply among different models of liquid
crystal screen; the details are shown in Specifications).
1. Image signal flow
RF signal is demodulated by tuner, then the obtained video signal and signals inputted from
COMPONENT terminal, AV terminal, S terminal, 2-way Scart interface and VGA interface are all
sent into video-processing IC SVP-CX32LF to be processed. Input signals with different format
are changed into uniform LVDS differential signals, and are sent to liquid crystal screen. In
addition, TV video signal is processed by SVP-CX32LF to generate 2-way CVBS signal, which
is used in 2-way Scart video output.
Signal inputted from HDMI interface is decoded into digital video signal, which is sent into
video decoding IC SVP-CX32LF to be processed. After procession, the obtained uniform LVDS
differential signal is sent to liquid crystal screen through LVDS interface to screen.
2.Sound flow
RF signal is demodulated by tuner, and then the obtained SIF signal (SIF-2 signal) and
audio signal inputted from Scart1 interface are directly processed by STV-8217. The output
audio signal of STV-8217 is sent into power amplifier TDA8944J, and the amplified signal is
finally sent to speaker or earphone output.
Signals from HDTV terminal, AV terminal, Scart2 interface and PC Audio are first sent into
74HCT4052D for choosing passages, and then sent into STV-8217 for demodulation and sound
effect processing. The output audio signal of STV-8217 is sent into power amplifier TDA8944J,
and finally the amplified signals are sent to speaker or earphone output.
After the demodulation and sound effect processing of STV-8217, there is an audio output
which is used for sound output of Scart1 terminal.
3. Whole machine power supply system
There are 4 ways of voltage output in the power panel: +24V
（
6A
）、
+12V
（
2A
）、
+5V
（
2A
）
and +5VS
（
1A
）
. +24V output is used by LCD screen; +12V output is used by power amplifier
TDA8944J, and is also changed into +8V output by AZ78L08ZTR-E1 to satisfy the need of
STV-8217 IC; +5V output is changed into 1.8V output and 3.3V output by DO (such as
AZ1117H-1.8TRE1 and AZ1117H-3.3TRE1) to satisfy the needs ICs. +24V, +12V and +5V
outputs will be cut off in standby state. While +5VS output is the power supply of MCU, infrared
receiver and EEPROM etc, and will be cut off when the AC is turn off.
5V output will be divided into two ways: one way is changed into +3.3V and +1.8V analogue
power supplies through DC/DC converter for STV-8217 IC, ANX9011LF IC etc and peripheral
circuits; other way is changed into +3.3V and +1.8V digital power supplies through DC/DC
converter for SVP-CX32 IC etc and peripheral circuits, and this way of 5V voltage will be cut off
in standby state, while the other way of 5V voltage supply power especially for MCU, infrared
receiver and EEPROM etc, and will not be cut off in standby state.
(1) Constitution and distribution of whole machine power supply
FB34
FB8
FB34
12
V
U23
AZ78L08ZTR-E1
8V
ST82X7
Pin 3
12V shutdown mute circuit
+12V（voltage of line to screen） CON1
Pin 1,2,3,4 (used by 37 inches screen)
12V power amplifier TDA8944J
Pin 3,16
+12V power
supply
CON11
Pin 1,2,3
+12V power
supply
CON403
Pin 3,4