Basic operation of a LCD TV

Download Basic operation of a LCD TV

 

In this article we will describe how all stages which comprise a LCD TV.
It is obvious that we are going to generalize in the circuits most used
by major manufacturers, is clear that not everyone will use the same
type circuits.



We will start with a block diagram and then try to explain the function of each block.



Source:



In the block diagram the first thing we see is the power supply or Power
Suply. The LCD TV power supply are similar to those used in TV but with
different voltages. All of these sources are power supplies.



The main tensions which are used in a LCD TV are:



* 3V3 micro-feed.

* 5V general circuitry and extrude the 3V3

* 12V to general circuitry and the stages of audio

* 24V circuit reverse

*-12V can be used any time for stages of audio that need symmetrical power



To get these tensions manufacturers often go through two different sources or even up to three times.



The first source is used to get the 5V and the 3V3 for the power of the
microprocessor and the eeprom, this one works on a continuous basis, I
mean, this source always works, so when the TV is operating as when it
is in Stand-by. This is done so that the microphone is always powered
and with remote control we can remove the TV Stand-By. The only way to
stop this supply is by unplugging from the mains or by pressing the main
switch if it exists. This source tends to be low since the micro does
not consume large amount of current.



The second source is often supply the 12V and - 12V if needed. This
source is only active when we get the TV Stand-By. This is achieved by
using the POWER ON line, can also be found as P ON, SUPLY ON or PS ON,
and is printed in the printed circuit board.

When the TV is in Stand-By POWER ON line usually level low and when we
turn on the TV goes at high level, about 3V. Once we activate this line
activates the octoacoplador from this source through a transistor or
more and the source starts.



The third source and more powerful is that removing the 24V to the
inverter. This source is that more heated since it is that has to offer
more power. This is so because inverter with lamps CCFL consumed on the
order of 4 to 6 amps depending on the size of the screen and the number
of lamps that incorporate.

This source does not work when the TV to Stand-By and only turns on when POWER ON is set at high level.



This is an example with three sources incorporated in the POWER SUPPLY
module. In the case of only two sources, thing which some manufacturers
choose to use the 12V supply is removed and is the source Stand By or
3V3 which incorporates one output for 12V.



The inverter:



A fundamental and exclusive part of the LCD TV is the inverter. This piece did not exist in the conventional TV or picture tube.



The inverter is responsible together with the display backlight CCFL lamps.



The circuit of inverter is based on circuit oscillators of high stress
such as that used in the lines of a TV picture tube circuit for MAT or
high voltage. The difference is that voltages of the order of 20 to 25
KV is obtained at the stage of MAT and an inverter circuit MAT provides
about 1KV. Another difference is that there is a circuit of MAT for each
lamp or two. Thus in an inverter you can find 6, 7 or 8 transformers in
more normal cases, and may be more depending on the size of the screen.



External connections that needs to run are:



* 24V

* BL ON

* There is another line called BL Current that is used to give more
light back, or less. There are times that it isn't used and although not
present the inverter works.

* There is another line of protection that activates when it detects a
failure in the inverter. This line used by some manufacturers to turn
the source off if there are flaws in the inverter, but few people use
it, with which if not used the TV continues to operate with sound.



The boot order is as follows:



* Press the main switch and get 3V3 and 5V.

* Press Stand-By or program and turns POWER ON with which obtained the second source 12V and 24V from the third source.

* Once all the Main Board integrated circuits are powered and the micro
you can communicate with them via the I2C Bus and check them.

** If the microphone detects that it is everything active the BL ON line, which can also call Backlight ON.

** This line is similar to POWER ON but does funcionael inverter.

A time with 24V and BL ON at high level in the inverter this can already
make flickering all phases of high voltage and light lamps.



Main Board:



The Main Board, plate base, control Panel, SSB or as we like to call
him, is in charge of all signal processing, since they come into our TV,
(either by the DVB-t tuner, SCART, HDMI, VGA or RCA inputs) until they
are processed and displayed in our LCD panel.



For a better analysis, we will separate the different sections that comprise the Main Board.



• IN/OUT; inputs and outputs section. Here we have all the signal
inputs to the TV (SCART, HDMI, VGA, component, CVBS or Composite Video)
and also have the (SCART) outputs.

• Tuner and video signal processing. Here we have included tuner
analog and digital tuner, though very soon only be manufactured TV with
digital tuner.

• Control which includes the microprocessor with your eeprom, which
are they also control all working the TV. Through the I2C Bus monitors
all the Main Board integrated circuits that possess this bus.

• Scaler that is responsible for converting all video signals that
receives and transform them into appropriate digital signals to be able
to be played by the LCD panel. This process apart from the own of the
Scaler circuit requires several SDRAM memories, who is writing and
reading at the same time to be able to perform the transformation. This
section is also responsible for making the control of brightness,
contrast, color saturation and other necessary corrections before
sending the signals to the display.

• LVDS; (Low voltage differential signalizing) cable that is in charge
of transferring the digital signals from the Scaler to the LCD display.
These signals are very low voltage (range from 0 to 1, 2V) and very
high speed, with which this cable must have special characteristics with
few losses and with a minimum of noise.

• Audio; almost all manufacturers currently opt to mount all audio
processing on a single integrated circuit called MSP (Digital sound
processor) which is responsible for all audio processing. You receive
all the audio signals of the inputs and switch between them to decide
what should be heard. It also processes the signals to get audio in the
SCART and headphones outputs. This MSP are obtained in low-frequency
audio signals and only need an audio amplifier that, according to each
manufacturer, there may be many options to choose.



REMARKS TO REPAIR:



The best option to repair are the measurement points to give us an idea of where the failure of the TV can be found.



1.-Location of the 3V3 and the 5V. If present are generally Stand-by (red) led will be lit.



2.-Location of 12V. If present are generally the (green or blue) power led will be lit.



3.-Location of the 24V to the inverter.



If these first three items are correct, we can say that the power supply is correct.



4.-POWER ON location; If we seek this tension is because we lack the 12V
and the 24V. If this line changes state to change from Stan-By to
ignition, it means that the source is wrong if it does not provide the
12Vy the 24V can occur that this line does not change State and the
problem would be located on the main board (control section)



5.-BL ON; If we measure this voltage it is because we do not have
lighting on the screen. If this voltage changes state when switched on
and we have no backlight is because the inverter or a lamp is defective.
If BL ON does not change state the problem is located on the main board
(control section).



6.-Measuring the micro swing. This point is important to measure it to
know if the microphone is working. It is measured with the oscilloscope
in the glass or quartz oscillation of the micro. If there is no rocking
the MIC does not work.



7.-Measuring I2C Bus; If there is no bus the MIC is not working. It is
measured with the oscilloscope and there should be activity. A good
place to measure it are legs 5 and 6 of the eeprom that correspond to
the Data and the Clock.