Basic operation of a LCD TV
2012-10-22
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In this article we will describe how all stages which comprise a LCD TV
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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.
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