TROUBLESHOOTING T'CON BOARD _ LCD PANEL _ SONY LCD TVs
A typical LCD panel and the
associated video processing circuits as found in the WAX3 chassis. The various
formats and resolutions of video signals are processed on the BU1 board. All
video signals exit the video processor in the native resolution of the LCD
panel. In this design, the resolution is
for a 1366 by 768 at 60HZ refresh rate panel. 48 horizontal lines are discarded
to match up to the 720p resolution of the ATSC specifications so the video will
exit as 720p. The LCD panel used in this
model processes 8-bit RGB video data. Before the video information can be sent
to the TCON board it must be converted to a format that allows for practical
and noise-free transmission. The large number of parallel lines to transmit the
8-bit RGB data would need to be sent on differential lines for noise reduction.
This would require 48 lines just for the video. The TCON circuit also requires
B+, ground connections, a communications bus, sync, and a clocking line
transmitted differentially so we can see that up to 60 lines would be required
for an 8-bit video signal and significantly more lines for a 10-bit processor.
The practical way to transmit this information is to convert the parallel video
data to a serial stream and this is accomplished by the Low-Voltage Differential
Signaling (LVDS) transmitter.
The LVDS transmitter contains a
circuit to serialize the parallel data. The parallel video information along
with sync and clocking data are transmitted via twisted line pairs. Depending
on the logic level, current is sent along one or the other of the twisted pair
of wires. The receiving end of the wires is loaded with a resistor (usually
around 100 to 120 ohms). The receiver detects the polarity of the voltage drop
across the resistor to determine the logic level. The current level swings in
the wire are about 3ma with a voltage differential of around 350mv. This allows
for transmission of the video signal with minimal EMI. The LVDS receiver on the TCON board converts
the serialized data back to parallel. This data is processed by the timing
control IC to allocate the RGB data into serial streams for processing by the
LCD panel. The TCON transmits the pixel
control data to the panel via flat, flexible circuit board cables which can number 2 or 4 depending on the bit rate and refresh
timing of the panel. A 1366 X 768 panel requires about 180 lines to transmit
control information and B+ from the TCON. This number of control lines is not
even close to the number of horizontal or vertical rows of pixels so the LCD
panel must use this information to further expand the ability to turn on each individual crystal. The process will be explained in
the gate and source driver paragraphs.
All of this is accomplished by the TCON board. The term “TCON” is short
for Timing Control. Other LCD panel manufacturers may have a different name for this particular circuit but the term
used by Sony will always be TCON.
Gate Drivers
The IC’s located along the side of the panel. These IC’s are mounted on a flexible cable(s) which are bonded to the LCD panel. Their function is to activate each row of pixels one at a time starting with the first line at the top. As each line is activated, the source drivers turn on the appropriate liquid crystals for the frame of video about to be displayed. This continues from top to bottom until the entire frame of video is displayed. The process is repeated for the next frame. This rate can vary from 60 times per second or be increased to 120 or 240 as found in the high-frame-rate panels.
The IC’s located along the side of the panel. These IC’s are mounted on a flexible cable(s) which are bonded to the LCD panel. Their function is to activate each row of pixels one at a time starting with the first line at the top. As each line is activated, the source drivers turn on the appropriate liquid crystals for the frame of video about to be displayed. This continues from top to bottom until the entire frame of video is displayed. The process is repeated for the next frame. This rate can vary from 60 times per second or be increased to 120 or 240 as found in the high-frame-rate panels.
Source Drivers
These IC’s provide the control voltages to turn on each RGB segment of the vertical rows of pixels. In this example, the panel has a horizontal resolution of 1366 pixels. Each pixel is made up of a red, green and blue liquid crystal which means there are 4,098 columns to control. The source drive IC’s contain shift registers along with buffer switches. Shift registers are used to convert serial data to parallel. By using this method, the TCON is able to transmit control information to each of the source drivers using serial data lines. If the TCON is transmitting 8-bit data to the panel, each data line is capable of controlling 256 lines exiting the source drivers. Understanding how the gate and source drivers work together makes it easier to observe a problem on the screen and determine if the failure is panel or TCON related.
These IC’s provide the control voltages to turn on each RGB segment of the vertical rows of pixels. In this example, the panel has a horizontal resolution of 1366 pixels. Each pixel is made up of a red, green and blue liquid crystal which means there are 4,098 columns to control. The source drive IC’s contain shift registers along with buffer switches. Shift registers are used to convert serial data to parallel. By using this method, the TCON is able to transmit control information to each of the source drivers using serial data lines. If the TCON is transmitting 8-bit data to the panel, each data line is capable of controlling 256 lines exiting the source drivers. Understanding how the gate and source drivers work together makes it easier to observe a problem on the screen and determine if the failure is panel or TCON related.
Diagnosing a Failed TCON
In order for this concept to move forward successfully, it is important that the service industry be able to properly identify the symptoms of TCON issues to avoid unnecessary service calls and repair costs. Accurate analysis of TCON failures will reduce costs significantly (both in parts costs and time) when warranty repairs are involved and will reduce the number of COD repairs that are lost. A good approach when determining a TCON failure is a good understanding of which symptoms ARE NOT caused by the TCON. Examples are as follows:
In order for this concept to move forward successfully, it is important that the service industry be able to properly identify the symptoms of TCON issues to avoid unnecessary service calls and repair costs. Accurate analysis of TCON failures will reduce costs significantly (both in parts costs and time) when warranty repairs are involved and will reduce the number of COD repairs that are lost. A good approach when determining a TCON failure is a good understanding of which symptoms ARE NOT caused by the TCON. Examples are as follows:
Video Process Failures:
All video inputs received by the video process circuits are handled on a frame-by-frame basis. The video frames are converted and scaled to 8 or 10-bit RGB information. It is virtually impossible for the video process circuits to cause a problem on a specific area of the screen. Failures on this board usually appear as distortions, colour level shifts, video level shifts, noise that involves the entire picture, or no picture at all. The TCON can generate symptoms that appear to be video process related but the video process circuits cannot produce the symptoms of a failed TCON circuit.
All video inputs received by the video process circuits are handled on a frame-by-frame basis. The video frames are converted and scaled to 8 or 10-bit RGB information. It is virtually impossible for the video process circuits to cause a problem on a specific area of the screen. Failures on this board usually appear as distortions, colour level shifts, video level shifts, noise that involves the entire picture, or no picture at all. The TCON can generate symptoms that appear to be video process related but the video process circuits cannot produce the symptoms of a failed TCON circuit.
LVDS Cable Failures:
Although problems with the LVDS cable or connectors can generate symptoms of TCON failures this usually tends to be intermittent and wiggling of the connectors will usually provoke a change in the symptom on the screen. LVDS cables and connectors have become rather robust over the past few years and most problems are caused by technicians who damage them and this is generally quite obvious upon close examination.
Although problems with the LVDS cable or connectors can generate symptoms of TCON failures this usually tends to be intermittent and wiggling of the connectors will usually provoke a change in the symptom on the screen. LVDS cables and connectors have become rather robust over the past few years and most problems are caused by technicians who damage them and this is generally quite obvious upon close examination.
LCD Panel Failures:
Some LCD panel failures could possibly be mistaken for TCON issues. Other than damage to the LCD glass, most panel failures are isolated to a particular area of the screen. Since the TCON disperses the pixel data to groups of line and column drive IC’s situated on the outer edges of the panel, it is unlikely that more than one of these IC’s would fail at the same time. Multiple columns of stuck on or stuck off pixels are, therefore, more likely to be the fault of the TCON circuits. The same applies to a single row of lit or unlit pixels. The TCON simply cannot cut out a single line of information.
Some LCD panel failures could possibly be mistaken for TCON issues. Other than damage to the LCD glass, most panel failures are isolated to a particular area of the screen. Since the TCON disperses the pixel data to groups of line and column drive IC’s situated on the outer edges of the panel, it is unlikely that more than one of these IC’s would fail at the same time. Multiple columns of stuck on or stuck off pixels are, therefore, more likely to be the fault of the TCON circuits. The same applies to a single row of lit or unlit pixels. The TCON simply cannot cut out a single line of information.
Failures involving the LCD panel are
usually displayed with the following symptoms:
* Physical damage such as cracks in the panel, a single pixel or group of pixels that always on or off, or random sections of the panel which are completely dark.
* Source driver failure. This symptom appears as a single vertical band around 1 to 2 inches (depending on the panel size) and can be black, white, or any other colour. It can also contain video information with distortion. A single vertical line that is dark or coloured. This may be due to a tab bonding failure from the IC to the panel but either cause requires the replacement of the panel.
* Gate driver failure. These IC’s operate in a “bucket brigade” fashion. As mentioned earlier, the gates drivers scan each horizontal line starting at the top. If any one of the gate drivers fails, all of the subsequent drivers below it will fail to operate properly. This symptom is usually indicated by normal video on the upper portion of the screen followed by distorted video from the point of the failed IC and downward.
* Any horizontal lines. The gate drivers are activated by a single source of timing information so any single horizontal line or groups or random horizontal lines are caused by an output failure from a gate driver or a loss of the tab bond to the panel.
* Physical damage such as cracks in the panel, a single pixel or group of pixels that always on or off, or random sections of the panel which are completely dark.
* Source driver failure. This symptom appears as a single vertical band around 1 to 2 inches (depending on the panel size) and can be black, white, or any other colour. It can also contain video information with distortion. A single vertical line that is dark or coloured. This may be due to a tab bonding failure from the IC to the panel but either cause requires the replacement of the panel.
* Gate driver failure. These IC’s operate in a “bucket brigade” fashion. As mentioned earlier, the gates drivers scan each horizontal line starting at the top. If any one of the gate drivers fails, all of the subsequent drivers below it will fail to operate properly. This symptom is usually indicated by normal video on the upper portion of the screen followed by distorted video from the point of the failed IC and downward.
* Any horizontal lines. The gate drivers are activated by a single source of timing information so any single horizontal line or groups or random horizontal lines are caused by an output failure from a gate driver or a loss of the tab bond to the panel.
TCON Failures
Failures in the timing control circuits of the TCON can produce symptoms of absolutely no video or generate lines and patterns that usually cover all or a substantial part of the screen. Determining if the TCON is the cause of a “no video” condition is a bit more difficult since there are no indications on the screen to analyse.
Failures in the timing control circuits of the TCON can produce symptoms of absolutely no video or generate lines and patterns that usually cover all or a substantial part of the screen. Determining if the TCON is the cause of a “no video” condition is a bit more difficult since there are no indications on the screen to analyse.
Troubleshooting a “DEAD” TCON
Many of the Sony television models over the last few years will detect a TCON that has completely failed. The communications data between the video process circuits and the TCON will cease to communicate if the TCON fails completely. This will cause the television to shut down and display a diagnostics code indicating a failure of the TCON. Not all chassis designs have this feature and it is not found on older models. The typical scenario when this failure arises is for the technician to bring a video process board to the repair location. It is usually safe to assume that the problem lies on the TCON board if the replacement video board does not remedy the problem since it is highly unlikely that a replacement board with the same failure was received. One trick to check most TCONS for functionality is to loosen the LVDS connector at the TCON (as shown In Figure 4-3) while the unit is turned on. Handle the LVDS connector with care and be certain to fully release the lock tabs. Gently rock the cable in and out of the connector while observing the screen for any response. Depending on the chassis, the symptoms of the screen may be gentle white flashes, intermittent coloured lines, or a screen full of random patterns. The idea at this point is to provoke some kind of response on the screen. TCON boards that have failed will not usually generate any type of response on the screen. Another helpful procedure is to rapidly heat and/or cool the TCON with hot air devices or circuit coolant and watch for patterns to appear on the screen.
Many of the Sony television models over the last few years will detect a TCON that has completely failed. The communications data between the video process circuits and the TCON will cease to communicate if the TCON fails completely. This will cause the television to shut down and display a diagnostics code indicating a failure of the TCON. Not all chassis designs have this feature and it is not found on older models. The typical scenario when this failure arises is for the technician to bring a video process board to the repair location. It is usually safe to assume that the problem lies on the TCON board if the replacement video board does not remedy the problem since it is highly unlikely that a replacement board with the same failure was received. One trick to check most TCONS for functionality is to loosen the LVDS connector at the TCON (as shown In Figure 4-3) while the unit is turned on. Handle the LVDS connector with care and be certain to fully release the lock tabs. Gently rock the cable in and out of the connector while observing the screen for any response. Depending on the chassis, the symptoms of the screen may be gentle white flashes, intermittent coloured lines, or a screen full of random patterns. The idea at this point is to provoke some kind of response on the screen. TCON boards that have failed will not usually generate any type of response on the screen. Another helpful procedure is to rapidly heat and/or cool the TCON with hot air devices or circuit coolant and watch for patterns to appear on the screen.
Figure illustrates 2 examples of a
loss of control data to the drive IC’s. In the first example, an entire group
of column drivers has lost the data stream for red. The second example involves
the complete loss of drive data for all RGB information to the right side of
the screen. This is sometimes caused by the flat cable connecting the TCON to
the LCD panel coming loose. The area of missing video can be dark or completely
white depending on the panel design.
Service Tip: Select an inactive input (or one that is known to be a 4:3 SD source) and toggle between the “normal” and “zoom” modes. If the lines follow the zoom changes, the problem is located on the video process board. If they stay in the same place, they are originating in the TCON or LCD panel.
Service Tip: Select an inactive input (or one that is known to be a 4:3 SD source) and toggle between the “normal” and “zoom” modes. If the lines follow the zoom changes, the problem is located on the video process board. If they stay in the same place, they are originating in the TCON or LCD panel.
Actual TCON Failures
The illustrations show other TCON failures that have been encountered in the field. The idea is to get a grasp of the concept of TCON induced failures to avoid unnecessary parts replacement.
The illustrations show other TCON failures that have been encountered in the field. The idea is to get a grasp of the concept of TCON induced failures to avoid unnecessary parts replacement.
LCD Panel Failures
Below are some photos of actual LCD panel failures. Note that most issues tend to be isolated to a certain area of the screen with the exception of failures of the source drivers. The source drivers can cause thin horizontal line issues and can also affect a large area of the screen.
Below are some photos of actual LCD panel failures. Note that most issues tend to be isolated to a certain area of the screen with the exception of failures of the source drivers. The source drivers can cause thin horizontal line issues and can also affect a large area of the screen.
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