Tuesday, January 03, 2017

T-CON board Troubleshoot – No picture on LCD TV

Troubleshooting No Picture problem.
When dealing with a suspected T-CON problem, the very first thing you should approach is “Does the T-CON board have it’s operational voltage” TV TURN ON PROCESS: When the TV turns on, it is a 3 step process.
Step 1) RL-ON called (Relay On) or PWR ON _ON called (Power On). This turns on the power supply which turns on all the voltages that the Main board needs to operate. It also turns on the backlight B+ even though the backlights will not be ON at this time. The Main board needs several voltages to operate. It needs the Stand-By voltage which is always present even if the set is not turned on. It needs what is called Video Processing voltage which is 12V. And it needs Audio amplification voltage which can vary dependent upon the wattage output of the audio amplifier. This voltage will be either 17V or 24V. This voltage can also be used to generate Tuner voltage.
Step 2) Panel_CTL called (Panel Control). This will be the command that actually turns on the T-CON operational voltage by activating a switch that takes the 12V from the Power Supply and routes it to the T-CON board through the LVDS cable. 3D model T-CONs also have a secondary 12V supply (which comes from the same switch) and sends it to the T-CON through a two pin connector for additional current demands of the 3D T-CON. It is not the fact that the T-CON 12V current is too much for the 12V supply, it is that the current demand of the 3D FRC T-CON is too much for the thin wire in the LVDS cable.
Step 3) INV ON _ON called (Inverter On) or DRV ON _ON called (Drive On) This command will turn on the backlights .This output from the Main is sent back to the Power Supply to activate the backlight driver IC. If the backlights have an external Ballast (Inverter), the INV_ON line will be routed through the Power Supply to the Inverter. The same thing holds true for LED backlights which may have an internal or external Inverter.
So the area to concern when dealing with a No Picture symptom is to first make sure there are backlights . If the backlights do not comes on, you still may be able to see if the panel is working by using a flashlight and looking (at close proximity) to the panel. Hold the flashlight close to the panel and look carefully for any movement within the panel. You may also be able to shine the flash light through any hole in the metal shield covering the back of the panel. If you see movement, this would indicate there is activity in the LCD panel and that you need to investigate the problem from the backlight. Remember the backlights (Florescent or LED) need a power supply (24V) and the turn on command called DRV_ON or INV_ON.
Troubleshooting a No Picture problem, checking Power Supply voltages.
If you have backlights but you have no picture, the fist step is to check the connector from the Power Supply to the Main board. Confirm the 3 voltages, STBY voltage, 12V and 17V/24V. You can also listen for audio which would confirm that the Main board is processing audio and it has the Audio B+.
In the Turn On sequence, Panel_CTL turns on the 12V to the T-CON. Once you have confirmed that the Main board is receiving the correct voltages, you need to confirm that the T-CON is receiving its operational voltage. The best way to check is to begin with the LVDS cable from the Main to the T-CON. It will have a test point for the 12V to the T-CON. Look on the left or right side for 4 pins tied together, this will be the 12V test point. In some cases when the set has two LVDS cables, look on the LVDS cable with the most pins (usually 51). (The LVDS cable with the fewer pins will not be carrying any DC voltage).
Tip: Some models use the first and last pin as ground. This is easy to verify with a DVM in ohms.
Here is an example of how the T-CON 12V is routed to the LVDS connector. You will note that pins 48 through 51 are carrying the 12V to the T-CON. In this case it is not possible visually to determine on the PCB that the last 4 pins are tied together. This is where you would measure the 2nd and the next to the last pin for 12V.
In the Turn On sequence, Panel_CTL turns on the 12V to the T-CON, look at the circuit shown in the example below.
Here is an example of how the T-CON 12V is switched on/off on the Main board. When the Microprocessor outputs Panel_CTL it is routed to Q505 which turns on. The Collector current goes high and the collector voltage goes low. This drops the base of Q506 low and Q506 turns off. This allows the 12V from the Power Supply routed through L511 to pull up the Gate of Q507 via R558. Q507 has the 12V on its Source. When Qp 507 switches on its outputs the 12V to the T-CON.
Panel power
Once you have confirmed that there is 12V on the Main board side of the LVDS connector, it is time to get to the T-CON itself. Since the T-CON is generally covered by a metal shield, you may or may not be able to get to the 12V TP without removing the shield.
There will always be a 12V fuse protection in 12V line. It will always be the first component the 12V passes once it enters the T-CON. So this is an easy test point to check for the 12V input to the T-CON. If its there the Check the fuse. If it is missing, check the LVDS cable for an open circuit from the Main board to the T-CON. Be sure (with the power turned off) to remove the LVDS cable from the connectors on both ends to investigate.
All T-CONs have a DC-to-DC converter which develops voltages for the T-CON board itself and for the panel. Some have more than one. It is important to check these voltages once the 12V input has been confirmed. To locate the main DC-to-DC converter, look for the larger coils on the board. In the example below, the two larger coils can easily be seen on the upper left hand side. The flat pack IC US1 is the main DC-to-DC converter driver IC. If you do not have any training material you can still pick out some key voltage source TPs from the DC-to-DC converter. Look for “VCC, VDD, HVDD, VGL or VGH” silk screened labels on the board. This T-CON actually has more than one DC-to-DC converter. In some case Replacing the DC-to-DC Converter IC can solve the TCON board fault. If its not then you have to go for a new TCON board.
When there is a video problem it can be difficult to determine if the problem is related to the Main board, the T-CON board or the Panel. Most of the typical T-CON boards (not the 3D FRC T-CONs) have a built in test pattern that can be run to isolate the problem. 
Also remember that to run the Panel Test;
# The Power Supply must be working normally.
# The Backlights must be running normally.
This test can also be used when there is video or the video has a problem.
If there is a Video Problem and the Panel Test works normally, before condemning the Main board be sure to investigate the LVDS cable.