Saturday, January 07, 2017

Tech troubleshooting – TCON board – Backlight Dimming through TCON board – Deference between TCON and 3D TCON board - Purpose of LED in TCON board

T-CONs Deliver P-DIM (Dimming Signals) to the Inverter
P-DIM (Called PWM-DIM which stands for Pulse Width Modulation Dimming) is a signal from the Main board that is generated by monitoring the Video content’s average brightness level. It is then sent to the Backlight driver IC to control the brightness of the backlights accordingly. The maximum brightness level is established by the customer through the Customer’s Menu in the Video selection under Backlights. This setting has a bar graph that can be set from 100% down to 0%.
The P-DIM signal will follow the percentile setting proportionately. The range of P-DIM is 3.3V to 0V. So when the bar graph is set for 100% the P-DIM line will be 3.3V and the backlight brightness will be maximum.
Most often the P-DIM line is routed directly from the Main board to the Power Supply, it is routed through the Power Supply directly to the Inverter and then to the Backlight driver IC.
(If the Power Supply has an on-board Inverter, the Driver IC is on the Power Supply itself). However,on some sets, the P-DIM line is routed to the T-CON board.The T-CON then generates a control signal that is then routed to the Inverter and then to the Driver IC.
Here is the T-CON from a 42SL80. The connector in the bottom left goes to the Inverter and carries the P-DIM Control Signals. The P-DIM signal from the Main board is brought in on pin 8 of the left LVDS and is then routed to the large MCM IC. Then it is output through the 8 pin IC on the bottom left to the connector.
Troubleshooting the P-DIM Line when Routed Through the T-CON
When troubleshooting this type of P-DIM routing, you will still use the same procedure. Enter the Customer’s Menu and go to Video, then select Backlights. Place a DC voltage meter on the output from the T-CON CN3 or the input to the Inverter CN2. You can check either of the two pins. While adjusting the Backlights from 100% down to 0% the P-DIM lines will vary accordingly from 2.4V at 100% down to 0.67V at 0%.
Pin
Label
Run
Diode Test
1
n/c
n/c
n/c
2
SCAN 1
0.67V>2.4V
Open
3
SCAN 2
0.67V>2.4V
Open
4
Gnd
Gnd
Gnd

Function of the Blue LED on Some T-CON Boards
On some of the LCD T-CONs there is a bright Blue LED that turns on for a brief moment during turn on and then it shuts off. These are only used in set with Florescent Bulbs as the Backlights. As in the example below (from 42LG60), it shows the Blue LED is on the lower right hand side.
The purpose of LD1 is to help excite the Selenium gas in one of the backlight bulbs. This helps the bulb to light when there’s little room light by pre-exciting the gas. With little room light, the gas in the florescent Bulbs tends to lie dormant, (Little movement). This LED help to get the Electrons moving just before the point of firing (turning on).  There is a small hole under LD1. This is how the light goes through the board to get to the Bulbs. (There is a hole in the back metal cover of the panel as well).

Differences between a T-CON and a 3D FRC T-CON
The typical T-CON’s basic responsibilities are to drive the TFT panel and to generate panel voltages.The video input through the LVDS cables is already formatted for usage by the T-CON board.
Typical T-CON
The 3D FRC T-CON basic responsible to; generate panel voltages. When not in 3D it generates the Tru-Motion, Motion Estimated Motion Compensated (MEMC) frames. When in 3D it unpacks 3D frame content. It generates 3D sync which is used by the 3D Shutter glasses. It formats the video for usage by the panel driver circuit and it drives the panel. The video input through the LVDS cables is 24 bit LVDS video.
3D FRC T-CON
Since the 3D FRC T-CON does so much more work than a standard T-CON, it has an additional 12V input via a two wire connector. The voltage is from the same source as a typical TCON board. It comes from the  power supply and switched on/off by the Main board.