LG - 47LG90 - LED BACK LIGHT TROUBLE SHOOTING

A minimum of 30V required to light all 12 LEDs in a block.
Bottom 2 pins on the M,
Top 2 pins on the S.
Panes disassembly is required.  A simple forward bias diode check will cause the diode to illuminate if it is good.   Check any pin on right to any pin on the left.

The picture below shows a close up of one of the 1536 individual LEDs contained within the backlights. There are two sides each with 768 LEDs (64 blocks) in each side.  And individual block is manipulated by the T-CON board. These blocks are manipulated Global (All at once) and in accordance with the video intensity at the specific location of the block of LEDs for that video content (Local). In other words, if the video is dark in the area of the screen that is being illuminated by a block of LEDs, they will be dimmed.

Not counting “Black” there are 3 levels of intensity of the LEDs.  The Overall intensity of the backlights is manipulated by the “Global” Dimming signal from the BCM chip called Digital-Dimming.

LED BACK LIGHT TROUBLESHOOTING.

When dealing with the LED backlights that have a problem: Remember, the LED backlights are controlled by two inverters, so the screen is broken down into two sections. Then each inverter controls 8 sections of ½ the screen.

THE ENTIRE PANEL WON'T LIGHT.

•  Is the 24V being supplied to the Inverters from the SMPS? (CN1/CN501 Pins 1 ~ 5)
•  Is the Inverter On Command arriving at both Inverters? (CN1/CN501 Pin 12) (Main to SMPS P201 Pin 20)
•  Disconnect the ribbon cables from the T-CON PWB to the Inverters, if backlights turn on, could be a T-CON problem.

HALF THE PANEL WON'T LIGHT.

• Confirm steps 1 and 2 above for the particular Inverter involved.
• Disconnect the ribbon cable from the T-CON PWB going to the particular Inverter involved, could be a T-CON problem. (CN8/CN9)

SECTIONS OF HALF THE PANEL WON'T LIGHT

• Locating the connectors controlling that particular section should be easy, just compare the section that is out and align with the particular cable.
• Looking at the connector going to the LED backlights, look at the bottom 2 pins on the M inverter or the top 2 pins of the S inverter, check for 39V? If missing, unplug the connector and recheck.  If still missing, continue;
• Using the Interconnect diagram, locate the fuse protecting the 24V to the DC to DC converter that’s developing the 39V for that connector, if open replace the inverter.
• Be sure to test the Backlight LEDs that attach to that connector for shorted condition. With the connector unplugged, put the Red lead on the bottom 2 pins on the M inverter or the top 2 pins of the S inverter and the Black lead on any other 8 pins. All should read open. Or apply 30~38V on the bottom 2 pins on the M inverter or the top 2 pins of the S inverter and the (-) lead to any of the other 8 pins while observing that section of the panel, the LEDs should illuminate across the screen in the section controlled by that plug.

ENTIRE PANEL BACKLIGHTS ARE DIM

 Disconnect the ribbon cables from the T-CON PWB to the Inverters, could be a T-CON problem. (CN8/CN9).