LG - 42 LH90 _ LED - BACKLIGHT INVERTER_ CHECK

WORKING PRINCIPLES 

     The Inverter receives 24V from the SM PS on CN1 pins 1~ 5. The Inverter On (INV ON) command from CN1 pin 12 starts the Inverter drive signals. PDIM is delivered from the Main board through the SMPS to the Inverter on CN1 pin 13.

    The Inverter is responsible for delivering B+ (27 .5V) to each of the 64 LED Blocks. This is accomplished by 4 DC to DC Converters (M1~ M 4 ) on the board, (easily identified by the 4 yellow capacitors on the lower left of the board.

• M1 and M3 are routed out CN4
• M2 and M4 are routed out CN3

     It also must deliver grounding pulses (Drive Signals) to each of the 64 LED Blocks. This is accomplished by the 32 switching components [Q19~ Q 50] each controlling 2 blocks.
The Left hand connector CN3 connects the 4 LED Boards (32 blocks) on the right side of the screen and the Right hand connector CN4 connects to the 4 LED Boards (32 blocks) on the left.

GENERAL 'LED' BACKLIGHT BOARD INFORMATION [AS VIEWED FROM THE FRONT]

There are 4 LED baords per / side, 8 total. 
There are two Rows of LED blocks per / board.
There are 4 LED blocks per/row.

There are 8 LEDs per/block. There are total of 512 lEDs

BACK-LIGHT BAISCS

The picture below shows a close-up of one of the 512 individual LEDs contained within the backlight boards. There are two sides each with 256 LEDs (32 blocks) in each side. An individual block is manipulated by the Main board Dimming IC IC3502 in accordance with the video intensity at the specific location of the block of LEDs for that video content (Local Dimming). In other words, if the video is dark in the area of the screen that is being illuminated by a block of L EDs, they will be dimmed.
Also, all blocks are manipulated simultaneously via (Global Dimming) by IC100 Video Processor. The Overall intensity of the backlights is manipulated by the “Glob al Dimming” signal from the Mstar (IC100) chip called Digital- Dimming (PWMDIM)

This represents one block.  There are 4 boards per side.  Each board contains 8 blocks.  Totaling 32 blocks per side, 64 blocks all over.

  
A minimum of 21V required to light all 8 LEDs in a block Pins 1,2,13,14,16,17,28,29,31,32,43,44,46,47,58,59 On CN3 and CN4.

In a Diode Mode check, each LED can be checked by a multimeter. [2.6V reading]

Panel Disassembly required. (Not during warranty period.)

A simple Forward Bias diode check will cause the diode to illuminate if it is good.

Check any pin on the right to any pin on the left.

LED BACKLIGHT DIMMING [LOCAL DIMMING]

The graphics below show the relationship between the LEDs reaction to real time video on the screen, this is called “Local Dimming.

The areas in the picture on the left shown by A, B, C and D show the dark areas of the screen. The picture on the right shows the backlight LEDs. Note they are dimmed out in the same area as the dark video on the left.

To see a representation of 'Local Dimming' the TV has to be set to the 'Store Mode'.

Actual Video Capture.

Actual LED reaction capture.

INVERTER COMPONENT IDENTIFICATION.

INVERTER 27V [M-1 - M-4] POWER SUPPLIES INFORMATION.

There are actually 4 power supplies developing the operational B+(27.5V) for the LED blocks.  They are designed as 'M1 through M4'.  Each of the 4 have same components.

Each of the 4 capacitors C12, C27, C212 and C227 have a Test Point [TP] just above the (+) leg of the capacitor.

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