Question LG 55GA6400-UD technical troubleshooting questions.

Sep 15, 2019
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I have a LG 55ga6400-ud. I don't see a standby light on the tv. The LG logo would illuminate when power button pressed a second or two later the LG logo goes off. It does not appear to blink though it does seem to fade and brighten but no apparent pattern to it. I've tried numerous outlets. I have no visible backlight, however I disassembled the tv and removed the one side mounted LED bar with 84 diodes on it. The bar was rated for 94v, I wired 4 9v batteries in series which gave me 38v output and tested the bar by touching each - pin while maintaining contact with the +pin, zero of the diodes lit up. I then wired 9, 9v batteries in series giving me around 86v total and tested the bar the same as before, all diodes lit up using the 9, 9v batteries in series. I have checked each individual pin on both of the power supply board outputs and all readings were normal with 120acv supplied. I'm not sure if the LED driver board input and output were correct; on the pin labeled "volt" I got a reading of 24.7v, on pins vr1 through vr6, I had readings between 0.9v and 1.1v, given that it took 86v to get the bar to light up I don't understand these readings. The main board has the correct voltage input values but only one of the three USB ports provide power (This could be part of the design I guess?), I know this from plugging in a Google Chromecast device into each USB port and a HDMI port. With the Chromecast attached to the USB that provides power I can press the power button on the and the Chromecast device status light lights up and shows as active and if I press the tv power button again it goes off so the main board and power supply board are powering on and off as they should with the press of the power button, or at least it seems they are. I do not see any backlight coming from the back of tv with the back panel removed, I haven't been able to hear any audio, and I do not see any image on the screen using a flashlight. Again, I removed and bench tested the LED light bar and all 84 diodes illuminated with proper voltage supplied to the corresponding pin. I haven't tested the input/output for the tcon board, didn't want to risk shorting something while checking those tiny pins, I did check continuity on the ribbon cable from main board to tcon with the cable unplugged and it checked out okay. Any ideas on what steps I should take next?
 

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