VGH and VGL production mode by the TCON board. The charge pump converter. 

the cause is either the TCON or the screen. You can try to repair the TCON. Be careful, all the manipulations described here can destroy the TV set if they are not done properly. To avoid this risk, it is better to entrust the television to a technician. First of all, it is necessary to find the voltages VGH and VGL. Then, it is necessary to see when the image is correct while measuring with the voltmeter the test points VGH_odd and VGH_even. It is necessary to note on a paper the voltage which gives a normal image. Example VGH_odd the image is correct if VGL (-7v) and VGH_even the image is correct if VGH (27,9v) or the opposite. It is necessary to remove the tracks which go to the screen VGH_ odd and VGH_ even on both sides then to connect the tracks sectioned with a very fine insulated wire to the VGH and a second wire to the VGL according to the preceding observations.

VGH and VGL are 2 voltages for the gate of the TFT transistor in the screen

@@elearningélectronique I must have a bad one somewhere is series in the display, The display is 2018 55" 4K. all glass, no side connectors. I measured these 2 voltages all the way out the the ends of the panel. last chance is a $12 working pull TCON board. Just a little gamble.