UN55H8000 UN55H8000AF UN55H8000AFXZC UN55H8000AFXZA
This TV would turn on with no backlight and no video. I could heard sound effects when I moved the toggle button which confirmed the main board was working and the TV was on.
I first suspected bad backlights which is the case for 90% of Samsungs with no backlight. Unplugging the main board cable at the power supply end forces the backlights on and they came on. This test confirmed both the power supply and backlights were good.
I checked the BL_ON signal from the main board and it was high as well indicating the main board is likely working. On this model, there is an extra cable from the power supply to the tcon board which appears to control the backlight on signal.
The tcon board only had two out of three LEDs lit up and when disconnecting both ribbons to the panel, the third LED still did not light up. Powering up the TV with only one side connected at a time also did not produce backlight nor picture. This test indicates there is no panel fault and the tcon is suspect.
I ordered a tcon on Aliexpress and with the current shipping delays, it may be a few months until the part arrives.
Update: Tcon board finally arrived and TV powers up with backlight and video. The new tcon lights up with 3 LEDs.