Which model is this ? I did spend a lot of time in fixing Samsung Plasma TVs.
Some Samsung Plasma TVs have builtin test function. To exclude the possibility of main board, you can disconnect the LVDS cable from main board to control board (the smallest one) and look for a jumper on the control board. Connect pin 2-3 (I hope I remember it correctly) and power up. This will start a self test on the display. If you see some patterns on the screen, then everything except the main board (with lot of video inputs) is the problem.
I have rarely seen a main board going bad, most of the time it is the power electronics, either Y-Sus or X-Sus or the Y-Buffers.