Tab no longer powers on after completely draining battery.

I have been trying to repair this tab for a few weeks now. This is what I have done:

- Replaced battery with new one

- Replaced usb connector with new one

- Replaced power and volume button.

And still nothing happens, no Battery icon on screen (Black Screen) or lit up buttons. Could it be the motherboard or maybe the lcd screen? Is there anyway to test this?

Help would be appreciated.