(If the red light is already on the board when you plug it in or you can see it turn on when you hit the power button you can skip checking the voltage at the main board first). ( check for voltage behind the display board)
Find the 2 (or 3) plugs behind the control panel (remove top to acces) and find the large plug and test for 12v dc between the red and orange wires. Then test for 5v dc between the orange and yellow wire. IF the voltage is correct here, the main board is okay. If voltage is not good change main board.
You can remove the display panel and visually inspect as well. Found one yesterday that had a burnt resistor.
Checking main board voltage,
When you remove the cover of the board you will see a black and a yellow relay. (some times its blue instead of black) check for 120v at pin 1 of the black relay to pin 1 on the yellow relay. If your voltage is good, hit the power button on the front and the red light on the board should turn on (if not on already from plugging it in.)
Hope this helps.