The yellow light is good. When the light is red on the power brick, that means it needs to be reset by unplugging it from everything and wait till the light goes out. If it turns red as soon as you plug it up to the wall, you know the power brick is bad. If you have it plugged into the wall and the Xbox 360, the system is turned off, and the light is orange, the power brick is good. Now, if you turn the system on and then the power brick turns red, the system is shorting out somewhere causing the power brick to short out.
Check the ALL the slots in the Xbox 360 and make sure they are not damaged or anything is stuck in them. This includes the memory slots, USB slots, power cable slot, AV and HDMI input, anything that a cable plugs into.
If all the inputs look good, then it's a motherboard issue.