jtag dead help

[matt1980]

hi all i have a jtaged 360 that was working fine arfter i had to reflash as had error 78 and has been working fine since. but today i turnd it on and the fans were running farst and then got three red lights. turned it off now will not turn on again the psu is fine and has a yellow light on and this dose not goto red at any time just the 360 will not power up. i have tested the two regualtors U5B1 (3.3V) and U5B2 (1.8V). they are fine but am now stuck dose any one know if u can meter any firther or any think else like diagrams of the stand bye power i can test i have tryed a new RF board and all the caps look fine. i have also got no power to my usb ports front and back. thanks