Toshiba Satelllite C55T won't post

[PurplePJ]

I'm dealing with a maddening problem.

Toshiba Satellite C55T won't post.

When pressing the power button, the laptop fan will spin up, the HD spins up, but it never posts. After 30-45
seconds, the system powers off. No Toshiba logo, no beeps, no nothing. Same result if connecting an external monitor.

HD tested good in another machine.

Tried multiple sticks of known good RAM, all DDR3L (low voltage as required by this MB).

So, I proceed to strip it down. I get to the point where I'm doing a benchtop boot of the MB. All I've got
left is the motherboard, RAM, power button, and power supply. I use a voltmeter to confirm correct voltage from
the power supply at the point it feeds the MB (19.4V). I get the same result, power button LED lights up, then
turns off after 30-45 seconds without posting.

Tried ordering and replacing the power button ribbon cable/board/led assembly. Same result.

Tried ordering a new motherboard. Same result.

Tried ordering another new motherboard. Same result.

At the moment I effectively have two systems in place... two MB's with separate RAM and separate power buttons. The
only thing in common is the power supply, but my voltmeter says it's providing 19.4V (it's rated for 19V). Both
systems behave the same and power off after 30-45s

I'm going nuts trying to understand what could possibly be wrong.

The obvious answer is the power supply -- what is the allowed variance of a 19V power supply? I'm assuming 19.4V is
perfectly fine, but I'm no electrical engineer.

 

[PurplePJ]

I ruled out the power supply and/or connector as well by trying to boot the systems from battery only. Same results. They power on and give all indications that things are fine when I have peripherals connected (HD and fan spin up, LEDs light up, etc). I think the graphics are working because the external monitor complains if it's lost a signal and that goes away when I attach it to the MBs.... yet nothing ever shows up on the monitor, and the boards power down after 30-45 seconds.

I should note that I always have a heat sink attached so they shouldn't be powering down due to overheating.

These are K000889110 Toshiba Satellie C55T-B5109 Laptop Motherbaords w/ Intel i3-4005U