So, as mentioned in the previous post I installed the system on another motherboard I had while the one I wanted to use I had to send back because of there being no output from the iGPU.

I had tried two different CPUs, RAM, all 3 display outputs with no success. The rest was working fine as a discrete GPU worked. I also know all the CPUs worked from where I was installing and testing it fine .

I sent the motherboard away for RMA and they got it and showed it working fine. What the heck? So I arranged for it back and it didn't work still.

But when I tried the new Pico PSU, I got and it started working. But the other power supply worked with the other motherboard so what's going on?

I then tested 2 other power supplies with success and the original didn't work again and furthered on with investigation.

So what caused it?

The most likely explanation I have right now is that it turns out the power supply I was using has no -12v rail. Now there's no reason I know of that causes the iGPU to not work without a -12v rail. 

The negative 12v rail is more a legacy thing from where some ISA cards used them and it was sometimes used to either generate +24v for some devices or the negative rail for Serial (altough even this is rare as some ICs that did the logic conversion have a built in negative rail generators).

All I can think is that whatever circuit this motherboard has, somehow uses the -12v rail I suspect just enough to activate whatever circuit its using and doesn't even draw a lot of power. 

However for now as I'm using a pico psu then this isn't a major issue now. But was an interesting issue.