need help my A4000 won't start with new A4000 ATX PSU adapter

[dalek]

* -5V is not needed on the Amiga
* always test voltages with a dummy load applied - you can't assume the voltages of an unloaded supply are 'in spec'

Also check the 3.3V Sense wire - apply a dummy load the 3.3V line and also connect it to the Sense wire - more info here: https://reprap.org/wiki/PC_Power_Supply#Voltage_Sensing

[dalek]

I'll disagree on the -5v being needed, as -5v runs though the video slot, the Zorro slots, and the RGB connector. 

I should have been more specific.  It's not needed to be supplied by the PSU to the A4000.  If you tell him he needs -5V from his ATX PSU that is wrong.

-5V on the A4000 is generated from -12V on the motherboard.  So it is not needed to be supplied by the ATX PSU he has.