No the wires are fine.
I've just wired it as a normal 12v psu and put the jumper for 5v back to int.
Everything works fine. The motors, heaters, fans etc. Looks like the problem is with the Ext 5v/PS on connector.
From this I looked at the schematics and the PS on circuit goes through TR7.
It took some finding as it is tiny but I have found it and it is burnt out.

I have another Duet Wifi that doesn't use PS on so I can swap them over.
Thanks for your help with this.

The power supply has voltages that are all correct 5v,12v etc.
I have tried another atx power supply and unfortunately get exactly the same results. All works fine until I press the ATX Power On. The voltage drops and everything on the 5v turns off. If I press ATX Power Off quick enough the 5v carries on working. The board looks to be shorting out the 12v rail on the PSU. Something must have blown.

Good call bearer.
I have an atx tester plugged in and everything is reading normal.
I may have to go digging around for an old power supply to see if that fixes it. If not I may have to order a new one.
I'll keep you posted.

Hi, this is my first post on the forum as I have always managed to sort out my problems reading the posts. But not this time.

I have an Anycubic kossel with Duet 2 Wifi (v1.04c) bought in Nov 19 + Duex 5. Power is provided by a 600w ATX power supply providing separate 5v and 12v using the PS on pin.
The printer has worked well for 7-8 months but I had a problem with it today. The printer initially turns on with 5v and the web server comes up fine.
As soon as I press the ATX On button, everything goes off.
To me it looks like the board has a short as the voltage drops when ATX is enabled and a message saying Vin has dropped briefly pops up.
I can smell that something has overheated but there is no signs anything has burnt out.
I have tried with all 3 fuses removed and only the positive wire and this still happens.
Is there an obvious component I can replace that could fix this or is it time for a new board?

Thanks in advance