Hey all here's another power issue tip not mentioned which I have encountered with two different 3400s. I you open yours up to do repairs or modifications and do not re screw the DC board it will be loose and pop out of the main board.
This will prevent any power from any source from getting to the board and it will not power on as a result.

So don't freak out and loose all hope, its just a loose daughter board.

This is most likely to happen if you upgrade the hard drive to say a CF card with adapter and don't re-use the drive bracket.

I found this:

***Can iMac 5k 2017 upgrade to i9 processor?

"In this case the best you could install would be a 4.2 GHz Core i7 (I7-7700K)"

Does anyone know the names of connectors for the fan? I would like to power one externally if possible.

Thanks :)

I found that my cable was not seated very well, but feel as though like its not a solid placement in the port like id expect

my display seems to drop out from time to time, when it receives a bit of vibration and/or USB cables touched or adjusted in some way. I am using a thunder bolt cable instead of the built in cable which makes the display even less reliable.

any ideas there?

I think my display needs this fix at some stage, does anyone know why this cable does need replacing?

I am currently using it with the external port and Thunderbolt 3 to 2 adapter on my iMac 5K 2017, but its the only way I can reliably use it.

If the built-in cable is used, the display is not stable and stops being connected to my iMac.