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I have built 17 rigs successfully.......but this next rig....whoa.....what am I doing wrong????  I have fireworks and dont know why?

its a 12 gpu 570rx rig.  I am using asus z270AR  motherboard.  I use all 7 pcie slots, both m.2 slots and a 3 card expansion board.
I have a coolmaster 1200 PSU hooked up with a evga1600  using a dual PSU adapter.
I have the first 4 cards, their risers,  expander board and motherboard on the coolermaster 1200 psu.  These first four cards run through the expander board.  The expander board is powered from a molex off the same power supply(1200 coolermaster).
i have the next 8 cards, and their risers on the 1600.
I start it up and POP with smoke and spark.  it was very loud and startling.  i unplugged quickly
The pop  came from a card that is plugged into the 1600 using a normal motherboard pcie slot.  There could be other damage.  I dont know. This card is powered by an 8 pin on top.  This 8pin strand also powers a 6pin riser on another card above it..... which is also on the same power supply.

What am I doing wrong??!?!?

I would start by asking if you used only Evga cables to plug into the EVGA PSU and same with the coolermaster.  Some PSU manufactures wire their inputs differently and could cause problem if you use a cable other than what is provided. 

Is the Evga PSU powering the Motherboard 24pin and 8pin power?  You don't want to power GPU's with an external power source if they are plugged directly into the motherboards PCIe slot recieving pcie power from another source. 

Thanks Poop.

Evga and Powermaster use different style that are not interchangeable so that is OK.
The coolermaster is powering the mother board 24 pin and 8pin cpu cable.
All the GPU's are plugged into risers....so none are plugged directly into the motherboard.

Any more ideas?

Time for some basic PC troubleshooting steps.  First simplify your hardware config to  the minimum required set of parts to boot.  I presume you can boot up with just one PSU with minimum hardware config?  Can you boot with both PSUs connected?  With expander board?, with just one RX 570?, etc.. 

Ah ok, the way you worded it sounded like you had a GPU plugged into the motherboard directly.  Risers are usually the first things to fail so I would just try a new Riser for the GPU that made a pop and double check the connections and cables.  

Does anyone see any issue with taking one of the VGA strands that has an 8pin and 6pin on it and using the 8pin on the top of the card and the 6pin on a riser of another card?  both cards powered by same PSU.

No that should be fine for rx 4xx/5xx GPU.

I think your right.  everything in this rig is suspect right now.   I am not sure how much stuff I burned up.  I am going to get out a fresh MB and PSU and test cards one by one to see what i have left.  then I will test the old PSU one by one.  Then I will try something different.  this is the second time I have done this now with this rig.  I personally think its either the coolermaster somehow doesnt play nice or the expander board and the Asus Z270AR dont play nice.  It was a major burnout though....not good at all.  All this money sitting here idle now.  >:(

It maybe possible in your rig, that if you normally underclock/undervolt/underpower and this is a separate step, forgetting this step might be an issue. and by forgetting this step runs all 12 cards full out. Especially if the PS doesn't trip fast enough.

Thanks 64.  This happened as soon as I powered the rig on.  immediately a large POP and smoke.  I think the pop came from one of the cards.  maybe more than one.....not sure.

I would check and make sure both power supplies are putting out the same voltage on the 12v rails.  Seen some weird stuff with multiple power supplies back feeding the board.