New PC will not boot with a Radeon GPU anymore

I built a new PC. It was working fine for a couple days.

I found the system unresponsive after using it this morning. The RGB lights on my motherboard were on and the numlock key on my keyboard was on. The system would not respond to anything including a long press of the power button.

I shut off my PSU at the switch on the back and disconnected everything except the video card and cpu fan. This includes all front panel connectors, sata connetors, etc... (new build - something could be broken). Still would not power on but my RGB lights were working still. I killed the power again and removed the Radeon video card and the PC came to life when I hit the power button with only the CPU and CPU fan installed. When I installed the RX470 again the system would show RGB but not respond beyond that. I tested all 3 of my PCIe slots with the same results. I also tried a different cable from my PSU box (I have 3 extras) with the same results.

I connected everything and installed a nVidia GT1030 from a Lenovo I have. The system works perfectly with the nVidia grahics card from my Lenovo. The 1030 does not require the 6 Pin power.

I am going to test the RX470 in a PC at work tomorrow if I can find a PSU that will work.

I used a meter to test the voltages on my 6 Pin connector and they are all correct; I can't test amperage. It appears my PSU is likely OK but my video card is dead. Would you agree? Is there anything else I can do to test the RX470? I don't have another GPU that requires pcie power so I cannot test the PSU outside of the voltage right now.

CPU: Ryzen 5 2600 - stock cooler until my H100i Pro arrives
Motherboard: Asus Prime X470-Pro
RAM: CORSAIR Vengeance LPX 16GB (2 x 8GB)
PSU: evga Supernova 750 G2
Boot Drive: Western Digital 3D Nand SSD 500GB (plan to upgrade to nvme) (Bought 3/2018)
Storage Drive: WD Black 2TB (Bought 3/2018)
Media: DVD-RW and Blu-ray Drive (used)
Original GPU: Visiontek Radeon RX470 (used - and no longer working after about 2 days in new system)
Current GPU: nVidia GT1030 (Lenovo OEM or add-on from previous owner)

I had every possible problem and often cleaning contacts helped. i am also cleaning contacts of new stuff the folling way. maybe also a half-dead PSU? yes even new.
The latest one is corsair CX650M -removable cables. would never buy something else. good invested 90 chf.
someone told its not ideal using a fiber-erazor to clean contacts. i am using a hard erazor instead, a round thin flat one.
then wiping dirt and final cleaning with special pc-contact cleaner(spire cleanser spray) or 90% alcohol. i am using a dental brush not used yet....lower than 90%/70% alcohol would do but one must wait until water is evaporated. 30 sec?

I think the card died. I tried it in two other systems at my work today. All the computers had the same issue with the Radeon installed - no response to pushing the power button. I can't even get fan spin.