3NT

Well, hope this helps others. I went thru the checklist, but did not reseat the CPU - too risky, from my perspective. My Seasonic Focus+ 550 lacks two cables that the 650 and higher models have: a second 8-pin to 2 x 4-pin CPU cable, and a second 4-connector SATA cable. I ordered both, because the existing 2- and 4-connector SATA cables might not reach everywhere I need them to. Cables from BTOSinte.com are approved by Seasonic; generic cables will void the PSU warranty. Cost of cables: $5 for SATA and $5.35 for CPU. Do NOT order the 4-pin to 4-pin, which is for older units and only saves a buck or so, anyhow. I upped the $6.60 shipping to $8.90 for priority mail, not clearly a win, but I did get USPS's confusing tracking. The system boots with the two 2 x 4-pin cables!!! (On one use only the one 4-pin connector that fits.) Conclusion: for an X570 mobo, be sure you have two 8-pin to 2 x 4-pin CPU cables, in addition to the usual 24-pin power. Most mobos only require three of the four 4-pin connectors, but the highest-end mobos use them all. Be smarter than me, and get a PSU with sufficient connectors in the first place! I took too much for granted.

New build, using components listed below. Thanks in advance for your help with this problem. Mobo has 24-pin connector, no problem; also 8-pin (2x4) and 4-pin (2x2). PSU has only a double 4-pin (two 2x2) connector on one cable, which I plugged into the 8-pin socket on mobo. Turned on power at PSU, then pushed power button on case: CPU fan and a case fan connected to mobo are spinning at apparent full speed. Display does not know there is a computer there. The mobo has four LEDs on the edge, labeled VGA CPU BOOT DRAM; the CPU LED is on (red). In "X570 motherboards means a new PSU!" topic on this site, Astyanax posted: This motherboard, and the Pro both have prior history of WHEA errors when only the first EPS is connected. Assuming this to be true, would it be sufficient for me to order an additional 2x2 cable from btosinte.com, and connect it up? The PSU has lots of empty sockets for such. If an extra cable is needed (that is, all of 24-pin, 8-pin and 4-pin), the "NEW PSU Tier List" topic should be updated to note X570-readiness. I also understand that the 4-pin connector is replaced by an 8-pin on some of the most "godlike" motherboards, so levels of X570-readiness might be appropriate. (I did scope out this last topic, before I started assembling my gear.) If this turns out to be so, I'll post here and on that topic. On the other hand, in topic "Ryzen 7 CPU led on", Zyndo posted just four days ago: I'll go thru this list as soon as I can. The Seasonic power calculator indicates this PSU would not be used much over 50% of its capacity, since I don't overclock, do action gaming or other heavy graphics. Seasonic Focus+ 550 full modular PSU GIGABYTE X570 Aorus Pro Wifi AMD Ryzen 3700X with wraith cooler ASUS GeForce 1030 CSM (fanless) Samsung 2343BWX monitor (old, for setup) 16 GB Corsair Vengeance LPX DDR4 3200 (2x8GB) Samsung 970 EVO Plus 500 GB Fractal Design Define R5 quiet case ASUS 24x DVD burner Kingwin KF-91 mobile rack More HDD/SSDs to be added later

There appears to be difference between X570 and X470, with multiple M.2 slots. On X570, two slots would both be Gen4, but the second might be throttled by the chipset, which it uses. Same setup on the X470, but the chipset is PCIe 2.0. If you think you might want to use that slot, X570 appears to be better, even for a Gen3 NVME such as a 970 EVO [Plus].