Replacement 3950X received under warranty. Back up and running! There were no hassles at all with AMD. Their warranty support is very good.
Confirmed dead 3950X. I've tried an alternate power supply, DRAM kit, GPU and motherboard. The 3950X won't boot with any of them. Ordered a Ryzen 3600 to try out. It works in both the original system and the 2nd one I built up.
Btw, the 3.65GHz underclock was at 1.1V Vcore under load. While the folding workload was stressful, the settings were not.
My hardware is in my sig. (3950X, Gigabyte X570 Ultra, B-die 2x16GB, 5700XT, Seasonic Prime 750W)
I've been running this system since January. Various PBO overclocks before settling on all-core OCs. While I found a 4.35/4.25GHz stable OC, I usually run a 3.65GHz all core underclock because I've got the CPU running folding about 60 hours a week. I mention this because that is pretty much the most stress the CPU has seen. Even my PBO days, I made sure the CPU never saw 1.3V under load.
So tonight, just sitting idle, the machine went unresponsive, black screen.
Turn off the PSU and Reboot: nothing
Reset CMOS: nothing. Looking at the status LEDs: "CPU" comes on briefly. It progresses to "DRAM" and stays mostly there, but occasionally starts over and CPU lights briefly before settling back on DRAM.
I swapped out DDR4 for another set I had. Tried both sets 1 stick at a time, one channel at a time. Cleared CMOS each time. Same result. Stuck at the DRAM light.
Tomorrow, I'm going to borrow another PSU and GPU to swap out, but I'm not too hopeful. I went ahead an ordered a Ryzen 3600. If the PSU or GPU solve the problem I can return it. If they don't, I'll use the 3600 temporarily while I going through the warranty claim attempt.