I'd reflash the bios one more time while you have the 1700x installed. Re-install the 3800x, clear the CMOS, and try to post again. Give it time. The 3000 CPU's will take an uncomfortably long time to post. Especially the first time. If it still fails to post shut it down, turn off the PSU, unplug everything (wall power and peripherals), and hold down the power button for 30 sec. Install only a single RAM stick, clear CMOS again, plug everything in, and try it again. I noticed the older boards can refuse to post with certain memory and 3000 CPU's. Even when they work fine with a 1st or 2nd gen on the same board. Again give it time. It will usually post cycle a few times and then run fans full blast when it can't post because of memory. Good luck.