*Troubleshooting* Wits End

[archvras]

I've decided to post on this forum after two months of being without a PC (it's been hellish, to say the least). Forgive me if this isn't necessarily the right subforum for my specific issue but it's the closest one I could find.

My problem first arose, as stated before, two months ago when I woke one morning to find that my PC wouldn't POST. When I powered on, I would get a brief flash of the motherboard light and a momentary spin of the fans but nothing beyond that. I was very confused because I had not had any issues for the last two years, not even the night before. That day, I drove to the nearest Best Buy and picked up a CORSAIR 650m to replace my EVGA fully modular 650w 80+ Gold, but I had no luck there. The PC in question also had an i5 6500, an MSI R9 390, 8gb G.Skill Ripjaws V, a Western Digital 1tb, and a basic ATX Gigabyte mobo that cost around $100 in early 2016, all packed into an NZXT S340.

Flash foward to a month ago, I ordered a Ryzen 1700x and a Gigabyte AX370-Gaming, figuring it was a mobo and/or CPU problem. I built the machine and, what do you know, I had the exact same issue as before. After that serious let down, I sent in an RMA at Gigabyte for the mobo, shipped it, let them do their work, and got it back only for it to still not function (I don't consider flashing lights to be worth over $100 haha). That was last Friday. I'm hesitant to do the same for the CPU as my original problem still remains persistent, and I don't really believe in coincidences. BTW I didn't replace any other parts in the PC besides the CPU/mobo. 

So that leaves me to now. I know it isn't a PSU issue, and I've tried booting it without the RAM, GPU, and SATA connected, all in different combinations, all to no avail. I hope there somebody reading who can assist me, so thanks in advance! 

[emosun]

so to clarify , the power supply turns the machine on for a second then shuts off right away?