Ok, this is long and very puzzling. I've built a couple of systems and am a computer engineer so I thought I understood these things, but I am stumped at what to try next.
Short story: breadboarded brand new motherboard with brand new CPU does not POST, boot or beep. And I've tried everything, including replacing every component including PSU and clearing CMOS.
It started with what supposed to be a high-end gaming rig. i7, water cooling, M2 etc. After putting everything together it didn't boot up, so I started to remove components, even tried with a spare i5 I had. Fans spin up, MB LEDs light up, but no boot or POST.
Breadboarded (put only motherboard, CPU, fan and memory together outside of chassi) but still nothing. Attached a chassi speaker, but no beeps, not even without memory.
Returned motherboard (Asus Prime H270-PRO), came back with one bent pin fixed (but they couldn't tell me if that was on the board or in the socket) but with "OK working" from them. They even upgrade BIOS.
Full of hope, put back the i7 (7700). Still the same, fan and LEDs turn on, but no boot, POST or beep. Swapped to the i5 (6500). Same thing.
Tried another PSU (from my day-to-day computer, so I know it's working). Still the same.
Gave up, got another motherboard, and just to be sure, a brand new Celeron G3900, in case the two other CPUs got fried by the first board. Breadbording this motherboard directly with the Celeron, attached the ATX and CPU-power lines from my new PSU, outside of chassi.
NOTHING! Still not even a beep...
The new board is Gigabyte GA-Z270P-D3. It spins up the fan, rew's it a few times, settles on a moderate speed for a few seconds, then turns off. Repeats this sequence over and over.
Tried my other, working, PSU, still same. Cleared CMOS, still the same.
So it seems like no combination of two motherboards, two PSUs and three CPUs doesn't even get to POST...
Is it possible that the problem with the first motherboard (bent pin?) fried my to CPUs and broke the PSU so that it in turn broke the other motherboard?
I haven't tried the i7 or i5 in the second mobo, since I'm afraid that they could damage this board if they are bad, I wouldn't think so, but I'm starting to doubt my senses here.
Question 1: What does the cycling of the fan speed on the GA-270P indicate? Malfunctioning fan header? Tried one of the other original fans, so it isn't that.
Question 2: So, is there a scenario where the two CPUs were fried by the bent pin (assuming it was in the CPU socket)? And could that have broken my new PSU so that it in turn broke also the new motherboard?
Question 3: Can I safely put one of the first CPUs in the second board without risking that board to go bad too (assuming it is not bad already)?
Question 4: Am I missing something else, really fundamental?
EDIT: Progress on this have been slow due to other commitments, but it turned out that the second board also had bent pins (Is it me? Starting to doubt my abilities...). So we know nothing about the state of the processors. Might still be that the first board is still not working, even stated so by supplier after "fix". Next step, send back board with CPU:s if supplier accepts our claim, or by another motherboard... Sigh...