PSU Help

[Danger]

Pretty sure my PSU is the problem but just picking the internet's brain first.  The system is below but the general history was:

 

COVID Build PC for home/office which ran strong for what it was.  First problem arose ~18M ago, PC wouldn't POST but this turned out to be the design of the Power Button on the front of the case (Phanteks 400S).  Replaced and no issues for a while.  Then the PC began to intermittently not response to waking up from sleep mode, couldnt figure it out at the time as work was hectic so i just disabled sleep mode and never got round to fixing it.  Always put it off, train of thought was... if i'm gonna open it up, might as well upgrade the board and processor but work + life didn't give me much time to set time aside.

 

Recently (~6M) there's been a fair bit of planned power outages as the hospital round the corner is having major constuction done.  When powers back on it would be hit & miss if the PC would power up first try.  Occasionally I'd resort to unplugging it entirely for an hour or so and this would fix it thinking that the CMOS battery might be dodgey.  The PSU never entered my mind at this point... 

 

I'd always get lights flashing on the ethernet adapter and the very minor slither of LED on the MB.  The fans not kicking over immediately was always assumed to be the cooling profile of the quiet set-up.

 

But the other day we lost power unexpectantly and can not get a rise out of the machine even with all the ol tricks... So i opened the case up and ran the usual no-POST checks... reseated cards, plug/unplug things and tried to flash up, new battery for the MB... but still no-POST so on to checking out the PSU.

 

PSU is still powering the MB but the quick jump of the PSU 24pin test yielded nothing.  The PSU fan didn't move.  Ran it back 4 or 5 more time and once I got the front fans to fire up.  Thinking it maybe an intermittant fault with the PSU cables or plugs I chased that down but still no obvious problem.  I'm guessing that the fault is in the guts of the PSU and this gradual degradation was actually happening for a while.

 

I was ready to say f$ck it and buy a new version of the same PSU to throw in and be done with it, but I thought I'd ask if anyone had any other fault finding to try before hand.  I don't have a spare MB or PSU, so I cant bench it to be 100%.  I've got amazon dropping off a cheap PSU tester today. I know its not the same as a bench test underr load, but atleast it will give me an idea what the voltages are across the pins and it'll be quicker that digging around in my shed to find my DMM.  I have a suspicion that my 8yo is using it as a grebble in his hidden base.

 

- ASUS Prime B365M-A Motherboard - be quiet! Straight Power 11 Gold Modular 850W Power Supply - be quiet! Shadow Rock Slim CPU Cooler - MSI GeForce GTX 1660 Super Gaming X 6GB - Team T-Force Dark Z 32GB (2x16GB) 3200MHz CL16 DDR4 - Intel Core i5 9400 Processor