Sometimes when I press the power button it will power the lights and fans for a second and then turn off again. Some other times it will do that repeatedly, eg fans get a spin then stop, another spin then stop again, etc. for a while then it stops completely again.
If I persist, after 10-20 tries it will start. And once it has started then it doesn’t turn off. It looks to me like whatever latching mechanism the motherboard has to keep power on (a relay or some transistor) is failing. Is this something I can fix on my own?
It is not the PSU, the power button or anything else. I had the motherboard totaly bare (just CPU, all cables disconnected and no memory or any cards) on my workbench with a brand new PSU and tried to start it by manually shorting the pins with a screwdriver. It still did the same thing.
In your typical ATX style PC power supply, the PS_ON signal just shorts two pins together. You could theoretically have a motherboard problem, but I think it is more likely a power supply problem. My guess is that the power supply is either shutting down because it isn’t regulating properly or there’s a fault somewhere in the system that is drawing too much current which is causing the power supply to shut down.
I’d probably start by swapping out the power supply.
While it could be a capacitor or chip on your motherboard it s more likely a power supply issue, you can pick up a cheap power supply tester on eBay and go from there. If it turns out the problem is your motherboard it would be easier to replace the entire board than to try to find the bad part and replace it on the board.
Double check the settings in the BIOS. I have had strange behavior like this due to the settings for what the power on switch is supposed to do. E.g., suspend mode, full power on, etc.
When I see something screwy with power on, I press and hold the button for 5 seconds. (There’s a BIOS setting for this, too.)
Also, could be the battery. Is the clock running slow?
As everyone else has said, that sounds more like a power supply issue than a motherboard one. Just because a power supply is new doesn’t mean it works properly - has this motherboard been tested with more than one PSU, or the PSU tested with more than one motherboard? Have you tried more than one power circuit in your house? What brand PSU are you using? In my experience, Corsair and SeaSonic are the most reliable. They’re not as cheap as some others, but they’re more than worth it. I worked for a custom computer builder for several years and saw a few thousand machines go through using a dozen different brands of PSUs, so I’m speaking from a large statistical sample here.
I’d put the RAM back in for the purpose of your testing, some motherboards don’t like to boot without it. Also make sure the motherboard isn’t sitting on anything conductive, such as stray screws or even the anti-static bag it came in - those can cause shorts.
Does the motherboard have a built-in speaker or one that you can attach to the speaker pins? If it’s booting far enough it might give you beep codes.
If it’s a BIOS problem, you can pop out the battery for thirty seconds or so to reset it to defaults. It’s worth making sure you don’t have a dead battery, that can cause all kinds of screwy motherboard behavior as well.
How old is the motherboard? My experience is that they usually die of old age somewhere between four and seven years.
When/how did this problem start? If it started suddenly, did anything unusual happen in the day or two beforehand? Heat, moving the computer, a sudden shutdown? There can be power surges when the power comes back on after an outage. (I worked at an office that had a bunch of five year old computers that were all dying of the same motherboard fault. I could always expect one or two of them to die within 48 hours after our monthly generator test, the power surge was enough to tip them over the edge.)
Once you’re convinced it’s the motherboard that is the problem, the only thing to do is replace it. It’s not possible to repair motherboard faults outside of the factory.