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PC power up / unreliable startup debugging

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So I built myself a PC a few weeks back and it has one annoying persistent problem. It doesn’t power up first time every time. You have to turn it on and off a few times with the soft power button to get it to come up. Once it’s going it’s absolutely rock solid and passes 24 hour memtest86.

Starting this thread as a “debug thread”. If anyone has any suggestions please let me know

Power supply is an Be Quiet Pure Power 11 and the board is an ASRock B450M Pro 4. Nothing is overclocked.

Debugging so far shows correct application of PWR_ON# but an RC curve on the ATX PWR_OK signal which is border line iffy. And it seems to be thermal related (probably crap caps)  as it starts up fine when warm.

So far I have tried:

1. Moving devices to non USB 3 ports. This improved the RC curve. But not enough. This was coinicidental.
2. Checking voltages. All fine when powered up. .
3. Visual inspection of caps. None looking dodgy.

Therefore next steps are:

1. Wait until it’s cold tomorrow morning
2. Blast the board with a hair dryer for a bit to warm it up.
3. See if it comes up. If it does, replace the board.
4. If not, wait until next morning, blast the power supply with a hair dryer and replace that if it helps.

Amy more ideas or thoughts appreciated.

Well, start with the basics. Is the switch actually ok? Because many a PC engineer has been caught out with bad case switches. (Me included!)

Second, you say it improved moving USB stuff about. It could be to do with which USB's get powered with what. A PC PSU is actually TWO PSUs, there is an always on one when AC is supplied +5Vsb, and then there is your main one with all the major rails, +12V +5V +3.3V and -5V (I think, or -12V, one was removed in the revised ATX 2.1 spec).
Now, some motherboards will power some USB ports from the +5Vsb, some only power from the +5V, some boards let you choose where some ports are powered from. The +5Vsb supply is limited, it can be anywhere from 1A - 3A (higher on better supplies) but it is possible if you have high draw USB devices to overload the +5Vsb, and in some cases it'll prevent starting cos it sags or trips out on protect. Though you usually need to remove the AC and restart the PSU if it's in protect.

Anyway, its a long winded way of saying, make sure you know which USB ports are being supplied by which power rail (I should be in the manual), and figure out how much the +5Vsb can supply (PSU manual), and how much your USB devices are drawing. (RGB keyboard by any chance?)

Hopefully that gives you a place to start.

Thanks for the reply. Yes the switch is ok. It does actually illuminate the power button when you press it but does not boot the machine. When you press the power button again, the power light goes off. Just no boot action takes place. I've got a backlit keyboard but I did try it with a generic keyboard and that didn't make any difference.

I've pulled all USB devices other than a generic non backlit keyboard and that made no difference.

Ah, if the power light comes on, that's a different problem. That means the BIOS isn't booting.

A few things you can check.(well things that can prevent booting) -

Check the bios battery is good. Check the BIOS Battery jumper  (CLR CMOS)
Memory could be an issue preventing boot, so try with one stick in the first slot.
Check your BIOS version, and see if an update is available - in some cases you need to update for some CPUs to be properly supported.
I'm not familiar with UEFI bioses, but I have heard there can be issues there, maybe worth a google. (I think they store drivers in them now??)
And there's the more obvious check all power connectors are tight, no loose ones shorting to the case (though a power light means this isn't the problem)
Maybe conductive thermal paste on the CPU perimeter?
Rogue stand off under the motherboard shorting?
Check the reset switch isn't holding it in reset?
That's about all I can think of, before saying to RMA stuff.

Check your power supply, this might help

Edit As LateLesley says, check your reset button has not either stuck or is short circuited, I've know that to happen before too.


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