Re: Troubleshooing advice, please...

Lotsa interesting info here, thanks. I've been poking at this thing all 
day with no joy. I can verify that the old PS is dead - no voltage on 
any pins, no matter what. I can verify that the new PS is good - it's 
one of the ones you mention that the fan doesn't come on unless it 
heats up to a certain temperature, but the expected voltages are on all 
pins, including the CPU connector. But that's as far as it goes. The 
motherboard has some lights on it that glow when the PS is connected 
and turns on, but nothing else happens, no fans, no drives spinning up, 
no display, not a peep or twitch. I can only conclude that the mobo has 
failed. Probably the PSU killed it while itself dying. Murder-suicide.

I wonder if it's possible that the PSU might test good with the 
multimeter but can't power up mobo + peripherals when all loaded up?

This is a bummer - I've always enjoyed building PCs for myself, but 
I've always known the current machine was ready to be replaced and I 
had time to leisurely deal with it. This is a catastrophic failure and 
I'm in the middle of some projects and don't have time to research and 
gather components for a new build. I'm inclined to go buy a Dell or 
something off-the-shelf.


On 20 Mar 2024, Paul <[email protected]> wrote in
alt.comp.hardware.pc-homebuilt: 

> On 3/20/2024 9:10 PM, Nil wrote:
>> I hope I can get some guidance here. My desktop computer that I
>> built several years ago just died on me - as I was watching, it
>> appeared to be  going into sleep mode, which it does after some
>> hours of inactivity. However, it could not be awakened, it was
>> dead. I immediately thought the power supply had gone bad, and in
>> fact, I tested it with a multimeter and it was, indeed dead - no
>> voltage detectable on any pin. So, I bought a new PS of similar
>> specs. Now, the motherboard (ASUS Prime Z270-K) lights up, but
>> there is no other activity, no fans going, no display, no disc
>> drives spinning. I can only wonder if when the PS blew it took
>> out the mobo or some other component.
>> 
>> So, my question is, 'what can/should I do now to confirm that the
>> mobo is truly, irretrievably gone?' If I can't, I will proceed
>> toward a new build or buy, but if I can revive the existing one,
>> VERY much the better. After all, it's tuned to a fair-thee-well
>> and I don't look forward to going through months of tweaking to
>> get it to that point again.
>> 
>> Aside... A year or so ago I replaced the mechanical boot drive
>> with an SSD drive. If THAT had gone bad, would the mobo not still
>> boot with at least a display and error message?
>> 
> 
> The indicator of PSU death, might be something like the state of
> Pin 9. If the PSU is plugged in, the PSU makes +5VSB which
> functions as a "supervisor voltage" and it's on Pin 9.
> 
> Anything on the motherboard needing supervision (while asleep,
> soft off, hibernating) then some +5VSB would be handy for that.
> 
> If PS_ON# is driven to Ground (logic 0), that's a Wired-Or capable
> signal that you can ground to test that a PSU works (the main
> portion, and its internal fan). If you ground PS_ON#, the fan
> should spin (on a PSU with a "normal" kind of fan curve -- some
> PSU don't spin the fan until the unit hits 50C inside which is
> unfortunate). 
> 
> https://en.wikipedia.org/wiki/ATX#Power_supply
> 
>    Purple   +5VSB    Pin 9
> 
> When the PSU is doing the job of making +5VSB (used by USB ports),
> it does not need to use the fan for that. Some PSUs get kinda warm
> doing that (a Bestek in front of me, blows a cloud of warm air out
> at startup, as proof the +5VSB thing was running). More modern
> supplies use an SMPS for +5VSB and are more efficient and have
> less waste heat. 
> 
> The integrated circuit that drives PS_ON# to ground, it needs
> power and that power source is +5VSB. You can drive PS_ON# to
> ground yourself, with a wire to a black ground, or a wire plus a
> switch (if you want a convenience switch for turning it on and
> off). 
> 
> A motherboard that cannot drive PS_ON# to ground, then that
> motherboard will never get to spin the fans or make any other
> happy noises. 
> 
> And PS_ON# signal failures are fairly frequent, for unknown
> reasons. 
> 
> At one time, the driver for that on some motherboards, was a
> Fairchild 74F with 64mA drive capability. Which gave me a good
> chuckle, because it said "this here motherboard is not taking NO
> for an answer". 
> 
> Later designs only had 8mA drive and were more easily
> pistol-whipped into failure. The circuit really should never need
> more than 2mA of sinking capability. But I think the individual
> who was using the 64mA drive, that guy deserves a medal :-) You
> want an IC that will "catch fire" rather than take NO for an
> answer. 
> 
> *******
> 
> You CAN do ohms tests between rails. With the 24-pin connector
> open, you could ohm from +5V to GND, +12V to GND and so on.
> But be aware there could be 2000uF of bypass caps on such a rail,
> and this makes it hard to distinguish what you're measuring
> (the dead short of a charging capacitor).
> 
> The PSU has a Power_Good signal. It indicates the rails have
> charged to min-voltages. Maybe Power_Good is logic 1, any time
> +12V is above about +11V or so.
> 
> The motherboard also has an internal Power_Good of its own.
> All the tiny power converters on the motherboard, that make
> 1.5V, 1.8V, 2.5V, and so on, those can have status indicators
> that say they are working nominally.
> 
> The motherboard can only come out of RESET, when all the
> Power_Good are logic 1. The Reset generator might have a
> half-second pulse on it, but the Power_Good extends the
> board-level RESET for as long as it takes for Power_Good
> to go to logic 1.
> 
> After about 35 milliseconds or so, the PSU starts to check
> for overcurrent. The 35 milliseconds gives time for the PSU
> to charge the motherboard bypass capacitors. The 35 millisecond
> period where it does not check for overcurrent, causes computer
> case fans to "twitch" and then you know PS_ON# was asserted,
> the PSU did try to start, but it tripped off on overcurrent.
> And normally, you have to (slowly( toggle the switch on the
> back OFF. Then wait 60 seconds and turn it ON again, to attempt
> another start. Seeing a fan twitch, is a "symptom" to record
> in your lab report :-)
> 
> Mobo VCore is a rather large SMPS, and capable of a couple hundred
> watts of output. On the machine I'm typing on, VCore varies
> between 0.35V and maybe 1.2V or so, at up to 200 amps. It should
> have a Power_Good indicator too, and the CPU cannot be started,
> unless VCore is nominally working. When the CPU socket is empty,
> the VID signals float to logic 1, and the VCore all-1s code
> is "shut off". The motherboard will not arm VCore, if the socket
> is empty. This also means the motherboard would not enable all
> Power_Good, if the CPU was missing. Not that it would matter
> in that case, of course.
> 
> You can see that all of the power conversion machinery must
> be in top-rate form, for the CPU to start running firmware code.
> Nothing short of perfection, will work, when it comes to power.
> 
> But if the +12V comes on, you should be hearing case fans spin.
> 
> *******
> 
> I've had two motherboard failures now (P4C800-E ICH, P5E
> X48/ICH10), and the failures seemed to be Southbridge/PCH and
> something to do with one of the five converters that feed the
> Southbridge. But once the SB is cooked, you'd have a load of fun
> trying to fix that (hot air station, on motherboards not intended
> to be repaired). 
> 
> *******
> 
> The PSU can fail if the slow-blow fuse inside it, blows open
> circuit. That hardly ever happens. If the 11kV outside your house,
> fell on the 230V wire, then that would be sufficient to blow the
> fuse. The fuse of course, is not rated for 11kV, so there would be
> mondo damage. In any case, it would take an extreme event to
> blow the fuse. Minimum would be a close lightning hit. That
> might do it. Plus fry a lot of other shit.
> 
>    Paul
> 
>