Re: Troubleshooing advice, please...

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