I repeated my testing that I just described, and now I get power to
the old machine from the new Gigabyte's PSU. Ergo, I guess the PSU is
okay. Now I need to ask - how can I determine if my problem is the
AMD CPU or Gigabyte MOBO? Or the Corsair Ram for that matter? They
came from different sources.
Thanks
Big Fred
It's the motherboard. Or rather, it could be.
*******
The steps in the process are:
1) Plug in power supply with switch off. Power supply makes
no power at all.
2) Flip switch to "ON", on the back of the computer.
Power supply makes +5VSB (powers USB devices, motherboard
wake on LAN logic, some motherboard chipset stuff).
If +5VSB is overloaded, then you'll have zero volts
on the +5VSB rail. The fans don't spin, and the power
supply relies on convection cooling while making +5VSB
for you. So in this state, the fans aren't telling you
anything.
3) When the motherboard has a working +5VSB source, it
begins to "listen" for wakeup events. The motherboard
monitors the front-mounted power button. A momentary
contact on the front switch, causes a logic 0 pulse
which is latched on the motherboard. That's why you
can poke the two header pins on the FP_header with
a screwdriver and get the motherboard to start. The
pulse is latched.
4) The motherboard then drives the PS_ON# signal to the power
supply. The fans begin to spin. This is independent of
whether a CPU is even plugged in. Unless something extraordinary
happens, you should at least be able to get a "twitch" out
of the system fans, when pushing the front power button.
And that's because the motherboard latches the pulse on
the front button, and turns it into a steady level on
PS_ON#.
So things we would suspect:
1) Too much loading on +5VSB, causes the power supply to
shut off +5VSB. Remove external USB devices, coffee mug heater,
iPad charging, USB reading lamp etc.
2) Motherboard is not getting pulse from front power switch. Substitute
another switch for testing. You can use the computer case
reset button, as a temporary replacement for the power button.
(But you already checked for this, by using the screwdriver
tip on PWR button pins, as a test.)
3) Motherboard is getting pulse from the front power switch,
but is refusing to drive PS_ON# to logic 0. This sometimes
happens because the open collector transistor isn't working
on the motherboard. Or, the power supply can fail in such a
way, as to "overpower" the transistor on the motherboard, preventing
a logic low from being asserted. That would be "too strong
of a pullup on the power supply". You checked for that,
by swapping power supplies, and seeing the same symptoms.
The fact you've tried another power supply, suggests the
motherboard is at fault. If you were the tech in a shop, you'd
be using your multimeter right now, to verify +5VSB, and also
to look for logic 1 and logic 0 levels on PS_ON# (to see if
the motherboard responded in a useful way). Both those
signals are on the main power cable. You can stick a meter probe
between the ATX PSU wire and plastic connector shroud, where it
plugs into the motherboard. You can touch metal inside the
main connector, while the system is running, to take
measurements.
The PSU can be overloaded two ways. If you overload +5VSB
(draw more than 2 or 3 amps), the power supply just shuts
off the +5VSB. There is no visible or audible indication
of a problem if that happens. When you use an Asus motherboard,
a visible symptom is the green LED is not lighted on the
motherboard surface. (The Asus green LED is tied directly
to +5VSB for monitoring.) Other brands, don't waste $0.10 on a
green LED, so you need to use a multimeter to verify
+5VSB is present. My Asrock doesn't have the LED.
Alternately, you can use a USB reading lamp LED, to verify
that some +5VSB is present. But bottom line is, a multimeter
can tell you exactly what the voltage is on +5VSB.
If you overload the +3.3V/5V/12V main rails, the power
supply has the ability to ignore overloads for the first
35 milliseconds of operation. That time exists, to give
the capacitors on the system time to charge up. The result is,
the fans on the computer may twitch for the 35 milliseconds
that the power supply allowed output to appear. So if you
push the front power button, and you see the fan blade
"twitch", that tells you there is an overload on
3.3V/5V/12V. And you might use your clamp-on DC ammeter
with peak detection enabled, to verify the condition
and figure out which rail is shorted.
Your symptoms say motherboard, but I'd be using the
multimeter, set to volts, to review the status signals
on the main power cable. Power_good, PS_ON#, +5VSB,
are all of interest, both before and after you press
the front button.
*******
If you had a bare motherboard, fresh out of the box,
you can do the following test case.
1) Connect main ATX power cable to motherboard.
2) Connect POWER switch from computer case, to PWR
pins on FP_header.
3) Switch on the power supply at the back.
4) Push the power button.
5) Note that the PSU fan starts to spin.
6) Push the power button again, and the power
supply should switch off. Fan stops spinning.
7) Switch off supply at the back, when test is complete.
The motherboard doesn't need components connected to it.
The CPU socket has VID pin encoding on it. And if
the CPU is missing, the VID pins go to the all 1's
state, which tells the VCore regulator to make
zero volts of output. And that keeps VCore switched
off automatically. So the motherboard can still be
operated with no CPU and no RAM, and be used as a
simple means to get the PSU fan spinning. As a means
to test that the power supply is listening to PS_ON#.
That would be a complete test of the mechanism.
If you just want to test the power supply all
by itself (not connected to computer), you jumper
PS_ON# to COM. That's the green wire as far as I
can remember. You can get the wire colors from an
offical ATX PSU spec from formfactors.org. Jumpering
PS_ON# to COM, is a way to send a logic 0 to
the power supply ("switch on").
A final word of warning. There are some power supplies,
that like a minimal amount of loading when you operate
them. The voltages won't be in spec, unless the
minimum loading is present. I don't happen to own
any supplies like that, so I don't need to put a
couple hard drives as dummy loads on the PSU. But
it's something to keep in mind with older power
supplies. It would be a particular problem
for a $20 supply without OVP, as the outputs could
rise slightly above the normal 3.3/5/12V levels.
Such supplies may have two rows of rating numbers
on the label, with one row being the "minimal loading"
numbers. That would be a hint, that when lightly loaded
(testing PSU all by itself), some dummy loading should
be attached.
Paul