w_tom said:
Many would consider a value of 4.85 at OK.
They'd have to since it well within specification.
That would be
too low and could indicate a problem with power supply.
There's nothing wrong with a power supply providing in spec voltages.
Around 5.03 means power supply is good.
True, because it too is within spec.
The other half of
that control system is therefore suspect - on motherboard.
Once started, the motherboard circuits could be a full +5
volts from the +5 supply line. This would explain why the
power supply controller cannot turn power supply on but works
OK once +5V is obtained from another source. By process of
elimination, motherboard based power supply controller circuit
is the only remaining suspect.
Not really because there's still the switch and wiring plus, while not
applicable in this case because there's no flicker of life, there's also
whether PWR-OK timing from the PSU is good (or premature as a premature
PWR-OK is often the cause of having to turn the system on and off more than
once to get a boot. The CPU goes insane because voltages are not stabilized
before being released to run) and then there is the on-board Vcore
regulator (and others) power good signal.
Appreciate why I could not provide an answer on a subjective
"+5VSB looks OK". Without the specific number, I still did
not know if +5VSB was OK. Numbers from a meter are also
required for voltages on red, yellow, and orange wires to
confirm power supply is not telling motherboard controller to
'shut down due to a bad voltage'. Also that message of bad
power would be seen on gray wire - power good declared some
seconds later when the gray wire rises to over 2.4 volts.
If power supply does not tell motherboard controler that
voltages are OK after a few seconds, then motherboard
controller will shutdown power supply. Meter can display
that.
I am not aware of any motherboard that times PWR-OK and 'shuts down' the
PSU if it doesn't promptly arrive nor can I find any specification for such
a function.
Not to mention you'd see, at least, a brief flicker of lights and fans
attempting to run even if it did.
One need not buy a supply to test. One uses the meter to
either prove a supply fully sufficient or define that supply
as insufficient. Two minutes with numbers from critical
voltages could say, "Power supply is good. Move on to other
usual suspects." A definitive answer without speculation.
Since it runs when manually activated the thing to check is whether the
motherboard is asserting PS-ON (low).
