CPU is not involved with your circuit. When power switch is
pressed, then power supply controller orders power supply on -
equivalent to what that paper clip does on the bench. Then
power supply has a limited time to get 3.3, 5, and 12 volts
turned on. If not, the power supply itself detects what it
believes is a shorted output - and shuts down all other
outputs.
With a meter, identify which output is shorted while others
would rise up before the power supply shuts them all down.
I believer pin 9 is the purple wire? Circuits that control
that 'paper clip' needs 5 volts always. If too low, then
numerous and strange problems will occur. Why a digital
multimeter? Is that purple wire above 4.88 volts? Only the
most expensive analog meters can measure 5 volts that
accurately.
You have properly described the functions of Power Good.
But Power Good tells CPU nothing. During power up, well,
strum you lips. This is what the CPU is doing during power
up. CPU does not yet have enough 'intellect' to control power
supply.
By far, a fastest way to determine power supply integrity is
to leave it inside the computer. If voltages don't come up,
suspect either too much load, shorted power lead, or defective
output from power supply. But then the meter will make it
apparent which voltage is having such problem. That voltage
will never rise up to value before power supply sees the
problem and shuts down other good voltages. This, of course
assumes the power supply controller is telling all three
essential voltages to power on. If no voltages rise, then
power supply controller and related circuits are now suspect.
Notice still no reason to remove supply or do bench testing.
One final point. If looking at $40 power supplies, then you
expect more problems. Minimally acceptable power supplies
start at about $60 retail. Those cheap power supplies sell
at lower price by forgetting to include essential functions.
Al wrote:
Hi. This is on a Gateway Essential, 950 Mhz Celeron, FlexATX, mfg
date in fall of '91. The Power Supply is SFX, Gateway's part# is
6500545, 90W. (AKA Newton NPS-145PB-117 A Rev 03).
The computer would start but then go only into standby mode (with the
power indicator being always yellow, never green). After a few days
of that behavior, it would then start only for a moment, and only
with a few spins of the ps fan (there is no CPU fan). Pushing the
start button again would have no effect - unless I removed the power
cord and let it sit for a minute or two. Then I'd replug and the
pattern would repeat, with only a momentary start for the first
button press.
With an analog voltmeter, I would see 1V for that brief moment, on
POWER_GOOD (pin 8). (I got the exact same result measuring between
pins 3 and 9, which should have given 5V.) When I removed all drives
and tried again, I got the same behavior.
So, I took out and opened up the PSU. There was some dust, which I
blew out. (Nothing looked burnt, that I could see.) But thereafter,
the computer behaved differently: when I merely plugged the power
cord in, I'd get the momentary light and ps fan spinning. It
didn't need me to actually push the power button anymore. Next, to
see if the power on switch might be shorted, I removed its leads
from the MB - but that didn't change anything.
It seems to me that maybe the CPU is not getting a good signal on
POWER_GOOD, and so shuts down the PSU. Or could it be that the CPU,
the MB or some other thing is bad, not the power supply? The more
I research on this, the more it goes round and round.
I'd rather not waste $40-50 on trying a new PSU, not to mention the
week or more it would take to be shipped to me, if it's not likely
to be a PSU failure. How can I tell if the PSU is bad or not? Thanks.
Oh, and one more thing: weird behavior showed up in the last few weeks,
like the mouse and/or keyboard or even drives not showing up on POST.
This msg would also sometimes be given by Windows: "An unknown device
has exceeded the current limits of its hub port". And a USB attachment
for an MP3 player had been added after Christmas. But the "current
exceeded" msg still sometimes appeared, even after the MP3 attachment
had been removed.
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