Mike said:
I also tried 2 other different psu's on the original motherboard before I
touched anything (they both work with other pcs)
and all give diffrenet results...
results
The original psu 180w = green light on motherboard blinks on and off fan
starts and stops in time with it.
first psu I tried 90w = no power nothing
second psu I tried 300w = fan spins but No light on motherboard? drives
start up but pc doesnt.
Any ideas why I get 3 different results from 3 different psu's
Cheers for any help
The green light on the motherboard should be tied to +5VSB. When the
switch on the back of the PSU is turned on, the green light should come
on, indicating +5VSB is present. You should not add or remove hardware
when the green light is lit.
The first two supplies were probably too weak to power the system. The first
supply could be detecting the overload, and shutting down for a moment in
response. The +5VSB supply might have been shut off too.
I would have expected better from the 300W power supply. If the green LED
won't light, nothing else should be able to run either. So that test
case doesn't make much sense. If the fans can spin, then the +5VSB (which
runs the supervisory circuitry on the PSU) should be present too.
In terms of reasons for shutdown in 3 seconds:
1) If a late model AthlonXP or a P4 system, or an Athlon64, this could
be a hardware CPU overheat detection doing it. Make sure the heatsink
has some thermal paste or a thermal pad on it. You don't want any
air gap between the CPU and heatsink. On S462 processors, the heatsink
fits two ways, and only one of the ways cools the processor properly.
2) If the CPU fan is not detected by the BIOS code (due to the RPMs being
too low, or the RPM signal not being present on the CPU fan header), then
the BIOS can shut down the system. But if the BIOS is running, you'd hope
to hear some kind of noise from the computer speaker.
If the system won't "beep" the computer speaker, then the CPU may not be
executing any BIOS code. AFAIK, you need to execute some BIOS code to
beep the speaker. While a silicon chip can be made to beep without processor
support, I think generally things are set up to need the processor, for a
beep to happen.
If the motherboard has a separate 2x2 power connector for +12V, having that
disconnected from the motherboard, will give spinning fans, but no beeps. The
CPU cannot run without power on the 2x2 connector, if the 2x2 connector is
used in the design.
The -5V option on a power supply, should not strictly be needed by the
motherboard designer. Yet, there are a few motherboards, and even accessories,
that use -5V (I think a certain DIMM booster needs -5V to run a build-in
voltmeter). If the -5V was really needed, it could be tied into the Power_good
logic, and prevent the motherboard from starting if it is missing. I don't know
of an easy way to determine if a motherboard has a -5V dependency - the current
it draws could be quite small (the -5V could be a bias source for something). Even
if you measured the current drawn, you might not be able to tell the difference
between leakage current and real current needed to run something. For example,
on some motherboards, the voltage divider feeding the hardware monitor
interface (that measure the voltage on -5V), might be the only load on the
-5V circuit.
On the last two systems I built, I started building them on a table. I added
components a piece at a time, testing for the expected response after each
component is added. For example, with the PSU and motherboard sitting on the
table, and using the PWR button from the case, I should be able to switch the
PSU on and off by using the case power button. You would expect to hear any
fans connected running (and the green LED on the motherboard should be stable
at all times, only going off if the switch on the back of the power supply
goes off).
With motherboard, CPU, PSU, case speaker, case power button - expect a "beep"
code for missing RAM. Make sure the CPU fan is plugged in, the main 20 pin power
is plugged in, and the 2x2 12V connector if the system is a fairly recent
design.
If you add one stick of RAM to the mix, expect a "beep" code for missing video.
(That is, if the motherboard doesn't have a built-in video in the Northbridge).
Once the motherboard has CPU, PSU, RAM, some kind of video, error codes after
that may appear on-screen. A missing keyboard/mouse may cause the BIOS to halt.
I built up my entire system, while it was sitting on a table. I put a phone book
with cardboard cover, underneath the motherboard. That allows the little tab on
the bottom of the video card, to hang below the plane of the motherboard, without
bumping anything. Be especially careful when working this way, as if the
monitor VGA cable is tugged, it could pull the video card part way out of
the video card slot, and ruin the video card or the motherboard. You need a
safe, undisturbed work area to be doing this kind of testing (no curious
small children present).
Somewhere along the way of reassembling the system a piece at a time, you'll
hear or see something that hints at the problem.
When the mouse was inside the computer, there are limits as to what the mouse
can achieve. A little mouse urine could short something (maybe a crystal
oscillator circuit could not take the abuse). If the tower case was upright,
then the bottom of the case is the horizontal surface most likely to
be occupied by the mouse. A few well placed bites, could ruin something.
(If the speaker wire had +5V on it, you can get a spectacular meltdown of
the wire, if it touches ground. A good speaker interface design would prevent
a direct short from doing that, by placing a current limiting resistor in
series, but I've read about melted wires if the speaker wire gets pinched.
Apparently not all speaker interfaces are defensively designed). Visually
inspect the parts carefully, and maybe you'll be able to see the damage.
When you assemble the system a bit at a time, on your table, that will give
you an opportunity to do the careful visual inspection required.
[Maybe I missed it, but what motherboard(s), and processor were being used
here ? Knowing that might focus the discussion a bit better.]
HTH,
Paul
