Technically_Jared said:
This is new to me - I built a PC and plugged in all connectors and my
system wouldn't turn on. I noticed it powered on the fan and power
supply for less than 1 second then turned off. I got the same result
with a different power supply.
Then as I was using my power supply tester I noticed the power supply
didn't work, but it did work when I unplugged the 4-pin 12-volt power
connector going to the motherboard. After I unplugged the 4-pin
connector all is well. The system turns on, bios indicate fans and
temp are good, cpu running at the right speed, it boots up, no
problem.
Any Ideas as to why this is happening?
My system -
Intel Celeron D 360 3.46ghz, socket 775 cpu
Mach Speed P4MSD-800D2 Motherboard
512 MB Ram
550 watt power supply
OK, the first issue is, you cannot leave it that way. The motherboard uses
a 20 pin main connector. There is only one 12V pin on that connector. It
is rated at 6 amps max. If the processor is being powered through that pin,
the Vcore circuit could not draw more than 72W. The output of Vcore, might be
about 90% of the input power (that is the conversion efficiency). That
means the Vcore output can drive a 64.8W or so processor. Your procesor
is rated at 65W. That means there is no power left for any other 12V
loads on the motherboard, like fan headers, video card or PCI slots.
(And this is why the 2x2 12V is supposed to be split off from the main
connector in the first place.)
http://processorfinder.intel.com/details.aspx?sSpec=SL9KK
Joining the processor power, to the main connector, is a Biostar trick. I've
read a few reports of Biostar boards operating without the 2x2 ATX12V power
connector in place. Notice on this page, that the serial number info is not
in the standard place used by MachSpeed, implying that perhaps, some other
company made the board for them. If the board was made in the same factory
as the other MachSpeed products, they'd all put the serial number sticker
in the same place.
http://www.machspeed.com/xsnpages/mobosn.html
Now, the second issue would be, why would a power supply get upset if
the two 12V outputs were shorted together. (I.e. When the 2x2 ATX12V
is plugged in, the 12V from that connector is now connected to the same
copper, as the single 12V pin on the main 20 pin connector.) Some power
supplies have a single output, and have overcurrent detection to check
up on 12V1 and 12V2. You'd think the rails could be joined together without
a problem. About the only other thing I can think of, is the ATX12V connector
on the motherboard, is rotated 180 degrees from the normal position. Compare
the drawing in the manual, to what is on the actual board. I cannot believe
the magnitude of the voltage on the two 12V outputs, would be such as to
cause a loop current high enough to trip the overcurrent.
If you have a multimeter, you could use the ohms scale, and see if the 2x2
ATX12V 12V rail, is connected to the 12V pin on the main connector on the
motherboard. But really, we already know that is the case, because the
motherboard runs without the 2x2 in place.
If this was my project, I'd be dumping that Mach Speed board, for another
brand. And not a Biostar either. I see no good reason for joining 12V1
to 12V2, in the copper of the motherboard. They are supposed to be
independent. A properly designed motherboard will not run, if the
2x2 is disconnected. The fact that your motherboard runs that way,
is a mistake in design, and could burn the 12V pin on the main connector,
unless the ability for current to flow is beefed up somehow.
The Biostar scheme would be fine, with an ATX 1.3 standard supply.
(I.e. If you find an ancient supply, that has a 20 pin and a 2x2 ATX12V,
and meets ATX 1.3, that might work OK.) But with all the ATX 2.2 standard
supplies out there, their idea is a mistake, and a dumb thing to do. I
don't expect this is something that can be put right with an Xacto knife,
as the 12V may be routed on an inner layer of the four layer PCB. If they
had separated the two 12V rails, the motherboard would have worked with
either a 1.3 or a 2.2 standard supply. Joining the rails, means it can
only be safely used with a 1.3 .
I would immediately dump any motherboard of recent manufacture, that
manages to run with only the main 20 or 24 pin connector in place.
No matter who made it or claimed to have made it.
Good luck,
Paul
