Faulty Power Supply or Motherboard?

[computereducated]

Hello,
Last Thursday I cam back from Uni to find that my computer would not turn on.  I tend to leave it with the PSU on and the motherboard on standby.  When I pressed the power button nothing happened.  Initially I thought that this was my housemates attempt at a joke, so I checked all the basics such as the PSU being switched on and plugged into the mains.  These were all fine so I proceeded to open the PC to investigate further.

On opening the case panel I saw that the motherboard standby light was on, so I thought it was obviously receiving power.  Then I removed the power button connector pin and shorted the connection manually, but once again nothing happened.

I then tested the power supply further, by connecting the green and black wires on the 24pin connector.  The power supply switched on and I decided that it was working fine.

I decided that the motherboard was faulty and ordered a replacement of the same model.  Once I received the board I fitted it into my case, assuming this would be the fix.  However once again it does not turn on.  I then removed the most basic parts needed and set them up on my table.

I was starting to believe that the power supply is the fault, but it seems to be working fine whether the green and black pins are connected (even when plugged into the board for a short period).

I am guessing that the fault lies somewhere in the connection to or from the green wire used to signal the power supply to turn on.  However I am unsure if this is due to faulty motherboards or faulty power supply.

I will shortly be ordering a replacement power supply, as seems to be the next thing I should test.  I was wondering if anyone can provide further tests so I can ascertain the component that is at fault.

[truenorth]

This is a very comprehensive report on what appears to be an unresolved issue. The O/P has indicated a request for additional help above and beyond what they have already done. I am wondering why this post indicates "solved"? truenorth

[Geek-9pm]

We're going to talk about computer power supplies and motherboards .

Even the experienced technicians work on computers daily wind it difficult to guess at a problem. A very common practice is to just try substituting different parts until the system starts to work . But  performing a simple test on a power supply is not enough to ensure that the unit works as it should. The best way to prove that a power supply works is to try another power supply. If a known good quality power supply does not work, we have to assume something is wrong either with the motherboard or with the connections.

Also, a defective hard drive, CD-ROM drive, or even a USB component may stop the system from  working properly. Additionally, some computers have interlocks on the case that will stop the unit from working if the case is open. Another unusual problem that is hard to catch is when the motherboard has been shorted to the case by using the wrong kind of standoffs to hold motherboard in place. In some rare cases the system will work on until  the motherboard is stressed or moved slightly and something is  shorted out.

Some experienced technicians will take the motherboard completely out of the cases and mount it on a ground plane (metal sheet)e and attempt to get it to run outside of the case with no peripherals connected, just the power supply and the motherboard. And even with no memory.