That would indicate to me there is a short or a very excessive load on the 5V line.
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The start circuit is controlled by a transistor on the 5V line. The PSU fans are running fast because the PSU is heating up supplying the load.
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Fans run on 12V. So, the 12V line is good.
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The start circuit is controlled by a transistor on the 5V line. The PSU fans are running fast because the PSU is heating up trying to supply the load.
TRY:
measuring the resistance on the 5V circuits at the MB connector with the PSU connectors removed.
Using I = E/R, verify R is high enough so I is less than 1/2 PSU rating.
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If R is too low, you must find the cause. Start by pulling the CPU and remeasuring. Continue removing parts till you find it. If you don't find anything that way, then it issue is likely a soldered in device on the MB or the power conditioning circuits for the CPU.
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If R is too low, you must find the cause. Start by pulling the CPU and remeasuring. Continue removing parts till you find it. If you don't find anything that way, then the issue is likely a soldered in device on the MB or the power conditioning circuits for the CPU. That may mean a MB replacement unless you can locate and replace the bad part. Start by checking capacitors. I would use a low reading Ohmmeter. I found a shorted cap on an old IBM 5150 the other day that way.
Two clues:
# it starts without pressing the power button
# The PSU fans run fast after a while.
That would indicate to me there is a short or a very excessive load on the 5V line.
The start circuit is controlled by a transistor on the 5V line. The PSU fans are running fast because the PSU is heating up supplying the load.
TRY:
measuring the resistance on the 5V circuits at the MB connector with the PSU connectors removed.
Using I = E/R, verify R is high enough so I is less than 1/2 PSU rating.
If R is too low, you must find the cause. Start by pulling the CPU and remeasuring. Continue removing parts till you find it. If you don't find anything that way, then it issue is likely a soldered in device on the MB or the power conditioning circuits for the CPU.
If this works, give me 5!!!!!!