Odd power supply problem...

Hey guys.

This guy from work gives me his PC to look at, saying it won't turn on, or rather, it turns on then turns off. After looking at it, it'll turn on, but then the front case light goes off, and basically the motherboard is powered down, but all the drives are still running and the power supply is on.

So i get a new PSU, install it, and now it's doing the opposite!

Now you turn it on and the motherboard powers on, light comes on, PSU fan on, but none of the drives spin up. I'm lost. I'm pretty sure a motherboard can't tell a PSU what to power up and what not to power up on POST, right?

Anyone seen this before? THanks guys.

While you were changing power supply....Memory, CPU,or an addon card has worked slightly loose causing the computer not to boot.

Check all power connectors, especially the floppy drive connector. Those little 4-pin connectors are easy to get one-off and will cause symptoms like you describe. Fan connectors are also easy to misconnect.

Unhook everything except main power and P4 connector then power on to see if it will go through post then plug one thing at a time in and you will find the problem.

Go back to the person and get the full story. If he says that he Upgraded his CPU and Motherboard. Check for the following;

1. I would hook up the power supply so the only thing is connected is CPU, Fan, Memory, Video Card. Power on the system. Did it stay?

2. If no. Unscrew the Motherboard from the case. Insure nothing is between the motherboard and case that is metal touching the MB. Count the studs that the screws connect to the MB. When you place the MB back on are all the studs accounted for.

3. Check MB to insure it has the correct type of Power supply

Thanks guys. I'll try all this and get back to you. For reference though, the story is simple - the guy bought the PC at a store, its a no-name one, store built, just out of warranty, during warranty period a power supply went and they replaced it. He hasn't done anything inside the case, just turned it on one day and it started misbehaving.

"So i get a new PSU, install it, and now it's doing the opposite!"

My response was based on the above line from your original post.