iMac won't start. Powersupply?

Hello,

I have a problem with my iMac and I hope you guys can help me.
It‘s an 24“ iMac Mid 2007* 2.8 Ghz Core 2 Extreme (EMC No.: 2134 Model No.: A1225).
The iMac suddenly turned off as if sombody pulled out the powercable. If I press the power button nothing happens as if the machine get‘s no power at all.
So I opened the iMac and removed all the dirt and dust. I thought it might be the PSU but I can‘t see any optical defects.
When I pluged in the powercable the first Diagnostic-LED light up shortly and went off again. When i pull the cord and plug it in again nothing happens...
I checked the cable itself and tried another one to be sure it‘s not teh cable.
Can I be sure that it is the PSU (because of the LED‘s) or could it be the logicboard either?

Any Experiences?

Thanks for your help. And sorry for my poor english.

Thank your for the tip eschw95458. I haven't had time to check the drive an I have one more question before I to that. Was it the HDD and the PSU or only the HDD??

Hello,

thank you for your answers, and sorry for the very long delay. I never thought it would be that hard to get one of those PSUs in germany. How ever I finally managed to get one.
After I replaced he the old one my iMac started up without any Problems. I had it running for a few hours for doing all the updates with no problems. But after I moved it from my workbench to my home office. It begins to boot and after 20-30 seconds it turns off. I can't turn it on again pressing the power button doesn't have any effect. When I wait a couple of minutes and unplug it from the power network I can start it again, but only for about 20-30 seconds and then the machine turns itself off again.

Does anyone have an idea what this could be? Thanks for your help.