my powermac dual g5 will not boot up and/or wake up from being asleep

Have you tried booting from your Tiger DVD? If that doesn't work then you have a hardware problem. It could be something simple like a defective power switch or blown fuse/breaker. Could be the main power on the computer needs to be reset (button on the back near the AC connector, I believe.) Or, it could be more serious like a blown power supply unit.

The PSU was indeed THE weak point in the Dual G5 desktops. I've had two blown PSU's in mine, and I've heard and read about many others. Last time I didn't replace it anymore. (300$ for a new one on ebay if I remember well). I've read many topics about it back then.

Blowing out the dust in the PSU might help. (If you open it you'll be surprised how much dust is in there.)

Here's the thread when my G5 died, maybe you find it interesting to read:

https://discussions.apple.com/thread/3301670?start=0&tstart=0

Don't think you can open the drive when your keyboard doesn't work.

Do you have a bootable backup clone of your G5 system drive on an external Firewire hard disk?

If your internal harddisk is fried, and the rest of your hardware is ok, you should be able to boot from this back-up clone.

Not if he has a blown PSU or other major hardware problem. A good clone won't help in that case. It will only help if the internal hard drive is shot. But if that were the case it would not prevent the computer from turning on and passing POST.

Wasn't I saying more or less the same in other words? :-)

except for the remark that " it would not prevent the computer from turning on and passing POST."

Didn't think about that.

Anyhow, what you are saying is what I meant. Sorry I'm not native English.

Then it was my misunderstanding. Flemish is not my native language. 😝

If you can get an external 3,5" HDD enclosure, I'd take out the disk(s) that are not backed up, and back them up first on another computer. (You actually should have had bootable backups in the first place, but it's never too late to learn, you learn the hard way ;-)

I could be wrong but I still have a strong feeling that it's a PSU problem.

Good news: if your fans still work, the PSU is not blown (yet). I suspect that dust could cause the symptoms you're seeing.

You have access to an air compressor? Preferably one that has a moist filter. (Canned air is way too expensive, but an option if you don't know anybody with a decent air compressor. Pressure of a can is lower too)

I'd first try to blow it out without disassembly. The PSU is on the bottom, so blow through the frontgrid at the bottom.

You won't be able to get all the dust out, but it might work allready.

I recommend you do this outside, and be surprised by the huge dustcloud that comes out, and be glad you didn't do it in your house.

If that didn't work, I would disassemble your G5, take the PSU out, blow it out more thoroughly, by opening the case of the PSU.

You will find links to guides how to do this in the thread I mentioned in my previous post.