My PSU in my 2006 iMac g5 burnt out 3 years ago while I was plugging it in. There was a tiny spark and the thing wouldn't turn on. It was too old to get it repaired by Apple.
It took about an hour to get disassembled. Once I did that, I promptly realized there was manufacturer error and had to call and get a new PSU sent out. They were very helpful on the phone and the service way exceptional. Shipping took like 2 seconds. Not literally but it was really fast.
While I was disassembling I was at the part where you disconnect the SATA cable. Apparently it's really easy to rip right off the board and then you are out of luck forever since it has to be soldered back on meticulously. The internet and my local Mac repair shop said it's not worth it to even try to fix.
Putting the PSU in was easy and putting it back together took probably 30 minutes. It powered right up beautifully. Of course it can't read the hard drive so after that it just blinks a flashing folder icon with a question mark in it.
Only after all that did I see there was a guide to replace the PSU without taking the whole thing apart. HAHA OOPS.
Conclusion, I fixed the PSU successfully. The guide was really good, the part worked great and then I broke my iMac forever in a different way. The End.
Don't break your iMac while you're fixing it.
Try to do the shortcut of replacing the PSU first.
If you feel you must take the whole thing apart, be VERY VERY VERY VERY VERY VERY careful with the SATA cable disconnection. It was EXTREMELY hard for me to unplug even with a spludger and instructions so I thought maybe I'd give it a little tug since the other cables seemed to need a little force to disconnect. That's where I went wrong and it just ripped away from the board completely.
Also, if you DO break your SATA cable off you can run your iMac off an external drive.
TR8 Torx Security Screwdriver
TR10 Torx Security Screwdriver
iMac G5/Intel Power Supply