The problem was that the iMac was shutting down suddenly after 2 hours or sometimes after just half an hour. I had to wait for it to cool down and than it would start up again. Browsing on the Internet learnt that it could mean that the power supply was reaching the end of it's life span after more than 6 years. Mine is early 2006 (and bought it second hand), so it sounded as if that could be the problem to fix.
I got a power supply and a Pro Tech Toolkit and with the help of the guides from ifixit and other YouTube movies to prepare me, had a go at repairing my first iMac.
I used the step by step ifixit guide on another PC to guide me.
It took me more than 1 hour to get one of the bezel latches from it's case. I tried it with the bend credit/identity card but after one hour I gave up on the second one.
Next day, at work , I took a piece of flexible metal with me (the sort that is used to tie boxes on pallets for shipment) cut it to size and made a slight bend at the tip. After that it was much easier to get the second latch free from the case! It turned out that the iMac was probably opened once before and the EMI shield had got in the way.
After that it was just following the step by step instructions from the guide.
The only small problem I encountered was that the four display retaining screws are to close to the casing so that it was a bit harder to get them out and in again. A small magnet against the screwdriver did the trick for me and a steady hand of course.
After replacing the power supply, cleaning the fan's, etc. and installing everything in reverse order I made two small markings on the bezel to indicate where the latches are to be found.
Then came the moment of truth!
Plugged everything in an powered up the iMac.
I forgot to throw the main switch below my desk, Ha,Ha!
That did the trick and hey presto, the iMac started up with no problems and works fine!
If you have no clue about electricity (Ohm's and Watt's) just like me and you encounter the same problem with your power supply here's my advice.
Open up your iMac and clean the inside first! This means the underside and especially the fan's which, in my case, can clog up with dust. Try and remove as much dust from the fan scoops as possible and use some sort of vacuum cleaner to get rid of the dust. Then start up your iMac again and see if the problem persists.
In took my power supply to a friend of mine who has more knowledge of electricity than I do. He tested the old power supply thoroughly for 3 days on end (with temperature's up to 115 degrees celcius) and it turned out that there was nothing wrong with it!
So my advice is: Clean first, repair (maybe) later.
I also would like to mention the special service I got from ifixit for sending me a replacement T10 bit free of charge.
Thank you Ryan Lutz!!
All in all it was a very enjoyable experience!
Pete Wolters - The Netherlands