IMac not starting up and no chime

Question

Hello,
My neighbor's computer is experiencing some problems: it does not start (the screen stays black, no startup chime, but the fans run, as does the hard disk. Also, neither the mouse or the keyboard seem to be powered (no light)).
I have tried usual commands (reset PRAM and reset the SMC). Also tried with no device connected. (well, if something so trivial than all these tips did the trick, I wouldn't had to ask here ).
So, without keyboard, I couldn't even try special modes (safe, target disk, start from CD, etc), should the Mac be able to start in some mode. I have then decided to open the iMac (well, the warranty is way out, so nothing is lost). I have never seen a so-hard-to-open Mac, by the way (something was likely blocked).
I'm thinking of 4 possible causes for the problem:
-the hard disk could be damaged. It was malfunctioning some days ago. I assume it could come to a point where the logical board of the disk is failing;
-the RAM is an obvious possible cause;
-the CPU is also obvious;
-another component on the main board/logic board.
-I have forgotten a 5th point?
Well, I tried with unplugging the hard disk, and it had no effect, so it's not the HDD. I'm puzzled for the remaining ones (not knowing the iMac enough is a cause).
First, about the RAM, it seems the RAM slot is empty, so the actual RAM is somehow built-in (I haven't seen something seeming like usual RAM bars), so, if I can't remove the RAM, how can I test whether it's damaged or not?
Then, the CPU. There are 2 of them. So I thought I may take one out (and then the other one) and see whether the Mac starts. Well, I can't figure out how to remove the CPU cooling piece (there are just 4 "buttons" that can be pressed but nothing happen). I prefer asking here first, not removing the entire main board without advices.
Lastly, if it's the main board, I know I won't be able to do much. I have read there are LEDs to possibly diagnose power issues, but I don't see them (no LED is showing light, it seems). I'd like to test my other assumptions before thinking the main board is the cause.
Else, is there something I could try I have forgotten?
P.S: sorry for my (possibly) bad english, and the troubles of explanation; it's not as easy when english isn't the native language. I just wanted to mention that I have already repaired computers, so don't hesitate to tell me complicated answers.
Thanks in advance for your help.

Answer 1

David.Austin.Allen wrote:
If the RAM slots are empty, someone has removed the RAM. There is no RAM on the logicboard.
This page does say some RAM is built-in: http://support.apple.com/kb/HT2266
It explains why the RAM slot (which there is just one) is empty as there is not further RAM installed.
No startup chime means no usable RAM was encountered during Power On Self Test.
It may also mean some other problems, if I recall correctly (for instance, a dead CPU will prevent the chime, because the chime indicates all has passed).
I have the feeling that we aren't talking about the same model, because, according to:
http://support.apple.com/kb/HT2341
the LED should blink when there's no RAM. In my case, the LED stays on.
Exactly what model of iMac is this?
It's an iMac third generation 20'' isight (put the terms in the order you want ). EMC is 2082.
Thank you for this answer. Does not solve my problem but is useful in progressing.