Imac not starting - help please

I bought an iMac G5 cheap to use for photography software. It was cheap because it wasn't working and I think the CPU was shot.
The model is an iMac G5 20" EMC number 2118 with a T7400 CPU(not sure cos I've misplaced it to check the number)..
I inserted a T2350 that I had spare and it fired up and worked fine except for some lockups and crashes that I assume is prob caused by incompatibility.
I figured if I'm going to buy a CPU I might as well upgrade at the same time so I purchased a T7600 for it.
Problem is, it won't boot with the T7600.
Fans run, 2 motherboard LED's on (but not the 3rd that shows screen communication), no start up sound, screen black, keyboard and mouse doesn't seem to do anything.
I know the new T7600 CPU works because I tested it in another computer.
I have tested with quite a few different rams and they all work well in another test computer.
I also checked that the ram speed matches the cpu.
A known good T550 doesn't work in the iMac, but a T2450 does.
Another weird thing I found is when attempting a reset.
Instructions say to hold the power button while plugging in the ac cable, then press the button again to start the computer. When I do that, the computer behaves much the same as the original post description except the fans run flat out. If I hold the button to shut down then press again it will make the fans spin fast each time. I have to unplug the ac to get the fans back to normal.

The only thing I found that is common is that the 533Mhz cpu's work and the 667Mhz cpu's don't. ... This is supposed to be a 667 Mhz machine.
Can anyone help ?

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Just to clarify.
In the first post I meant T5500.
So five cpu's tried in it but only the two 533Mhz cpu's work.

I have repaired more computers than I can count but this is my first Mac.

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The G5 iMac was one of the worst hit models

during the bad capacitor era a few years back.

The symptoms were never the same but they were always bad and usually different on days.

Racing fans one day, good fans but streaky picture the next.

I note you say that you have repaired more computers than you can count but I would suggest that you take a look at the caps on that MB and see what the result is.
Remember that Apple used non-lead solder on those boards,


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I see the note from mrmacfixit

The thing is I rarely see a burned out Intel CPU. If that happens I usually find the motherboard and much of the rest of the machine has failed such as a lightning strike or fire.

But the BAD CAPS cause so much trouble and never the same trouble.

Be sure to inspect all boards including the one in the power supply. The BAD CAPS issue has plenty of pictures and story on the web.

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