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.
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 ?