Need help with iMac

[loopyloo]

Story goes :
I bought an iMac G5 cheap to use for photography software. It was cheap because it wasn't working and turns out the CPU was shot.
The model is an iMac G5 20" EMC number 2118 with a T7400 CPU.
I inserted a T2360 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, no keyboard lights.
I know the new CPU works because I tested it in another computer.
Here's a page from wiki that shows it should be completely compatible so what's going on ?
Can you help ?

[Uncle Fester]

Sure you didn't stuff up a ribbon cable or something is not seated properly? Reseat all the cables, if no avail try the other processor again?
Never heard of a T2360 but whatever you have might be running a slower FSB speed (T2060 and T2370 have 533Mhz) but at least it booted up for tests.

[loopyloo]

Oops my mistake , it's a T2350 and is the only "T" configuration I had spare for testing.
The T2350 is 1.86 GHz with 533 MHz bus.
I've had the T2350 in it a few times and it works each time.

[loopyloo]

I just found a T5500 that I didn't know I had and tried that.
The T5500 is a 1.7 @ 667Mhz.
Same thing, doesn't work.
So I've tested with two known good 667Mhz cpu's that don't work in the iMac and one 533Mhz that does.
It looks like this 667 meg machine only wants to run at 533 for some reason.
I put the recovery disc in it to see what would happen. I can hear it search and I think load up not sure but it spins and ticks a bit. I can't remove the disc because the keyboard and mouse doesn't seem to do anything.
I also checked that the ram speed matches the cpu.

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.


Any more ideas ?

[Uncle Fester]

Have you done a PRAM reset:

• Turn on the computer.
• Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
• Hold the keys down until the computer restarts and you hear the startup sound for the second time.
• Release the keys.

...but it might be a logic board/power supply problem. Capacitors could be failing.
The more powerful CPUs are putting more load on their ability to filter the supply rails.

[loopyloo]

Have you done a PRAM reset:

• Turn on the computer.
• Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
• Hold the keys down until the computer restarts and you hear the startup sound for the second time.
• Release the keys.

...but it might be a logic board/power supply problem. Capacitors could be failing.
The more powerful CPUs are putting more load on their ability to filter the supply rails.

Can't do PRAM reset cos the keyboard is out, anyway , it doesn't get as far as a grey screen, only black.
I did consider bad caps and their possible influence on a faster cpu so will be having a look at that tomorrow.

[mandc]

Is the ram the right speed or perhaps faulty? Maybe why the machine will only run at 533MHz

[loopyloo]

Is the ram the right speed or perhaps faulty? Maybe why the machine will only run at 533MHz

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.

[lsemmens]

Maybe the apple is rotten.....

[loopyloo]

Maybe the apple is rotten.....

You may be right haha.

[loopyloo]

Ok so I finally got round to checking caps and they seem to test just fine.
The T7600 still doesn't work but either of the 533Mhz cpu's do, so I though maybe a bios update might help.
I found this firmware update which I believe is a bios update. Correct me if I'm wrong.


So considering the machine is working, can I do any harm by running this update ?
I have it loaded onto a disc just waiting for a go or no go reply.