"Late 2005" Power Mac G5 CPU upgrade

[firebottle]

I have two PCI-Express-based "Late 2005" PowerMac G5's

One has a 2.0ghz dual core CPU, and is a perfect-looking (new, mint) case.

The other has a 2.3ghz dual core CPU, but it's a scratched up case.

I want to put the 2.3 in the 2.0's case. But I am wondering...

Is the motherboard in these G5's tied down to the CPU it was originally installed with?

Or can one just drop a faster CPU in and have it clock at that faster speed?

Better yet, are all Late 2005 PowerMac G5 power supplies the same power output rating?

I have no problem going through the process of swapping the motherboards and/or power supplies (yes, I have the service manual)...

But then again... it would be ALOT less work if I didn't have to.

Anyone out there ever attempt this?

 

[Macdrone]

CPU and motherboard are matched on the G5's. I have heard of minimal success putting a faster G5 on a slower motherboard but it slows the CPU to twice that of the bus speed (half the CPU speed) so no gain.

 

[firebottle]

Looking at the service manual in the "Exploded View" section there are only two logic board part numbers: one labeled as dual logic board, and one as quad logic board.

Also, there are only two different power supplies, one is a 710 watt the other is a kilowatt (!)

If the CPU wasn't what controlled the motherboard bus speed, you'd think there would be three different board part numbers?

 

[Macdrone]

Motherboard bus speed controls CPU speed. I'm sorry if I was not clear. I was just trying to show motherboard bus speed is set, and minimal success using faster processor on a slower motherboard but CPU is crippled by motherboard to double bus speed.

 

[Cory5412]

If you are going to take apart your two machines anyway, it may be worth looking at the part numbers on the motherboards. A quick Googling suggests that there may only be part numbers for the "dual" PCIe board (which used one CPU socket) and the "quad" PCIe board, which used two CPU sockets. If this is the case, probably each motherboard was certified to run at the top 1250MHz implied for the G5 by a 2.5GHz processor. The slowest PCIe systems were dual-core 2.0GHz models if I remember correctly, so it's not that big of a stretch to imagine that the boards were certified to run between 1000MHz and 1250MHz.

Although beyond that I have no idea what's necessary and whether it's just popping out a daughtercard and swapping the new one in (like the PowerMac 7300) or if it's a socket directly on the board with a heatsink that attaches separately and needs to be re-equipped with heatsink paste (which could turn a "simple processor swap" into an evening's project.)

If you are going to dedicate an evening or so to it though, I'd try just the processors first.

 

[firebottle]

If you are going to take apart your two machines anyway, it may be worth looking at the part numbers on the motherboards. A quick Googling suggests that there may only be part numbers for the "dual" PCIe board (which used one CPU socket) and the "quad" PCIe board, which used two CPU sockets. If this is the case, probably each motherboard was certified to run at the top 1250MHz implied for the G5 by a 2.5GHz processor. The slowest PCIe systems were dual-core 2.0GHz models if I remember correctly, so it's not that big of a stretch to imagine that the boards were certified to run between 1000MHz and 1250MHz.

This is what I was thinking too. And the CPU card sets the speed of the bus.

Although beyond that I have no idea what's necessary and whether it's just popping out a daughtercard and swapping the new one in (like the PowerMac 7300) or if it's a socket directly on the board with a heatsink that attaches separately and needs to be re-equipped with heatsink paste (which could turn a "simple processor swap" into an evening's project.)

Looking at the pictures in the service manual, the CPU cards seem permanently attached to the heatsinks.

The heatsink/CPU assembly pulls straight out once you loosen the screws.

If you are going to dedicate an evening or so to it though, I'd try just the processors first.

I was going to try swapping CPUs, but first I have to find a 4mm "ball-head" hex wrench...

 

[beachycove]

I have removed the processors from a couple of G5s using ordinary — but long — hex bits, with the only "ball head" in sight being the one on my shoulders. The machines were older, but I doubt that the final revision would require different tools than the 03-04 models.