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#1 2014-06-11 16:19:11

iMic
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From: Adelaide, Australia
Registered: 2014-05-12
Posts: 877
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Power Mac G5 - U3 Heatsink Clamp Mod

Here's a useful little trick for the Power Mac G5 owners out there. It's no secret by now that G5s have problems with the BGA under the U3 System Controller / Bus Bridge IC. Recently I bought a G5 with this problem, and after removing the Logic Board, discovered that the U3 heatsink had completely separated from the board. The barely adequate little plastic clips had failed after several years of use.

When I installed a new Logic Board, I made a small modification to the U3 heatsink to prevent this from happening again. You will need:

  1. Side Cutters

  2. Stock Power Mac G5 Processor Retaining Screws* x2

  3. Small Nuts for Retaining Screws x2

  4. Your existing U3 heatsink

* Processor Retaining Screws refers to the screws that hold the CPU to the Logic Board in a standard air-cooled Power Mac G5. You may need to scavenge some spare screws from another Power Mac G5 to do this mod. If you're purchasing these new, these screws come in a kit including the screws and standoffs that the screws screw into. They have an Apple part number of 922-6325 and look a little like this:

2v9sym8.png

The head of the screw unscrews from the standoff (the hex-shaped part with the split up the middle). You don't need the standoff, just the screw. This is where you would find them on an already-assembled Power Mac G5:

vndyyu.png

Of course, don't use the ones already holding your processor in. See if you can get your hands on a couple of spares if you're looking to do this mod.

In addition, you could use any generic screw with the same thickness and length - it doesn't necessarily have to be these processor screws. Really the aim is to have something long enough to go through the heatsink, the board and with enough thread on the other side to slip a nut on to.




Using the side cutters, I cut the old plastic pins that secured the U3 heatsink to the board. You can see what plastic pins I'm referring to here:

25jd36a.jpg


But don't throw away the springs from the plastic pins - instead, remove the springs from the plastic pins and slide them over the processor retaining screws. With the nut on the end (but don't go ahead and put the nut on yet!), you'll have something that looks like this:


90zx8x.jpg



Slide the screws and springs through the holes on the heatsink where the plastic pins used to sit. Slide the screws through the existing holes in the logic board, then on the opposite side of the board, screw the nuts onto the threads to tighten the U3 heatsink to the board. You don't need to overtighten them - just snug will do. The springs will help tension the heatsink to the chip.

2lkxlp4.jpg

3583tj9.jpg


You may wish to use a small nylon or rubber washer under the nut to avoid damaging the board. If the nut you're using is large enough to come in contact with any surrounding electronics, then using a rubber or nylon washer is extremely important.

On the opposite side of the board, the threads will stick through a lot more than the existing plastic pins, but if you're using the stock processor retaining screws like me, they're still short enough to clear the processors, heatsinks and fans.

290wui1.jpg


You can guarantee that these screws won't snap like the plastic pins will, and if you ever want to adjust the tension on the U3 heatsink, you can tighten up the nut from the front side of the Logic Board without having to remove it from the enclosure.


Cheers

Mic.

Last edited by iMic (2014-06-21 07:12:35)


Resident Professor of Alternative Methodology
Faculty of Macintosh Restorations & Modifications - "It works, let's fix it!"

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#2 2014-06-12 17:54:03

cc333
Member
From: North S.F. Bay Area, CA
Registered: 2014-05-23
Posts: 562

Re: Power Mac G5 - U3 Heatsink Clamp Mod

Neat! I had this problem when rebuilding my 2.0 GHz DP G5 last year, but I didn't know what to do, so I just wedged some cardboard in there (far from ideal, but it seems to be working). Next time I'm in there (which won't be for awhile, as I hate working on the wretched thing), I'll do this.

Thanks!

c


Main Macs: Early '09 Mac Pro, Mid '12 MacBook Pro 13"
Secondary Macs: Early '08 Mac Pro, Mid '12 MacBook Pro 15"
Playthings: Mac SE/30, 3.0 GHz Mavericks-based HackServe, Many others....
Desired: Lisa, Kanga PowerBook G3, Apple IIc, Apple II, Spare parts, etc.

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#3 2014-06-12 19:11:42

uniserver
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From: Sf, Mi
Registered: 2014-05-15
Posts: 955
Website

Re: Power Mac G5 - U3 Heatsink Clamp Mod

i like this... much more solid and sturdy!


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#4 2014-06-15 03:11:54

cc333
Member
From: North S.F. Bay Area, CA
Registered: 2014-05-23
Posts: 562

Re: Power Mac G5 - U3 Heatsink Clamp Mod

Yes! I like it as well.

I must do this next time I'm in there working.

c


Main Macs: Early '09 Mac Pro, Mid '12 MacBook Pro 13"
Secondary Macs: Early '08 Mac Pro, Mid '12 MacBook Pro 15"
Playthings: Mac SE/30, 3.0 GHz Mavericks-based HackServe, Many others....
Desired: Lisa, Kanga PowerBook G3, Apple IIc, Apple II, Spare parts, etc.

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#5 2014-06-15 14:42:39

ClassicHasClass
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From: Electron Alley
Registered: 2014-05-26
Posts: 1,083
Website

Re: Power Mac G5 - U3 Heatsink Clamp Mod

I wonder how much this problem affects the quad. It hasn't seemed like a problem on the two I own, but they're babied, so.

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#6 2014-06-16 03:48:59

markyb
Member
From: Bedford, OH USA (216)
Registered: 2014-05-16
Posts: 182
Website

Re: Power Mac G5 - U3 Heatsink Clamp Mod

I had to do a similar fix when I replaced the heat sink compound on my i5. Plastic tensioners all broke, so I used screws and rubber washers to get the right height (and so that I'm not using metal ones to prevent shorts), and then loctite on the nuts to prevent slippage over time. Plastic screws are so unreliable.


http://markyb.applefool.com for a list of my computers, my blog, and some random resources.

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#7 2014-06-16 15:24:38

ScutBoy
Member
Registered: 2014-05-25
Posts: 156

Re: Power Mac G5 - U3 Heatsink Clamp Mod

Nice solution! Like ChC, I hope it doesn't come to this on my Quad. I've moved out all my other G5 towers.

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#8 2014-06-16 17:00:59

iMic
Administrator
From: Adelaide, Australia
Registered: 2014-05-12
Posts: 877
Website

Re: Power Mac G5 - U3 Heatsink Clamp Mod

For reference, there were three revisions of the northbridge chip used in the Power Mac G5:

CPC925 - Also known as the U3. It supports up to two PowerPC 970s or 970FXs, has two 550MHz unidirectional processor buses, a 400MHz DDR memory controller, x8 AGP and a 400MHz 16-bit HyperTransport tunnel. It's fabricated on a 130nm process.

CPC925 - Also known as the U3H. Same as the standard CPC925/U3, but supports ECC memory.

CPC945 - Also known as the U4. It supports up to two PowerPC 970MPs, has two 625MHz unidirectional processor buses, two memory controllers that support up to 64GB of 533MHz DDR2 SDRAM with ECC capability, x16 PCIe lanes and an 800MHz 16-bit HyperTransport tunnel. It's fabricated on a 90nm process.


Presumably the Single and Dual Processor (Single Core) models running PowerPC 970s and 970FXs used the U3 chip. This includes the following models:

  • Power Mac G5

  • Power Mac G5 (June 2004)

  • Power Mac G5 (Late 2004)*

  • Power Mac G5 (Early 2005)

* The Late 2004 Single 1.8GHz used a different, smaller Logic Board from the other models and likely has a different heatsink.

The only Power Mac G5 to include Dual Core, Dual Processor modules was the Power Mac G5 (Late 2005) - also known as the "G5 Quad" - used PowerPC 970MP processors and likely used the U4 chip instead.

I have a Quad 2.5GHz, but it hasn't been torn down for any kind of repair yet so I'm not sure how the heatsink is mounted or whether this fix would also apply for those models.

Since the U4 is a 90nm process chip, wouldn't it have a reduced heat output over the 130nm U3? If so they may not be as prone to failure.


Of course I'm simply speculating here, feel free to correct me on any of this.


Resident Professor of Alternative Methodology
Faculty of Macintosh Restorations & Modifications - "It works, let's fix it!"

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