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G5 2.7 downgrade success(?)

cjtmacclassic

Well-known member
Hello all,
I've had this early '05 G5 2.7 marinating for a number of years. I worked up the motivation to revive it recently, but found that the LCS had done exactly what anyone would expect. Despite my best efforts, I couldn't revive either of the CPUs. Thoroughly encrusted.
Rather than try to find an exact CPU match on ebay in working condition (ha), and thus continue to operate a leak prone cooling system, I chose to do something different, but worse.
I spent some time comparing the motherboards and CPUs of the G5 family. I needed to find a motherboard that possessed:

1. The same ports and port arrangement
2. The same (dual) CPUs, but in a more tame, air cooled 2.0 or 2.3GHz configuration
3. Support for whatever flavor of PCI the G5's stock graphics card is. I tried to understand PCIx, but it just gave me pain.

Items 1 and 2 are purely mechanical. You could certainly opt for a single CPU configuration, but I haven't taken apart enough Powermacs to determine if there are differences in screw standoff/thread locations; It's entirely possible some may not be there at all, which would compromise CPU mounting and motherboard fitment.

The donor I chose was an absolutely wrecked June '03 dual 2.0. $40. No RAM, no graphics, no hard drive. I already have these, and simply needed the motherboard and CPUs within.

IMG_0285.jpg

I gutted the LCS, CPUs and motherboard from the 2.7 and did the same on the donor. During disassembly, I found no real difference in the construction of the housing save for one mounting pin, which isn't crucial, but you can tell it's unpopulated.

IMG_0232.jpgIMG_0233.jpg

I found that the power supplies share the same apple part number, and just kept the one I already had installed in the former LCS machine. The housings do have different P/Ns, but are entirely compatible for the purposes of this downgrade.
In addition, I found that all the accessory components (cpu fans, speakerfan, airflow cover) fit without issue.
Post swap, everything looked fine. No mechanical issues mounting two air cooled G5s and their motherboard in place of two liquid cooled G5s.
IMG_0295.jpg
Upon attempting to start up, I got two clicks as the power supply faulted. I attempted to trace the problem and disconnected everything but the bare minimum needed to run the motherboard. Two clicks, an audible arc, and a faint wisp of smoke from the topmost CPU. In my rush to assemble this, I hadn't carefully examined the CPUs.
I found electrolytic fluid from the twelve 100uf/16v aluminum caps on both CPU cards. One CPU was obviously spent and had a scorch mark below this International Rectifier component I haven't googled. The other, while leaking, hadn't yet immolated and I cleaned it thoroughly.IMG_0292.jpg

I cautiously reassembled and attached the cleaned, unburned CPU card in slot 0 and was pleased to find this machine is perfectly content to chug along with a lobotomy.

IMG_0293.jpg
IMG_0312.jpg
notes:
I have two more G5 2.0s on order. I'm also getting some very squat electrolytic caps that I hope match the footprint limitations and exist in quantities greater than 10 without a 43 week lead time. I realize cleaning the goo is only a temporary measure and that the caps are very much falling out of spec, and intend to recap this. The spare CPUs are for when I screw up.
I would suggest taking a look at your G5's CPU card. They may be reaching the age where recapping is as much a necessity as it is on a beige Mac. Mine set itself on fire, ymmv.
 

demik

Well-known member
Good work, G5s are not really easy to work with.

You might want to check the LB caps as well, Although there is a million of them on some revisions...
G5 were build during the cap plague era...
 

davidg5678

Well-known member
My first post on the post-crash forum software:

Thank you for writing about your G5! This post was interesting to see. I'll have to take a look at my G5's CPU cards one of these days. I hope that the capacitors aren't all leaky yet, but I do love a good recapping project anyway. :) As @demik has said, I think that this particular CPU failure could just be a result of the early 2000s capacitor plague and not entirely representative of all Power Mac G5s. Then again, the computer is over 15 years old. We'll have to see how this pans out, I guess...

On a side note, if you have not done so already, I'd recommend taking your G5's PSU apart and cleaning it out. (Disassembling power supplies can be dangerous, so don't attempt this unless you know what you are doing and have discharged the capacitors first.) As these computers typically sit on the floor and have no air filtration, the PSU can get absolutely filled with debris that can cause it to overheat. I have seen some awful pictures online of G5 PSUs completely stuffed with thick black dust, and so I've been meaning to take a look inside my own computer to make sure that nothing burns up.
 
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