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Rebuilding a Powermac G5 power supply using ATX partsI

I have recently acquired a dead Powermac G5. The previous owner said that he brought it to the service center and its going to cost ~$600 to get it a new power supply, and that was all that it needed to be able to run again.
Well after getting the unit home. The first thing to do is test it without doing anything. Who knows, it might actually NOT be broken. In goes the plug... Pressing the power button results in... nothing.
So next step would be disassemble the unit. This is the intake fan: This is the side of the unit with the cover off: These are the covers for the heatsink Because I did not have the correct service manual for this unit, I took the logic board out before taking out the power supply. I realized later that I only needed to take out the lower CPU to accomplish this (and yes I did take it out again ... but more on this later). I also read somewhere that there is supposed to be a cover on top of this power supply that is now missing, the service center must have forgotten to put it back. Some of the cables were also disconnected, again, my guess is that the service center did not bother to plug it back in as it is broken anyway.
This is the power supply. It is the biggest computer power supply I have ever seen. The middle connector is as big as a standard 24 pin ATX connector. This is what's "under the hood" of the power supply. This is the label stating how much Amps is needed by each line. A standard 600W ATX supply should satisfy these values. The only thing missing is the 25Vsb line. From what I have gathered, only the ADC out (for special apple displays) would use this. At this point I was not too concerned about the 25Vsb line. This is the whole power supply's internals: After the past experience with blown capacitors, I was hoping this is another case... No such luck. Poking around with my voltmeter revealed that the fuse was blown. I have a spare 250V 10A fuse that i quickly put in. Who knows, it may just be the fuse. I replace the fuse and then plug it in... poof: smoke comes out and the fuse is blown again.
A Careful inspection of the power supply board reveals that there are switching transistors whose legs are melted, there is also a crack on the board, and that there are more surface mount IC's at the bottom side of the board: Some of the IC's are burnt too much to be able to read the writing on them. Ok... I was going to try to "component-level" repair this one, but at this point, it looks like it would be more feasible for me to substitute an ATX power supply.
After getting a 550W power supply from a nearby computer store and testing this with a spare PC motherboard and verified everything working, I cut all the wires coming from it, and desoldered the cables from the original G5 power supply board. I connected everything to the corresponding voltage but left out the 25Vsb.
This is how it looked after the operation: I used shrink tube to insulate the individual wires to keep it clean (electrical tape would leave a sticky residue). After double and triple checking my connections with the voltmeter, I am ready to plug it in. Before I put all the screws back I should test it and see if it actually would work: So, it's the moment of truth. Plug it in and press power button... Power supply fan runs... red LED lights up on motherboard... but no startup chime. Repeat... same...
Of course (duh). There is no RAM. (double duh). After installing a pair of 1Gb sticks of PC3200 DDR memory (from my iMac G5): Pressing the power button now gives the easily recognizable apple startup chime, or as a good friend calls it, the "jeng" sound. That's all I needed to hear, out comes all the cables, out comes the 2nd CPU... put the power supply and everything else back on... And it all works. Both CPU's detected, About this mac says it's a "Dual 2 GHz PowerPC G5". I did notice it was a bit noisy. My wife described it to a coworker as "it's as if there is an airplane about to take off". After doing some more reading, I think I may have put the processors back the wrong way (swapped the 2). The logic board seems to "remember" the CPU ID of what was there before, and keeps a thermal profile stored in non volatile memory. This thermal profile would be specific to that particular CPU. If you were to replace any of the CPU's (or like what I did, swapped the 2) the logic board was programmed to "play it safe" and spin the fans (all of them) at full speed. I have 2 options, take the CPUs off and swap them back (no way). Or run what is known as "thermal calibration". Running thermal calibration took about 30 mins (about 15 mins for each processor), after which the fans are running at their normal speed now.
After all that I used the computer for a few days, and noticed that the power LED was not lighting up. It wasn't lighting up either during sleep (it's supposed to look like it's "breathing"). After taking out the front panel connectors and power switch, testing the LED reveals that there is nothing wrong with it, nor with the cables connecting it to the logic board. The only thing that could be wrong would be, apple must have used the 25Vsb line for the LED. I found a 24V switchmode power supply at a local electronics store, and after taking out the power supply again, I was able to put it inside (there was lots of room left). This power supply came with a voltage adjustment knob which I changed to 25V with no problem. This unit is only rated for 25W so as long as I dont go about plugging in an ADC monitor, I should be fine. After plugging it in, The LED now works.
I used the DVI connector to connect it to my monitor. I wanted to hook up 2 monitors, but the other connector was ADC. Well... as this is a different topic, I will write about it another time.
Posted by Jonnyboy at 1:22 PM 
Thanks for the write-up.

I will be removing and opening up the PSU from my machine (Dual G5, "Late 2004") to try and replace the fans, which have become very noisy. Hopefully the 40mm fans I have ordered are suitable.

Did you have any trouble removing the cover that sits on top of the CPU heatsinks? I have read people say that it is impossible to take it off without snapping some kind of plastic clip holding it in place. Do you know of any guides for removing the cover/heatsink/cpus properly?


March 14, 2009 at 6:54 PM Peter said...

Finally, an alternative solution! Thanks for all your work publishing this. I have a Powermac G5 that won't boot, but the fans do start when I press the power button. It won't respond to keyboard start-up options. Should I assume I have a working power supply?

March 21, 2009 at 10:09 PM Jonnyboy said...

Did you mean the cover that says "G5" on it ? When I got mine, *someone* already opened it up before and lost some parts. But from my reading, there should be a plastic clip that resembles a nail/screw that should be visible from the AGP/PCI slot area. What you want to do is pull out the middle part of the clip and the whole thing should come off. This must be the clip they are talking about. It is missing from my unit so I'm not quite sure if my description is accurate. After that, you will need a torx screwdriver with a very long handle. Each cpu/heatsink is held down by 4 screws deep down the 4 corners. losen the correct 4 screws and the CPU pulls out easily.

Does it at least give "jeng" chime? If your fans start up then it shouldn't be a power supply problem.

April 8, 2009 at 5:16 PM baitshax said...

This is really like the only info on the net I can find on the subject at all. I'm having trouble finding which screws actually attach the CPUs to the board.
I too need to replace the power supply and I have the cover off thing thing. I took out the four screws at the four corners, but thing thing still isn't budging.
Any help would be appreciated.
baitshax at gmail.com

May 1, 2009 at 1:58 PM dadwarf said...

Thanks for your post !

It helps me to try to replace my 1000w PSU by an ATX. I said "try" because i'm still at the beta test of my hack because my PSU is different than your : 1000w and 710w have 2 connectors and metal bars screwed to the logic board and it makes it harder to replace (i also need to remove totally the logic board to take the PSU appart).

May 31, 2009 at 3:57 AM 
Can i ask you something : How had you connect the 25V power supply to the ATX's one (i know where to connect the +25v but for the ground ?) ?
Thankes, regards.

June 3, 2009 at 8:40 AM 
I didn't hear a startup chime, so I tested the power supply, and it looked good. I eventually pulled a processor and it chimed and booted. I reseated the processor and things were back to normal! Until the 4th, when someone took out the transformer across the street. It won't even power on when I pull a processor.

July 7, 2009 at 1:23 PM TheSpark29 said...

@ Jonnyboy
I just wanted to thank you for that comment on how to remove that cover with the G5 logo on it. I have spent many hours trying to figure that out. I've also searched online for a solution, but you are the only person on the internet that knows that AFAIK.

The screws that hold the CPUs down in my G5 are hex screws. Where did you find a hex wrench long enough to get them out. I have three on each CPU out, but the two that are in the middle of the heatsink are impossible for me to reach. I just got back from Home Deopt and they do not make hex wrenches that long. Any suggestions? Thanks alot!

July 14, 2009 at 3:52 PM Aris said...
July 19, 2009 at 4:20 AM 
Just in time! My dual G5 stopped working a few days ago. Will try getting it into the power supply repair extension program and if that fails, I will go down your route.

By the way, sb or stb seems to stand for stand-by voltage. I didn't know :)

July 19, 2009 at 4:52 AM 
Firstly, excuse my ignorance but I must somehow learn :)

I have been looking at power supplies and I had a question. Must the ATX power supply chosen have multiple +12V lines? (+12V1, +12V2, +12V3) or is it OK if I use a PSU which has a single 12V line that can deliver at least 36A max then draw all the +12V lines from there? (36A = 13.5 + 9 + 13.5 as indicated in the label you photographed)

July 19, 2009 at 5:38 AM 
@ Aris
I don't think it matters if the PSU if multi-rail or single rail. A computer can't tell the difference. Most mult-rail PSUs have a single 12v source that is split up into multiple rails. So it's all the same afaik.

@ Jonnyboy and anyone else
I figured out how to get the heatsinks out. I've documented the complete disassembly of a G5 at the following link:

July 19, 2009 at 1:32 PM Kirill said...

How is the "Thermal Calibration" done??? I have a Dual 2.0Ghz which sounds like a jet engine when its turned on... I would like to fix this but I couldn't find any info about it online. Thanks.

July 25, 2009 at 4:17 PM 
Forgot to ask... I also just acquired 2 PowerMac G5 units (2.5Ghz/water cooled) The guy sold them because they wouldn't power up. I thought it may be the power supplies, but after taking one of them apart and switching the power supply with a working one, there was still no power... Any ideas???

July 25, 2009 at 4:22 PM 
@johnyboy My CPU cover had the plastic bit. It was a two part component. A plastic nail driven through the middle of a plug making expand. Unfortunately, there was no way of pulling the nail so I had to sever it badly to remove it.

July 29, 2009 at 3:34 PM Yichen said...

How did you figure out the FANtach wire supplying 3.3V?

August 5, 2009 at 4:00 PM 
Hey, thanks for the tutorial. I mustered up the courage to take apart my own G5 power supply which is also busted. I confirmed the fuse is blown, but the only other thing that looks off seems to be resistor R302, which resides under one of the heatsinks and looks completely burnt out. I want to replace it, but unfortunately the resistor's outer casing seems to be burnt/cracked off and I can't find the resistance value. Anyone with a working R302 happen to know the specs of this component? Here's a picture of the resistor: [link]

September 13, 2009 at 6:12 PM David said...

My Late 2004 Dual G5 power supply died a few weeks back too. Just about when everyone else's did! I had just installed the 10.5.8 update. The system rebooted and was gone.

Many, many thanks for your blog and the pointer to the http://preview.tinyurl.com/kmq5c4 posting! A very. very big help!

September 14, 2009 at 1:30 PM 
The official service manual for the Power Mac G5 (Updated 20 June 2006) is out there. It's got clear step by steps for all G5 models.

Google Power_Mac_G5.zip. 23.8 MB.

September 15, 2009 at 6:16 PM Kevin said...

Can you tell me what you did with pin 3 (the fantach). Where does this go?



September 20, 2009 at 10:49 PM 
wow, fantastic help.
I'm trying to confirm that my power supply is bad and that the used one i'm looking to buy works.
Which pins should I look for the 25V and ground?

many thanks!

October 15, 2009 at 8:07 AM 
Thanks you the Blog,

I did exactly the same thing you did with a G4 dual 1.8 PS. The previous owner purchased a replacement 600W PS and after that didn't work he decided to sell and then I came along.

I use the brute force method and so after reading an open across the fuse, I went on and put a jumper across the it and powered the PS up with the same results you got.

And so I since this failed I concentrated on the replacement PS. After examining it, I saw a rip on the PS P3 Connector 5V wire insulation. The copper is exposed. I don't know if this cause or not. I do know that I can read 5V between Pin#1 & 23 on P1 which leads me to believe the PS maybe ok.

I am planning on buying another connect and remaking the P3 connector.

October 17, 2009 at 2:46 PM 

I am planning on buying another connector and remaking the P3 connector.

October 17, 2009 at 2:48 PM steste said...


Jonnyboy, please, do you can tell me the correct wiring (yellow, orange and black wires already connected) schmematic of the "poweron" section? I connect the wires as writen above to the atx powersupply, but the g5 does not start.
I think it´s a bug in the wiring.

Many thanks !

October 30, 2009 at 1:46 PM 

I have been looking at 550W power supplies but they seem to be falling short of the required amps for +12V (36A required VS 20A) while they have ample amps for +5V (22A req. VS 42A).

If you still have the ratings of the 550W psu would you mind posting them as a guide?


November 5, 2009 at 3:27 PM Sgt_Oddball said...

Got the same 600W power supply with an exploded IC on the board (with it's silicon on show for all to see) just wondering if anybody knows part number it is or what would replace it. It's list as U801 on the board.

@Matt it's a 100 ohm resistor.

November 6, 2009 at 4:52 AM 
For those needing more info on taking the CPUs out, I would refer you to TheSpark29's pictures of how he took them off: http://preview.tinyurl.com/kmq5c4

Thermal calibration can be done using a G5 specific utility disc. This is sometimes (unintentionally) left in a drive after a service center does some repairs on your unit. You can ask an Apple service center to perform this procedure for you or if you can locate the appropriate disc, you can do it yourself.

The 25V power supply's ground is common ground with the ATX power supplie's (you connect them together).

The 550w power supply had 2 12v rails. The total is less (but almost) close to the Apple unit. I could not find the original case with the label anymore. I think one is 20A and the other is 10A. I just wired 12V1 and 12V2 to the 20A one, and 12V3 to the 10A one

Regarding the wiring of the power supply, there are several places on the internet documenting the G5's power supply pinouts. Do a google search for "powermac g5 power supply pinout".

The FanTach wire goes to the tach line of the original fan (I used the original power supply's fans). You would find 3 wires coming out of one fan, and only 2 on the second fan. I think black was ground, red was 12v. The remaining wire goes to FanTach. You can try to trace the original PCB paths if you are not sure.

November 13, 2009 at 10:57 PM Ollie said...

I'd like to thank you for the write-up. It was just the motivation I needed to do this myself....

My first attempt happened to be with a dead atx power supply that I didn't test first...

The second attempt with a new ps worked wonderfully.

I have a question regarding the fan connections. I have the P2 connection, pins 6 & 7 wired to my power supply's +12V lines, and pin 16 (rtnfan/ground) to the power supply's ground. I then have my power supply's fans wired to +12V and ground. This makes the power supply's fans run at a constant max speed. How should I wire the power supply's fans to enable regulated variable speed?


November 28, 2009 at 10:55 PM 
Since I never saw the original power supply working, I'm assuming in all that fancy additional circuit was something to regulate the speed of the fans. Unfortunately for me, I have no real idea how this worked. I did try to use a pulse width modulation (PWM) motor speed controller to run the fan at lower speed, but without some feedback mechanism (based on heat) there is no real way to make it slower or faster. In the end, I just wired it like you did at full speed. I also connected the TACH wire back to the logic board connection (I guess the logic board wants to know if the power supply fan is still running). other than being a little louder than normal, the system works fine.

November 29, 2009 at 3:14 AM 
Finally got around to getting a PSU, gutting the old one and getting ready for the organ transplant.

Question: the 24ping connector has +3.3V & +5V sense pins on it (pins 9 & 20). Does that mean the motherboard has voltage sensors? Do we simply connect the corresponding voltages from the PSU to these so the mobo can monitor them?


December 30, 2009 at 8:43 PM 
Hi Aris,
Since I no longer had a chance to probe a working power supply, I just connected the like-coloured wires together. I hooked up pin 9 to 3.3v and pin 20 to 5v. I guess something in the motherboard keeps track of these voltages and maybe turns the computer off if it goes beyond tolerance. It's almost a year and so far no smoke has come out of the unit. :)

January 3, 2010 at 2:26 AM alibi said...
January 23, 2010 at 8:09 PM aris said...

Yep. I probed the sense pin on the PSU and indeed it is on the same track as the corresponding voltages.

At the moment, my G5 is down. It worked quite well for about 3 weeks but it would switch off spontaneously once every few days. Slowly the frequency at which it would switch off increased. For a few days it was switching off once a day. Yesterday, it switched off 3 or 4 times and the last time it did, it just stayed off. Not sure why. Unplugged it and left it unplugged for a while but nothing.

Will have to open it up and have a look. Maybe the PSU I used (650W) was not quite up to the task.

Since I have no experience with PSUs degenerating like that, any suggestions drawn from your experience would be very much appreciated.

January 23, 2010 at 8:18 PM 
@ aris:
The PSU you used may have been faulty. And I can firmly guarantee that 650W is PLENTY of power.

January 23, 2010 at 9:20 PM 
Yeah, thought so even though it was a brand new one. I agree that 650W, especially with no add-ons apart from one extra hard drive, should have been enough.

Will try my luck once once more in the next few days.

PS. Achieve Zen state prior to handling sensitive electronics. I managed to damage one of my memory DIMMs. There is enough of a gap between the slots to actually fit in a DIMM. If you are not carefull or are a little stressed (me) and you manage to put your DIMM in the gap then you press it down with the regular amount of force... bye-bye surface mount capacitors and resistors. Memory downgrade in one swift move... ouch...!


January 23, 2010 at 10:02 PM Howard said...

Man, that write up on the ATX psu for G5 is awesome. Bad thing is I am not an electrician at all. I am scared as heck to do such a thing. Anyhow, any chance on a schematic or maybe pay you to make one? I have a dual 2.0 G5 with the 600 watt psu. I live in the states. let me know. thank you. Howard

February 20, 2010 at 12:15 PM 
You don't need a schematic. The power supply connectors pinouts are available in the Apple service manual.

February 21, 2010 at 9:31 PM 
Hi, I need some help to try to repair my PSU. Only one component was burnt on it : the one on the heatsink on the photo. Could some tell me what is it ? Thanks a lot, Thierry

April 22, 2010 at 11:18 AM Customcomp said...
April 22, 2010 at 5:51 PM 
For dadwarf - The component is a TOP249Y on my power supply. Here is the spec sheet link


April 22, 2010 at 5:53 PM 
Thanks a lot for this quick answer !!!

April 22, 2010 at 11:30 PM 
I have replaced the burnt TOP249Y in my PSU but saddly it still doesn't work.

There is no other burnt component. Could you give me some clue to test the PSU ?

There is no sound at all from it, no current on the output...


April 30, 2010 at 12:56 PM 
I would find out if there is any voltage on the +5vsb and the +25vsb. That should give you a start.

May 3, 2010 at 1:46 AM 
Thanks for your answer. No as i said with my awfull english : there's no voltage on (all) outputs. I check all diodes and resistors for short...but nothing. The fuse is ok. The PSU is seems dead. I really want to try to save it before cut it connector !!

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July 21, 2010 at 2:54 AM faris hussain said...

Great work! Thos people who is having G5 must read it its will given them an extra mile.

Apple Blog

July 25, 2010 at 2:54 PM fantutu said...

you always have here an interesting things about cameras though,i really appreciate that louis

July 29, 2010 at 2:29 AM J said...

Hai Johnyboy, My G5 wont do anything after I replaced the CPU's. No start,led,sound nothing.
So I checked the PSU with a volt meter, I am reading 4.5 Volt by the pinout no.1. It should be 5 volt does this mean my PSU is broken or could this be normal.
Is it also possible that my torch screw driver which came with a magnetic top could damage the CPU or motherboard.

I hate to say it but I am totally in the mist at this point.

Thanks anyway, Jo

August 6, 2010 at 7:15 AM 
Hi J,
Could it be your voltmeter? Can you try to put some small load on it? (light bulb?) Also is there any reading on the 24VSB? If that was 0, then you wouldn't get power LED to light up, and could mean something is wrong with the PSU.
I Don't think a magnetic tip torx screwdriver would damage your logic board/CPU unless you accidentally scraped off some traces.
Also, have you tried zapping the PRAM? And did you check if your battery still has power? Any of these can cause non power up.
Lastly, what was the reason for replacing the CPU?

August 6, 2010 at 7:34 AM Douglas Bischoff said...

Worked like a charm... thank you VERY much for posting this!!

September 6, 2010 at 3:41 PM vincent said...

this is great!!!! I went through 3 of these piece of crap POWERPC....not power mac!!! Which is the only thing that this needed to be different. Other than that it is one hell of a great tutorial and one hell of a great job, I am definitely planning on doing this, I am gonna buy a 700 watt power supply, I got the service manual on Scribd, and I think those "Geniuses" @ Apple, suck, and they did not put a new power supply in my machine, just fixed the one there, and no Joke, this problem is sooooo bad, that when I first plugged my G5 in from the store, bam it smoked, no good, Back to a store now an hour away, then 3 years later right after apple care, bam it goes out, but luckily its cause they admitted to having a sub par component in and replaced all the models in between a certain serial range, I then blew it 2 more times and of course the third time was right after there Used Car salesmen routine of 90 day "Genius warranty". I had to go back to my decade old dell, dude I Still got a dell, and I am so much happier with Linux, I am gonna take Linux to the next level with my ideas for a competing tablet, and its uses in the music industry for education and for writing. But really thanks! Oh and the only reason I dont like the "POWERMAC" thing is how they left the PowerPc all to push the 3 benchmark and from what a friend says that works there, support windows, what a shame, long live, free, as in speech,

October 15, 2010 at 6:19 AM seigio said...

Do you have a step by step list on how to do it, including parts and connectors needed?

That would certainly make you a legend.


October 23, 2010 at 2:12 PM macguy said...

there are several different choices for 600W ATX power supplies out there. could someone help me out and be more specific on which one i should modify for my dual 1.8 g5 power supply conversion. Thanks!

December 1, 2010 at 10:03 AM nomad said...

It'd be great if you can tell me the model # on the resistor positioned at R302 right below the longest aluminum heat sink(?). Mine was burnt that you can't even see all the numbers... I suspect that was the problem. Also, how can you tell if the fuse has blown? It appear to be ceramic type so how does a blown ceramic fuse look like...?

December 3, 2010 at 1:11 PM robert said...

The value of resistor 302 is 100 ohms
2watts ,
Q301 also need to be replaced
and C301,C302 both 104K 400v
and fuse F1 10 amp

December 4, 2010 at 4:15 PM 
robert, this is very interesting to me as I have noticed some of the parts you just listed are blown on mine. is this the entire list of parts? i have a 1.8 dual core power mac g5. i would much rather order and solder in new parts than atx convert or buy new supply.

December 5, 2010 at 10:42 AM 
Thanks for the info!! I tested the burnt resistor and of course the meter shows nothing is passing through. However the others that you have mentioned are all registered on the meter. I was wondering if this means I don't have to go fiddling with them especially since Q301 is a pain to replace. You would have to remove the big transformer (?) in order to access the other screws that holds the heat sink...which it needs to be removed in order to gain access to Q301...

December 6, 2010 at 9:31 AM Mark said...

Johnnyboy... first of all.. Merry Christmas or Happy Holidays or Happy Hannakah which ever applies.
I got a G5 and it wouldnt power up. fuse blown in power supply.
bought a 650 ATX, and have gone the route of you and your description. after installing the power supply, matching colors from old supply wires to new supply wires, soldering and heat shrinking... no joy. i did connect the green wire from the new powersupply to power on pin 14 of P1, a red+5 to the 5vsb, and the 3v sb to a orange 3v line.. no joy.. any hints you can email me at email@example.com

December 23, 2010 at 10:25 AM 
one other thing.. maybe this is a clut.. the atx power supply has a grey wire - power is on - output?.. should that be connected somewhere? or the green - turn dc on - input which is presently connected to the green from p1 connector

December 23, 2010 at 3:58 PM 
Very helpful tips in rebuilding PowerMac G5 power supply using ATX parts. This tips is very needed nowadays because PC repair are usually using ATX parts. Keep on posting more about hardware repair and troubleshooting.tall sheepskin boots

January 31, 2011 at 4:26 AM youlacka said...

March 25, 2011 at 6:44 PM burns154 said...

If I were to get a 4 rail PSU. Could I just use the 4th rail in place of the 25Vstb? The specs of the Power Supply are

July 2, 2011 at 8:21 PM 
If you meant to use a 12v rail on the 25vstb, It may or may not work. First of all, you should make sure that nobody would be using an ADC monitor on it, the 25vstb powers ADC monitors and I'm pretty sure 12v is not going to work.

Second, besides the voltage, this 25v stb is supposed to be always on (even when sleeping). If you do this, you will only get a power light when the unit is on. No pulsating light when sleeping.

Only thing you would really get out of this is the power LED might light up when it's on (I have not tried so I couldn't be sure). I ended up using a standalone switching power supply that's always on on the 25vstb.

July 4, 2011 at 6:26 PM 
Could you post a model number of the stand alone power supply that you are using? Just for reference.

July 17, 2011 at 1:50 PM sophie said...

That little baby is adorable. And I am sorry to know about what happened.
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July 29, 2011 at 7:05 AM 
Woohoo! My G5 is up and running again after the last PSU attempt failed after two weeks. The fuse then got cracked. This time, I decided to make the PSU external so it is easy to replace plus it gets to use it own cooling fan design. Looks very hacky and the G5 has been nicknamed "Frankie".

- Note that the P3 connector (12v2) is only for the DVD and HD bay. You can connect all the drives directly to the ATX PSU connectors if you pass its cables through an expansion slot opening.

- The P2 connector carries all the power for the rest of the system and P1 carries virtually no serious power.

- ATX 12v2 normally powers hungry CPUs so:
- ATX 12v2 -> G5 12v1
- ATX 12V1 -> G5 12v2 + 12v3

- The airflow design of the G5 is soooooo tight there is no space to pass any extensions from the bottom to the top. I now have a 24pin and a 4 pin molex hanging out of the hole for the AC power. Had to gut the G5 PSU to do that.

- To connect the FANtach, I only powered the one of the 2 little fans with the 3 wires.

- After I gutted the G5 PSU and left its top half off, I still had to somehow mount the top cover on it as botht the inlet fans and the CPU cover latch on it. I ended up drilling two small holes on and screwed it on the bottom half of the gutted PSU.

The fans are still going all the time but not as loud as possible. Ran the service diagnostics and found no problems.

July 29, 2011 at 5:59 PM gregtuco said...

Great blog.
You all sound like tech people- I am not. Here is my problem: I too have a late 2004 G5. I had Text Edit doc on my desktop. The computer froze.. no spinning pinwheel...could not do a force quit...turning off and on failed to work... could not boot from Leopard install disk...smu reset failed to work...disk drive needed be opened w/ paperclip method...fans go into overdrive when I take plastic cover off...when I turn on I get jet sound-but no chime...monitor reads "power save mode"...power light stays on and blinks 3 times every 4 sec...is it power source?...logic board?...very sorry for bad typing, doing this from phone, no internet. Thank you! Greg

August 2, 2011 at 8:16 PM 
Hi Greg,

I'm sorry but I do not know enough to figure out what is wrong with your G5 from the information you provided. What I can guess is that the power supply and the circuit that monitors the power button is still functioning (thereby allowing you to still turn the unit on and off). What I can suggest is you take out all the add on cards (video card too) and try to power up if there is a chime. Beyond that, you may have to bring it to an authorized service center.

August 3, 2011 at 4:48 AM 
Hi jonnyboy-
Taking components out of the thing is probably something I shouldn't be messing with. I guess I'm at the mercy of Apple. But thanks for the help. Greg

August 3, 2011 at 6:27 AM 
FanTach update.

The ATX PSU is still working fine. However, the fans were driving me crazy! I ran the diagnostics, no problem, no thermal calibration needed. iStat menus was reporting 300rpm or so. No reason for the noise.

Then it hit me:

The 60mm G5 PSU fan I left connected to have FanTach signal was supplied by 12V which made it run at FULL SPEED. I calculated the resistor needed to drop the voltage to drop the RPMs and then it hit me again: just disconnect and see what happens.

Well, the system booted up fine and the little demon is no no longer wheezing. The whole computer is humming at a nice range.

So, no need for FanTach either.

August 6, 2011 at 2:47 PM 

6 days or so later and the disconnected PSU fan & fantac are causing no problem. Something tells me that the PSU fan speed is regulated by the original PSU and the fantac is only read by the G5 logic board during diagnostics to verify that the fan is spinning at the specified rate.

Separate issue: my firewire 400 is refusing to see my iSight but my Lacie is recognised OK (on firewire 800 also).

Win some lose some. Happy to have the G5 back on-line.

August 12, 2011 at 3:11 PM 
I followed every step, and did what I could. The problem is the power button doesnt work. If I bypass the power switch by grounding the line(the green cable to ground cable) the red light comes on and no chime... needless to say I'm frustrated...

September 8, 2011 at 10:41 PM G-Money1005 said...

Hey Johnnyboy!
I can't tell you what a help this was during a teardown and rebuild of my recently acquired G5. It is my first and it went well. The only problem I have is that it is as loud as a jet engine... still! I will be replacing some fans as I go along which seems pretty straight forward, but I was curious about the thermal recalibration. Do I have any other choice other than lugging the mammoth case into a shop to have them do it? I was hoping to find a copy of the diagnostic disc online but to no avail! Are you a tech that you happen to have one? Is there any where I can obtain an image to burn to disc? Just wandering in the dark on this one right now... lead only by the sound of nine loud fans...


September 15, 2011 at 6:07 AM 
@G-Money Have you checked the RPM of the fans? Which fans are spinning loudly? I had a fan noise problem but it turned out to be the fans from the old power supply and not those from the CPU.

September 16, 2011 at 1:20 AM Medline III said...

Worked perfect... thank you a lot for posting this solution for the Apple user of the G5.
I made this video with some photos , hope it help to others to do this:


October 9, 2011 at 1:03 AM Mac Maven said...

What a helpful blog page! Thank you, Johnnyboy and aris! A friend has asked me to repair his G5 and it seems like replacing the power supply with a 600 watt ATX is the way to go. I am electronics literate, but (aris) I am not familiar with P1 and P2 designations. Johnnyboy, how did you know which wires supplied what voltage if your power supply was dead? I can give you my email if you wouldn't mind answering my questions.

October 19, 2011 at 8:31 AM 
Mac Maven, the P1 and P2 designations should be written on the plugs of the original power supply. The pinout of the connectors are found on the Powermac G5's service manual. The voltages are listed in the pinout. http://img212.imageshack.us/img212/8477/g5psupinoutp1cn8.png

October 19, 2011 at 1:51 PM Jonny Jones said...

Fantastic information! I've got a Power Mac G5 Quad 2.5Ghz (late 2005) and the PSU has just gone. I've stripped the internals by about 60% but I need to take off the complex water cooling/cpu cluster. After this a friend is going to take a look at the PSU for me, hopefully it can be fixed as a 'new' one on eBay is £200. Hopefully its something simple that can be replaced. Symptoms were a click when power went to the PSU (normal) then a another 'clonk' when you pushed the power button - the machine wouldn't turn on. So I'm hoping its the PSU and not the motherboard. A fantastic computer that I don't want to scrap.

October 30, 2011 at 5:55 AM 
November 19, 2011 at 12:04 PM Kyrie said...

this post saved me. I have 3 g5's and hae re-wired 3 atx power supplys. The easiest way was with 'chocolate block' connecters - with no 25v supply the only downside is not being able to sleep the machine - otherwise everything else works fine - i leave the power supply outside and duck tape it to the side of the g5 - the internal fans spin heaps less as the power supply is now external.
Thanks for starting this post. chur chur

December 30, 2011 at 12:25 AM adam said...

My G5 wont power on at all but when i checked the power supply the female pins on the p1 p2 and p3 connectors they all have the correct voltage is it possible that the problem could still be the power supply unit.

March 9, 2012 at 2:44 PM 
Adam, did you check the voltages under load? A lot of power supplies will give correct voltage with no load but not under load. Try plugging in a hard disk and check again. If the voltages are correct even with load, then I would think it's not the power supply.

March 13, 2012 at 12:50 AM