ATX PSU in iMac G4 - help please

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Joined: Sep 14 2006
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I have an iMac G4/800 MHz/256 MB RAM/SuperDrive/60 GB HDD/GeForce2MX, bought in February 2002.

Recently the PSU died and now I have bought an ATX PSU 400W (thats all I could get).

Now I have to modify the ATX PSU to connect it to the main power connector on the Logic Board.

As far as I know, the connector is a 14-pin one and looks like this:
WITH THE CLIP ON TOP

RED, +5v, to HDD | YELLOW, +12v, to HDD | BLUE | YELLOW, +12v | YELLOW, +12v | YELLOW, +12v | YELLOW, +12v

BLACK, COM, to HDD | BLACK, COM, to HDD | WHITE | BLACK, COM | BLACK, COM | BLACK, COM | BLACK, COM

The only thing I can't figure out are the voltages of the BLUE and WHITE wires.

I wasn't able to get one even with much googling.

Apple follows different color coding methods for different models of PSUs (I think).

I have connected the YELLOW (+12v) and BLACK (COM) from the ATX PSU to the iMac's main connector.

The iMac seems to show signs of life.

The only ones remaining are the BLUE and WHITE wires.

Mine is the lamp shaped iMac, the one that looks like a volleyball at the base and a sunflower on top.

Does any one have a complete pinout for this model ?

PLEASE HELP.

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eeun's picture
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Joined: Dec 19 2003
Posts: 1891
I also searched for pinout in

I also searched for pinout info on the power connector and had the same problem you did.
Nearest I could figure was the power supply was always on when plugged in, and perhaps the two mystery wires were part of some power-saving control. If that's indeed the case, then it's part of the iMac logic board that controls power-on, as well as converting the +5 and +12V to any other required voltages.

edit: From a previous posting here, I noted - and since forgot - one of those two lines on the iMac PSU I measured as 3.3V.

What 'signs of life' did you achieve? Since I couldn't quickly figure out how to handle soft power on, my plan at the time was to switch to an older AT power supply that would give me a hard power-on.

Discovered too late for my plans, it is possible to modify an ATX PSU to an always-on state: Link.

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Joined: Sep 14 2006
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eeun, thanks for the reply

I have already connected the GREEN and BLACK wires of the ATX PSU together, thus the PSU is in an "always on" state. At this point the four YELLOW (+12v) and BLACK (COM) wires are connected to the ATX PSU. I just tried switching on the computer and it did power on with a blue screen with a flashing question mark. But the CPU became real hot and so I switched it off immediately.

I now suspect the WHITE wire should also be +12v and the GREEN one +3.3v (power good ?). Most probably the ORANGE wire in the ATX PSU carries +3.3v. Therefore I will be testing it out again this weekend. Will let you know the proceeds.

I have one more query:
I am thinking of rehousing the whole system in another case, but the funny thing is, the top dome serves as a heat sink for the CPU. If I cut the top dome in half and just fix the lower portion, will that be sufficient enough for cooling the CPU ?

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Joined: Sep 3 2004
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you got it!

if it powers on with a flashing -?- then you got the power to work, you just need to get the hard drive powered up, and most importantly, get a heatsink of the proc!

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eeun's picture
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Joined: Dec 19 2003
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Re: eeun, thanks for the reply

macrocosm wrote:

I am thinking of rehousing the whole system in another case, but the funny thing is, the top dome serves as a heat sink for the CPU. If I cut the top dome in half and just fix the lower portion, will that be sufficient enough for cooling the CPU ?

Hmmm...been too long since I was inside a G4 iMac. IIRC, for testing I had an old G3 or Pentium heatsink and fan balanced on the CPU, or maybe the heat pipe that goes between the CPU and iMac case metal.

I would think it would be easier to rig a heatsink as above, but more permanent, rather than cutting up the metal carcass of the iMac. Depends on what tools and goodies you have access to, of course.

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Joined: Nov 14 2006
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but what about video?

Hi,

I'm looking at purchasing a 1.25Ghz G4 iMac logic board. This thread has shown me I can power it, and I know using the external video 'mirroring' I can see stuff, but I can't find any info on if the LCD is digitally driven or driven via a VGA signal - like the eMacs and previous iMac with conventional CRTs. Does anyone have any info regarding this?