G4 MDD ATX Power Supply Conversion
A friend of mine recently passed on to me a Dual G4 1.25GHz MDD. The machine itself is a bit old by today’s standards, but can still run Leopard and could be a nice addition to any home office. Upon opening the machine, I checked the front switch as the button felt “dead”.
Ultimately, I determined that the button was supposed to feel that way. Further testing led me to the power supply. I removed the power supply and started to troubleshoot. I found that a fuse had blown in the power supply. It was a 250V 8A fuse and it was totally blown. Figuring that a quick fuse swap would fix the issue, I embarked on what turned into an adventure in locating the fuse. The closest thing I found was a fast-blow fuse at Fry’s. I got home and anxiously put it into the PSU and hooked it up. A light flicker, pop and flash of electrical light later, I had deduced that the power supply itself was bad and blowing fuses irregardless of how new they were.
I did some reading and found that the power supplies in this unit were unreliable, loud and expensive. I had an extra 700-Watt PC ATX power supply sitting at my workbench staring at me. I did a quick bit of reading on applefritter and a few blogs and found that an ATX power supply could be converted to work with the MDD.
I used the following pin-outs.
24 Pin ATX Pinout
MDD Power Supply Pinout
Be extra-extra careful and always double check your work before plugging anything in. Also, one thing I didn’t see covered in all of my reading was the Yellow +12v wires. There are two more on the MDD PSU than there are on the ATX and one more Black Ground cable. I simply spliced the ATX pin 10 to MDD pin 10/21 and ATX pin 11 to MDD pin 22/24. I also doubled up on one of the ground pins as well.
To preserve my existing ATX power supply and ensure easy changing in the Mac should the PSU fail, I used an 24-pin ATX extension cable that I purchased at Fry’s for $7.95. This also gives you a little more length on your cabling that comes in handy when you want to mount the new power supply inside the case. I was able to mount my power supply in my case without having space issues with my video card. With regards to other cards, I had to remove a Digi001 PCI card as it would hit my new power supply. I may swap it out for a mini-atx or attempt some other solutions for mounting to remedy this. I’ve toyed with the idea of removing the optical drive bay and mounting it there, but I can install the card in another PC and salvage this one as stock.
PSU Wiring at ATX Extender/Apple PSU loom before shrinktube
Overall look. PSU is mounted above optical drives.
PSU mounted
The one thing that will not work without some sort of external 25v power to the logic board is the ADC/apple display power. You should be able to supply power to a monitor without it though. It’s one caveat to saving a $100 and probably more headache with a used power supply from eBay. These power supplies seem prone to blowing up/failing so you’re rolling the dice on spending $150 on eBay and it would be ridiculous to spend $300+ on one from Apple.
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