Replacing An MDD's Power Supply With an ATX Power Supply

As the years go on, its not hard to see that parts are getting hard to find for all PowerPC Macintosh computers. Its unfortunate, but we have to live with it. So when it came down to replacing the power supply in out Power Mac G4 MDD that was bought used, we had two options. Get a used, but working, MDD PSU, or modify an ATX power supply. We decided to go with the latter, because of price and if the issue occurs again, we know how to do fix it!

Sourced from a fellow MacRumors' member, we received a used Bestec ATX Power Supply pulled from an older HP computer. This was the only way we could keep Molex power, since this machine does not have a SATA card. Rated at a maximum of 300 watts, it out puts 100 watts less then the max on the original Samsung Power Supply that failed. We believe this will be okay, since quite a bit of that is dedicated to the ADC (Apple Display Connector) on the video card which will not work after this modification. Thats alright though, since we didn't use it anyway.

What you will need:
24+4 pin ATX Power Supply (rated at the same or higher wattage then the dead one)
Dead Power Supply's main connector cable
Wire Strippers

Optional:
Crimp Connectors (We used closed ended ones)
Wire Crimpers
Double Sided Tape (Needs to be strong)
Wooden Paint Stir Stick
Electrical Tape
ATX Extension Cable
Original ATX connector
Molex Extension Cable(s)

(More Pictures are Coming soon!)

Step 1: Remove the original PSU from the MDD. To do this, first remove the lower Hard Drive caddy, main fan, and Optical Drive Caddy. Then, remove the phillips screw from the front of the PSU inside the case, then the hex screw on the back (a Hex 2.5 or Torx 10 will work)

Step 2: Cut the ZIP ties and cut the main power supply cable as close to the power supply as you can get. I cut mine about an inch from the actual PSU its self.

Step 3: Cut the ATX connector off the power supply. (If you don't want to mod the actual PSU itself, you can use an ATX extension, so if things go south, the power supply is unmodified.

Step 4: Strip all the wires on both the power supply and cable. If you are using crimp connectors like we are, don't make these too long. All you need is about 1/2-6/8 of an inch of exposed wire. Some will not be needed, for instance the gray "Power OK" cable on the ATX PSU itself, so these do not need stripping.

Step 4: Color match the cables together; green to green, red to red, black to black, etc, etc. Do sets one at a time, and make sure not to get the colors wrong.

Step 5: Take a crimp connector and your crimping tool, and crimp the connector. Make sure its is not loose, one came loose during the installing on the PSU itself and took a while to get a new one installed.

Repeat Steps 4 and 5 until all the wires are connected. There will be a few left overs, but the goal is to make sure the ones from the connector to the main board are connected to the PSU properly. Spares from the power supply, do not worry about them. Just cut the internal wires and  stretch the insulation to cover them plus about a 3/8 of an inch extra just to insure it doesn't short out.

Step 6: Making sure your wiring is correct, plug the power supply into the main board itself.

Step 7: Boot the machine. Note that it might not boot right away, because if the machine has been sitting for a while without power (like ours) then it take a second to "warm" everything up. Wait about 1 minute, then try again.

Your machine should now successfully boot! If it does not, recheck the wiring and make sure the power supply you are using is good. Is all these things check out, the issue could be further into the machine.

Step 8: Yay it works! Now to deal with the wiring. Unfortunately, a lot of these power supplies (like ours) does not have very many Molex power connectors. If needed, you could get a splitter it you choose, however we do not like this idea because it put additional stress on an already stressed PSU. Now is a time to make a tuff decision. You can either have 2 optical bays and 2 hard drive bays, or four hard drive bays. There is no longer room for all of these. This guide follows option A, 2 optical bays, 2 hard drive bays.

Step 9: First, fish all the wires through and plug everything in. We were able to just about plug it all in in the original holders, minus the HDD bays which will need a Molex extension. Now, try putting your PSU in place. Since most/all ATX PSUs are the same size, we found that by double side taping the PSU to the wall of the case that held the original display that it would stay in place fairly well.

Step 10: Make sure the power plug you want to use is already plugged in to the PSU (and not the wall) and secured to the top of the PSU. Because of the wiring, this is how it has to be installed.


Step 11: Install the HDD bracket into the main slot in front of the CPU heat sink. Make sure no HDDs are in it.


Step 12: Run the wiring through the HDD bracket. Make sure the optical drive carrier and main fan will clear the mess. Installing the wires in the HDD bracket gives them a space to reside in without taking up air flow to the CPU(s).

Step 13: Clean the back wall of the case and put double sided tape on the side of your power supply. Make sure none of the vents point to the top of the case. We used two pieces that extended the full length of the PSU. Take the protector off the tape and adhere the PSU to the case. Make sure it is as close to the top of the case as you can get it. Keep holding until you feel it is sturdy enough.

Step 14: Take a ruler and measure out the space where the old PSU used to sit. Now take your paint stick, and cut it about 1-2 inches shorter then that.

Step 15: Put a single piece extending the length of the stick of double sided tape. Stick it right under the PSU within the space cut out for the hold one.

Step 16: Take that ATX plug you kept and shimmy in standing up between the PSU and optical drive carrier.  This will support the power supply in center. Make sure airflow to the PSU is not blocked by the connector after installing it.

Step 17: Your video card should have no problem clearing the power supply. Check to make sure.

Step 18: Plug the machine in to make sure nothing came unplugged.

It is easier then it sounds.
So far, our tests have shown the computer is still as reliable as it was before. No problems have occurred.


Pictures. Click to enlarge.