The other day... the Radeon PCI card in my G3 MT suddenly stopped outputting graphics. At first, I thought the card had failed—but after testing it in another machine, it worked just fine. That’s when I began to suspect the real culprit: the power supply.
When you’ve heavily modified an old Mac—loaded with extra hard drives, CPU upgrades, and other power-hungry components—it’s not uncommon to see issues caused by a weakening PSU.
A Brief History of Mac Tower Power Supplies
Starting with the Power Mac G3 MT, Apple switched over to PSUs that were basically just modified ATX units.
Earlier machines like the Power Mac 8600/ 9600 used custom Delta or Astec PSUs. But the G3 MT (DT *1) actually uses a pinout that is fully compatible with ATX1.X (20-pin model) PSU. Apple even included a jumper: J28 on the logic board to switch between Mac PFW (Mac Supply, for DT) and ATX PFW (PS2 Supply, for MT), since their polarity differs.
*For a teardown of the G3 B&W, see the ifixit link below. It's easy once you get used to it, but you can access the PSU by sliding the front bezel and DVD chassis forward.
Later models, such as the G3 B&W and the early G4 PCI Graphics, also remained largely compatible with ATX—only the −5V rail and PWR_OK behaved differently. This reuse of ATX pinouts continued through the G4 DA and QS, which were basically ATX with an added +28V rail for ADC displays.
*-5V rail is omitted from ATX 12V V1.3, https://blog-imgs-37-origin.fc2.com/d/u/a/dualsocketworld/Main-Diff-V2-smb.jpg
Things changed with the G4 MDD: although the pinout still resembled ATX, Apple switched to a long, custom PSU design with special airflow considerations. From that point on, the ATX compatibility era was over.
*1 G3DT is always used Mac PFW (Mac Supply), but I don't know what the purpose of that was. Incidentally, the Early iMac models(Rev. A to D) uses the Mac PFW method, which suggests that development of the iMac likely began quite some time ago. This is something I first discovered 20 years ago when I built the SE/666.
Why Used ATX PSUs are Perfect for Old Power Macs
The used market is overflowing with ATX PSUs. When people build modern ATX-based PCs, they rarely reuse old PSUs, so plenty of units end up secondhand. Even high-end 80 PLUS–rated power supplies can often be found for dirt cheap.
For example, I’ve bought working 80 PLUS units for as little as 330JPY (about 2.50USD), with the most expensive one costing only around 2000JPY. Considering that the original G3 MT PSU was just 171W, swapping in a 400W–class unit gives you plenty of headroom for upgrades that were impossible back in the day!
Choosing the Right PSU
Modern ATX PSUs tend to be deeper than vintage ones. The original Mac PSU was 140mm deep, matching older ATX designs. That means oversized units (180mm+) won’t fit inside a G3 MT.
My recommendation is to choose a power supply of around 400-600W. It's easy to install because the screw holes are aligned, and if you choose one with the same height as the AC inlet, you can easily adjust it. *Genuine Mac’s PSU also have AC outputs, but they're not popular these days.
If you manage to find a 140mm-deep unit, you can even attach the bracket to the chassis and use it as a retainer—just like the original setup. *The bracket cannot be screwed into an ATX PSU because there are no screw holes, but it provides a sufficient retainer.
BTW, recent PSUs exhaust air from a 120mm side fan, which seems a little inefficient when placed in a G3MT case... Also, since most fans are variable, if the fan doesn't turn at the initial setting, you need to connect it directly to +12V to stimulate the exhaust.
20-Pin vs 24-Pin
In the ATX12V v2.0, connectors have used 24 pins instead of 20. Thankfully, compatibility is maintained—the extra pins just provide additional +5V, +3.3V, +12V, and GND lines. The −5V rail is no longer present, but that’s not an issue since Macs like the G3 never needed it.
You could just plug things in loosely and hope for the best, but if you care about both reliability and aesthetics, it’s worth doing it properly. Don’t forget you’ll also need to wire the +12V case fan properly—since the Power Mac 9600 and G3 MT, while similar, have different fan placements.
The cleanest solution is to use a 20-pin male to 24-pin female extension cable. This keeps maintenance easy down the road and ensures everything looks tidy.
Wirings:
Custom Extension Cables (20-Pin Male to 24-Pin Female) Available
If you’re handy with wiring, you can make your own adapter cable using the pinouts I’ve described above. If not, we also sell ready-made 20-pin male to 24-pin female extension cables in our store. They make for a neat, secure install—perfect if you want your vintage Mac upgrade to look as good as it runs.
Thanks for reading to the end!