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A guide for doing a SuperMac S900/C600 ATX power supply conversion

CircuitBored

Well-known member
Hi all,

Yesterday I finally got around to doing my SuperMac S900 ATX power supply conversion and it was a roaring success. I wanted to share my work so that others might do the same. The SuperMac C600 uses the same supply as the S900 but at a 200W capacity.

Parts and tools needed:
  • SuperMac S900 or C600 power supply
  • A standard ATX power supply with a 24-pin main connector (at least 250W, 350W recommended)
  • REC5A-0505SW/H2 DC-DC converter
  • Both black and white 16AWG wire, at least 50cm of each. Any colour will do, really. Do not buy cheap aluminium wire.
  • Wire strippers
  • Soldering iron
  • Plenty of 3mm 2:1 ratio heatshrink tubing
  • Lots of solder, lots of flux
  • 2mm width zip ties (buy a set of various lengths, they're handy for all sorts of things)
  • Patience

The S900's power supply has a bit of reputation for being underpowered and unreliable so it is a very good idea to replace it. I thought I was doing this repair preemptively as my PSU was still "working" but upon disassembling the original PSU I found a number of indications that it was well past its peak. More on that later...

IMG_4056.jpeg

I used a 350W Seasonic SS350-ET (kindly donated by @joshc) as a replacement, which should provide more than enough overhead to properly supply multiple PCI cards and power-hungry optical drives.

First of all: this modification is much, much simpler than ATX-ifying other "real" Macs because the S900 uses a standard ATX power supply, albeit with a few caveats. There are no tricky soft power circuits to worry about – you just need to connect up all the pins correctly and then you're good to go!

The first major obstacles are, rather obviously, the connectors. The S900 uses the older AT connectors and not the 20/24-pin ATX connector that is now ubiquitous in the x86 world. Surpassing this is easy if you are handy with a soldering iron. Below are the pinouts for the S900 and a standard 24-pin ATX supply, cross referenced to illustrate the way in which they should be connected. The S900's connectors are numbered left-to-right as they appear on the logic board, meaning pin 1 is the pin closest to the rear of the machine. The pinout for the ATX connector is easily found online.

Screenshot 2021-12-03 at 15.22.37.png

It is important to note that the colours of the S900's original PSU wires are not consistent across different machines. If your S900's supply matches the colours and positions listed above then you can use these tables as a reference, otherwise you will have to make note of your own specific colour-to-colour connections.

The most common variation I have seen online is the 3-pin J26 connector, which on some S900s is bizarrely connected with brown for ground, purple for +5V standby, and black for PS-ON. Again (I cannot stress this enough) make your connections based on pin positions, not wire colours.

Important note:
the main 12-pin connector on the C600 power supply (and some S900s) is split into two separate 6-pin connectors. The pinouts are the same, just use pin 7 from the table above as pin 1 on the second connector.

This brings us on to our second major obstacle: -5V supply. Most modern ATX supplies do not have a -5V rail as it is superfluous to nearly all modern motherboards. This is a problem, because the S900 uses this voltage for its serial circuitry (from what I've read, don't quote me on that). I have read mixed reports that you can run an S900 without -5V power but I would not personally recommend this. If you're going to the effort of doing this mod, do it properly. For a while I was scratching my head on how to turn a +5V rail into a -5V rail when @cheesestraws clued me in on a very handy little DC-DC converter, REC5A-0505SW/H2. Any DC-DC converter capable of supplying the right negative voltage and amperage will do but this one is good because it has a 600mA capacity – a perfect replacement for the 500mA -5V rail on the original PSU.

Here is how you should connect up the REC5A.

Screenshot 2021-12-03 at 16.59.03.png

I placed this component on a small proto-board to make it easier to mount inside the PSU but you can just solder directly onto the chip if you like. I suggest that you do a dry run with this chip hooked up to a molex connector outside of the PSU before starting to take everything apart. If the voltage between pin 2 and pin 16 measures at -5V then you have connected it correctly.

Be extremely careful with how you mount this inside the PSU. I zip-tied it to the rear grille of the supply with the REC5A facing outward so as to not short it out on the chassis. There's plenty of clearance from the main PCB so no risk of short circuits from there. Be sure to mount this in such a way that you can actually put the PSU back together again!

Once you have familiarised yourself with the information above (and made appropriate notes for your specific system) you can go ahead and start chopping off connectors and soldering on the S900's connectors. I recommended disassembling the original PSU and desoldering the wires from the PCB to give yourself as much wire length to work with as possible. Be sure to let the PSU discharge before disassembly.

Important note:
many modern ATX supplies (including the Seasonic I used) have voltage monitoring connections that must be connected in order to run the PSU. You can normally spot these in the 24-pin connector – they're the thinner wire on the pins with two wires running into them. Make sure you don't ignore these like I did initially.

Make sure that any unused connections from the replacement power supply are properly safeguarded by either desoldering them from the PCB or simply folding them back on themselves and heatshrinking over them. Do not leave bare chopped-off wire ends loose inside your PSU.

This is not a soldering tutorial so I won't describe how to actually connect the wires properly. It is a basic skill which I recommend you learn as it is immensely gratifying. That said, if you do not understand the information above (and how to apply it) then that is a good indicator that you should not do this work yourself.

It's worth remembering that the power supply in the S900 is extremely close to the power connectors on the logic board so your main connectors' wires don't need to be very long at all. Measure up and decide for yourself how long you want your wires to be.

You will likely also want to swap some of the SATA power connectors from your ATX supply with the Molex connectors from the original supply. These are nearly universally coloured as red for 5V, yellow for 12V, and black for GND so it is usually a simple colour-matching job. However, I say again: go by pin positions and measured voltages, not wire colours.

I cannot write a guide like this in good conscience without stating the following:

THIS IS A POWER SUPPLY THAT HANDLES HUNDREDS OF VOLTS. DO NOT INTERFERE WITH EQUIPMENT YOU DO NOT HAVE AN ABSOLUTE UNDERSTANDING OF. IF YOU ARE NOT CONFIDENT IN WHAT YOU ARE DOING, DO NOT DO IT.

Despite the big red warning, this is a fundamentally simple procedure that amounts to little more than soldering in a safe and well-organised fashion. Power supplies are dangerous but this danger is mitigated by awareness and the resulting safe working practices.

Once you have soldered on all of your S900 connectors you can test the supply by shorting pins 1 and 3 on the J26 connector to power it on. Don't forget to reconnect the PSU's fan – many supplies get very angry if you try to run them without it. With that done, use a multimeter to check all of the voltages on every pin. This is not the time to be rushing for the finish line. The AT connectors are a little fiddly to measure so you might need a paper clip to help you make the necessary connections. Don't panic if something doesn't look quite right, you probably just forgot to connect something or made a slight mistake in one of your connections.

With the testing completed you can go ahead and install your newly-modded ATX power supply.

IMG_4086 2.jpeg

Success! Now I just need to fix these blasted ATI drivers so I can harness the full power of my Radeon 7000...

Thanks for reading this guide; I hope it was useful to you. If you do not want to do this modification yourself, drop me a PM and I will do the work for you for a small consideration.

Please let me know if it seems like I have forgotten a step!
 
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CircuitBored

Well-known member
As an immediate follow up I should mention that this power supply has given my S900 a new lease on life. Multiple recurring system errors have now vanished and the system is now much more responsive at boot time. PRAM resets are now reliable and my PCI cards are not constantly dropping in and out of existence on the OS side. The built-in ethernet now works first time, every time, whereas previously it would never work after a cold boot.

All in all, this is a pretty strong indicator that the original PSU was far, far more poorly than I had thought.

Best of all: I can now confidently load this ridiculous machine up with lots of exciting new things.
 

demik

Well-known member
Nice job. The old PSU was definitively on its last legs given how unstable your S900 was. BTW, was the old PSU ATX sized ?
 

CircuitBored

Well-known member
Nice job. The old PSU was definitively on its last legs given how unstable your S900 was. BTW, was the old PSU ATX sized ?

Yes, the original power supplies are ATX power supplies with different connectors on the ends of all the wires. Beyond that, there is no difference between them and any other ATX power supplies as far as the physical dimensions and output pins are concerned.
 

Angelgreat

Well-known member
If someone wants to sell the adapters on eBay, that would be great for those who need a new power supply.
 
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