Introduction: Macintosh II(fx) Modern Power Supply Replacement


This is my documentation on how I adapted a model TFX power supply top power my vintage Macintosh IIfx. The original Astec stopped working and I did not have the expertise or patience to try and fix it beyond replacing capacitors and obviously failed components.

So I bought a TFX power supply because it fits into the space that the original one occupies.

There are a couple of things we need to consider:

  1. The specifications for both supplies differ, especially for the +5V and -12V the new one comes up a bit short, but that is only going to be relevant if you max out the system.
  2. We need to make the soft-power-on/off of the Mac work with the PS_ON signal of the modern PC power supply.


  1. 1x TFX Power Supply, including C14 IEC cable
  2. 1x Molex KK 396 2139, 15 way receptacle, no ramp connector housing and contacts (I got mine at Farnell)
  3. 1x BC547 bipolar NPN transistor
  4. 1x 220 Ohm resistor to pull the soft-power-on/off low
  5. 1x 1k resistor for the power indicator LED
  6. 1x LED 5mm through hole (amber)
  7. Shrink sleeve
  8. 1x BULGIN PX0575/15/63 Power Entry Connector, Plug, 250 VAC, 10 A, Panel Mount, Solder
  9. 1x BULGIN PX0695/15/63 Power Entry Connector, Receptacle, 250 VAC, 10 A, Panel Mount, Solder
  10. 6x Female Push On Crimp Terminals Blue 16Am 6.3mm 3D filament

Step 1: Wiring It Up

Low voltage side

Ok, so the Mac, whenever it wants the power supply to switch on, it pulls pin 15 of the power supply connector to the Logic Board high(>3V) from the battery and then, when the power supply switches on, this voltage is maintained (5V).

The PS_ON signal on a modern PC power supply needs to be pulled low in order to switch on the PSU.

So we need invert the signal. I've done this using a BC547 NPN transistor. The base is tied to the power on signal on pin 15, the emitter is tied to ground and the collector to the PS_ON signal of the power supply. It turned out the circuitry on the Mac side needs a pull-down, so I added a 220 ohm resistor from the base to the ground.

See schematic.

I needed to find a place for this little circuit and discovered that in the power supply there were some unoccupied places that I could use (see photo). Your power supply might/will be different. Also, if you're not comfortable opening up power supplies, then you can put it on a small pcb external to the PSU case.

The contacts in the connector to the logic board are crimp on.

Mains power side

For the mains power side we need a splitter to feed the TFX power supply (and the Mac) and routs power to the monitor. I cut the power cord and reused it to make this splitter (See photo).

Step 2: Make It Fit in the Mac

To make the new power supply fit in the slot I designed 2 parts and printed them on my 3D printer.

There is a front part, that fixes the power supply on the front and provides a press-fit hole for the power LED and fixes the supply in the case.

It is screwed to the power supply with two (PC case) screws.

Then there is the back part with fittings for the external power connectors.

The back part is screwed with 3 (PC case) screws to the power supply.

The finished supply slides in just like the original and is fixed the same way with the retaining screw.

That's it. To be sure you can fire it up by connecting pin 15 to the STBY voltage and the power supply should switch on. Check all the voltages. Then connect it to your Mac.

I would love to hear from you if this worked for you.

If you have questions, suggestions or improvements please let me know too!