I just received a Performa 476 and was shocked at the amount of rust in the computer. I believe it literally sat underwater for some time. The motherboard looks like it can be salvaged but the power supply is completely covered in rust. So much so that I would never put power to it.

I may one day try and locate a replacement power supply for it but in the meantime, I have an extra PC ATX power supply (340W) I could possibly use for testing. It's too big to fit in the Perfoma's case but I'd like to use it to confirm the computer works. I don't have any other PSU's at the moment.

So my question is, can this ATX power supply be altered to work with this computer temporarily? I'm very handy with a soldering iron but I'm not that familiar with Performa's. I suppose I could look up the schematic online. But, I don't know how the Performa's "soft power switch" would work with the ATX PSU.

Any tips would be appreciated.



The Performa 476 doesn't have a soft power switch. I was thinking about my Mac IIvx when I typed that sentence. The Performa 476 has a rocker type PSU. Still, I'm unsure how the ATX PSU could be adapted to the Performa.

Is it as simple as just matching the cable colors? There are two black wires, and one each of blue, orange and yellow. I know there are standard colors for power supplies but Apple sometimes does things "differently". :-)

Thanks to Cody Gray for pointing out that information.

  • Performa 476 doesn't have a soft power switch. It has a regular old "rocker" power switch on the PSU, sticking out the back of the machine. The power key on the keyboard does nothing with this and other slimline LC models. Performa 476 is just an LC 475 with a different badge, which is just a Quadra 605 without the cute little feets. Also worth noting that these machines won't initialize the display with a dead PRAM battery. You either need to replace it, or power-cycle the machine once (turn it on, back off, and back on rapidly). Commented Jul 21, 2017 at 14:06
  • Oh my...you are correct. I was thinking about my Mac IIvx. I will update my question.
    – cbmeeks
    Commented Jul 21, 2017 at 14:16

2 Answers 2


Yes, you can use a standard ATX power supply. Do not just match the wire colors. Look up the pinouts. Then (preferably) verify them yourself with a multimeter before wiring things up.

In order to turn on the ATX PSU you will have to short PS_ON with COM (ground). You can wire up a momentary switch for this purpose.

ATX pinout: https://en.m.wikipedia.org/wiki/ATX#Power_supply

LC 476 pinout: http://old.pinouts.ru/Power/mac_lc_power_pinout.shtml

  • 1
    Thanks for the links. I've read that I need some type of load on the ATX PSU so that it doesn't overheat. Is that true? If so, I would imagine a 220ohm resistor and an LED on one of the 5V lines should be fine. Would you agree?
    – cbmeeks
    Commented Jul 21, 2017 at 17:35
  • 2
    No, it is not true that the PSU will overheat without a load. Most power supplies require a minimum load (different for each rail) in order to turn on. These requirements can (sometimes) be found in the specifications or printed on the side of the PSU. The computer is your load. If the computer doesn't meet the minimum load current, an LED isn't going to either. You would need to find the minimum load that isn't being met and use an appropriately sized power resistor. I suspect you don't need to worry about this situation for this machine. Commented Jul 21, 2017 at 17:48
  • Scott: That makes sense. What I had read was that if you manually turn on an ATX power supply (by jumpering PS_ON to COM) and there is no load (computer NOT connected) then it would overheat. I can't remember where I read that. Anyway...thanks.
    – cbmeeks
    Commented Jul 21, 2017 at 19:21
  • 1
    That LC 476 pinout you linked shows a -5V pin and ATX power supplies haven't supported -5V for a long time now. The computer may still boot without -5V, but possibly with sound and/or serial ports not working. See thinkclassic.org/viewtopic.php?id=306
    – user722
    Commented Jul 21, 2017 at 22:00
  • 1
    If there is a -12V rail available on the replacement PSU, a 7905 and a few capacitors will give you a -5V rail. Commented Jul 24, 2017 at 22:00

The power supply is mainly 5V 3.75A (for all the logic), with +12V 0.78A to run the hard disk, and -5V 0.075A to run the serial ports (printer and modem). Ethernet cards exist, but the hard disk drive is parallel SCSI and those are no longer being made. The floppy drive is Apple-type, also no longer available.

Power connector J12 has (1) black (GND), (2) black (GND), (3)nc (4) orange (+5V), (5) yellow (+12V), (6)nc, (7) blue (-5VDC). Unless it's very dirty, you can read that from silk-screened labels on the logic board.

Any ATX power supply is overkill, you only need 30W, even with accesories plugged in. I'd consider yanking the power supply (it's held in by two plastic latches), disassemble, clean, and just paint all the rusty surfaces. The important power bits don't care how it looks.

If the hard drive isn't working, though, and if you can't remote-boot from a server with an operating system (7.5.3 is a good version) bootable image, it'll be a tad disappointing. The original disk drives (0.080 GB) would be about 23 years old today.

A workable HD substitute would be a Macintosh booted up with the T key held down, which would make the Mac HD available as a SCSI target just like it were an external drive. That hardware/software combination only needs to offer a HFS drive image (because the Performa won't read the format of the post-MacOS 8.1 HFS+ disks), and can do that from IDE disk. Target-disk firmware for the T key trick is supported on 'most all native SCSI Macintosh models.

  • Parallel SCSI adapters are still available, though, so you could convert a modern SCSI drive to work on the old Macintosh 50-pin narrow SCSI1. Commented Jul 22, 2017 at 12:40
  • @CodyGray That can work, but a Macintosh host (SCSI ID=7) with such an adapter would conflict with the Performa (also SCSI ID=7), in addition to needing some odd software to perform as a disk drive target (so that it boots up a Performa just like a hard drive would). The modern SCSI drives I know about are SAS (not a compatible electrical interface). There's a solution from Apple that does the deed; I'll add it to the answer.
    – Whit3rd
    Commented Jul 23, 2017 at 0:18
  • Wait, what? I thought enterprise Ultra320 SCSI drives were still being sold. That's what I'm talking about, you just have a fairly simple adapter that converts the newer-style SCSI interfaces to the old Macintosh-style SCSI interface. There's no SCSI ID conflicts, because you just jumper the adapter to have whatever SCSI ID you want (just like you would jumper the HD itself, if you weren't using an adapter). Commented Jul 23, 2017 at 6:03
  • Oh, I was thinking you meant to share a SCSI bus with a second chassis. Which does work (it's how target disk mode operates). To the best of my knowledge, though, the only SCSI drives today are 'serial attached SCSI', which is not compatible with parallel SCSI. WD site, Seagate site, show no parallel SCSI options.
    – Whit3rd
    Commented Jul 23, 2017 at 6:55
  • @Whit3rd painting and cleaning the PSU on this computer is out of the question. When I say it's covered in rust, I mean it's covered everywhere. Inside and out. I would have to rebuild 70-80% of the entire PSU in order to use it. Not worth the effort for this particular computer. Also, I'm only using the ATX PSU as a temporary PSU to see if the motherboard works. It won't be a permanent part.
    – cbmeeks
    Commented Jul 24, 2017 at 12:14

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .