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How I Built an ATX Case Beige G3
By Timothy A. Seufert
Published: 3/13/2000

    Disclaimer: This page is for entertainment only and is not guaranteed accurate. Performing modifications or other work inside your Mac will void any Apple warranty, may cause damage to your computer or result in personal injury. You assume all risk from the use of any of the information in this article.


I first got this idea while reading one of Mike's forums. One of the threads had some links to one of Takashi Imai's pages. He's got some information about using ATX power supplies in both the beige and blue & white G3s. The pages were in Japanese, but the implication was clear enough: ATX supplies can be used with the beige G3 without modification, and with a slight modification for the blue G3. So, I thought, why not use the whole ATX case? Some of them have a lot more drive bays than Apple offers.

So far I've done a beige G3. I'm going to do a blue G3 soon, and hopefully will be able to supply pictures for that conversion. Be warned that it is not as simple to make a blue G3 board work with an ATX power supply.

Even with the beige G3 board, this is not a drop-in operation. You will need to make modifications to the ATX case for sure, and possibly to the G3 motherboard as well, depending on what choices you make.

What you get in the end is pretty good. The motherboard will be mounted securely, the standard Macintosh soft power on/off will still work, you will be able to turn the machine on and off via the front panel power button, and you will be able to hear sound through the PC speaker.

Aside from drives, RAM, etc., you will need:

  • Beige G3 motherboard, any revision.

  • One 2mm jumper (the small kind commonly used on hard drives, and you probably already have one on the board)

  • Pentium heatsink/fan compatible with the G3 heatsink clip, or a 2" fan and four #8-32 UNC screws long enough to screw it onto the original heatsink.

  • Some kind of PCI video card (optional but recommended).

  • PC ATX case with the following characteristics:
    • Removable motherboard tray (important!)
    • Uses threaded offsets to mount the motherboard (also important!)
    • Tray should not have many (if any) large holes, molded depressions, etc.
    • Preferably at least a mid-tower; roomier is better.

    I used an Antec KS-288. It was OK, but the tray wasn't removable, and that made it harder to do the necessary modifications. [ Note: Inwin and many other PC cases have removable MB trays-Mike]

    Tools - besides the usual screwdriver and so forth, you will need:

    • Small round or half-round file
    • Hacksaw (optional, some may need, some may not)
    • A drill (preferably a drill press)
    • Matched drillbit and tap for making #6-32 UNC threaded holes
    • Light-duty wire cutters with narrow tips
    • Soldering iron and desoldering tools (optional)
    • Scratch awl or centerpunch
    Some kind of deburring tool might also be useful, though I got away without one.

    The Modifications:

    1. Motherboard Prep

    Behind the leftmost PCI slot there is a 2-position jumper block. If you look at the board, one position is marked "Mac Supply" and the other "PS/2 Supply". Make sure there is a jumper on the PS/2 position and none on the Mac position. This will configure the board to work correctly with ATX soft power on/off. (According to Apple's beige G3 service manual, beige G3 minitowers will have it on the PS/2 position, implying that these machines have power supplies which are almost if not fully ATX compatible.)

    Next, you have to remove the motherboard's DB15 monitor connector. It protrudes outside of the I/O panel into the PCI slot area on a standard ATX case, so it has to go if the motherboard is to fit.

    There are alternatives. I always planned on using an ATI Rage 128 video card instead of the wimpy Rage II on the Rev. A board I was using, so I just cut off the connector. But if you really want to use the onboard video, you can do it. You could cut up the back of the case to make room for the connector. Or, you could install some other kind of connector in its place. One option is to install a vertical PC-board mount DB15 in place of the original right-angle connector and snake the monitor cable through an opening in the case. Another is to solder wires into the holes and run them up to a panel-mount DB15 or HD15 VGA connector.

    The easiest way to cut off the connector is to take a wire snipper with a thin tip and cut each pin. After snipping the visible row of pins on the connector, you can bend them up and out of the way to access the inside row. Once all the pins are snipped, you can rotate the whole connector back and forth until the big strain relief tabs break. At this point you can desolder all the pin stubs if you want to use the motherboard video, otherwise just leave them in.

    2. Motherboard Mounting

    [Update:]
    An update to the G3 ATX article: I forgot to mention one difficulty. The mounting hole in the right rear of the motherboard is too close to the 50-pin internal SCSI connector. There's not enough room to get a normal screw in. I was able to use an extra threaded motherboard standoff instead of a screw, using a pair of pliers to turn it. Another option might be the use of a cable tie to secure the motherboard to the tray in that corner.

    The beige G3 board has some screw holes which match the standard ATX pattern. Unfortunately, its PCI slots aren't in the correct location, so PCI cards won't line up with the rear panel slot openings if you just install it as is. You have to make your own mounting holes to get everything to line up.

    You can spend a lot of time measuring things to make sure everything will work, but I found that a quick way of getting alignment is simply to install PCI cards in all three slots and use them to self-align the motherboard. I installed standoffs in the pre-existing holes to support the board at about the right height, and then lowered the board into place, lining the PCI cards up with the openings. (It's best to move the board up one full slot's worth from the original ATX mounting holes.) I screwed the PCI cards into place to get some stability. With the cards perfectly vertical and snug against the back panel, I used the scratch awl to mark the position of each mounting hole on the motherboard tray. Only use those holes which have a black border around them.

    With the hole positions marked, it's time to drill some holes. You can do this with a hand power drill, but it will be very difficult to keep the drill at right angles to the tray. A drill press will make life a lot easier. Once a hole is drilled, carefully cut threads in it with the #6-32 tap and a tap wrench, and then try to remove the burrs so the offset can sit flush.

    If you've never cut threads with a tap before, it's a good idea to drill and tap a couple holes in random useless spots for practice.

    Some holes may be impossible to drill from the motherboard side of the tray, because there isn't enough clearance for the drill. In my case, I could only drill two holes from the original marks. I used a pattern to do the rest from the back. I used a pencil to mark the hole positions to a sheet of paper, using the motherboard as a reference again. I then used the awl to punch holes in the paper at each screw position (I made them just large enough to pass a screw). I attached this pattern to the back side of the tray, using a couple screws through the already completed holes to hold it down. That allowed me to mark centers for the remaining holes on the back side.

    Now you should be able to put in the offsets (make sure they're tight!) and install the motherboard. Amazingly, all the holes lined up OK when I did this, despite the fairly crude technique I describe above. For the B&W G3 conversion I'll probably break down and do it right. "Right" means using the technique of sticking the board in with PCI cards installed for only one reference hole position, and doing the rest by laying out a pattern using a steel rule and measurements taken from the board.

    If you have the tools and some sheet metal stock, it would be nice to make a cover plate to fit in the ATX I/O port hole, to prevent RF interference and promote good airflow. I don't have access to a machine shop, so I didn't bother. This is probably not real friendly to nearby radios. :) A sheet of cardboard cut with an X-Acto knife will at least promote good airflow.

    3. Personality Card

    The Beige G3 has a "personality card" slot, with either an audio-only card or an audio/video card. I had the audio only card, so I can't speak as to how to mount an A/V card. I also didn't have the internal modem which plugs into either kind of card. Making the modem and/or AV card fit will almost certainly involve more modification than is necessary with the audio only card.

    The metal bracket on the card will not fit, so I removed it. After that, what I found was that while the card is on the same center as a PCI slot, the components are on the wrong side and the 1/8" audio jacks don't naturally fit in the slot opening. The card can be installed if it is bent upwards, but this puts a lot of strain on the jacks. I marked the center of each jack on the PCI slot opening, then removed the board and the motherboard and used a file to cut indentations for the jacks.

    With that done, the personality card fit nicely, but I wanted to do something to keep it from flopping around and possibly working loose. I ended up using a thin plastic cable tie looped through the hole in the personality card's corner and the PCI slot screw hole. This keeps the board just about as secure as a screw would, though you will have to cut it and replace it if you ever need to remove the motherboard again.

    Barring modifications, you'll have to live without slot covers on either side of the personality card; its PCB juts too close to fit with them present.

    4. Cooling

    Apple's case designs always ensure airflow past the CPU, but you can't guarantee that in a PC case. One option is to install a Pentium heatsink/fan (make sure to use the original G3 clip in the correct orientation, not the Pentium type clip). Instead of doing that, I used the original heatsink and installed a fan on top. I found that the tines were spaced about right that the threads of a #8-32 screw will engage them. With four screws, the fan is held down securely. I did have to drill out the mounting holes in the fan to make them wide enough to accept a #8-32 screw. (I did this with a small hand drill, not a power drill.)

    5. Hooking things up

    The front panel power LED, speaker, and power switch are all easily connected to the G3 motherboard. Apple uses 0.1" spacing headers for this stuff, so does ATX. The only difference is that while each of these functions needs only two pins, Apple makes each connector a different width to avoid confusion. This is easily fixed; just pop the pins in the ATX case's connectors out of their housings and install them on the headers individually. Use the original connectors in the G3 case as a reference for what goes where (best if you write it all down before first removing the G3 board, of course).

    The ATX power supply just plugs in.

    6. Fit and finish

    As mentioned previously, it would be nice to make a port cover for the rear of the machine. Another thing here which I didn't do is to make a faceplate for the floppy drive (I just left it sitting there looking all ugly). A PC floppy drive faceplate may be able to fit the Apple floppy drive, particularly if it's from a mechanism made by the same company which made your particular floppy.

    I think that's about it.

    One interesting tidbit about floppy drives: you may have noticed that there are holes for a 34-pin ribbon header right next to the Macintosh 20-pin floppy header on the G3 motherboard. PC floppies use 34-pin cables. Hmmmm....

    I asked somebody who knows about these things, and apparently the Beige G3 motherboard can in fact use a PC floppy drive attached to this connector, or at least it could during the early prototypes.

    Some history here: the beige G3 motherboard was codenamed "Gossamer", and was actually designed to accept either a 603e or G3 processor. There is an unused L2 cache slot to the right of the CPU which would have been used with the 603e. Gossamer was originally intended for low cost systems and for use by Mac cloners, which was the reason for the ATX similarities and the potential use of PC floppies. But when Apple discovered that a Gossamer with a G3 had much better performance than the high end 6-slot "Power Express" board also in development (at about the time that cloning went down the tube), Power Express was canned and Gossamer got used for Apple's whole product line.

    Thus, the PC floppy capability wasn't used in production machines, but it may still be functional. There are downsides. The original 20-pin Mac floppy header must be removed to use the 34-pin header (they're too close together), you won't get autoeject with a PC floppy mechanism, and you almost certainly cannot read or write legacy 400K / 800K Mac floppies with a PC mechanism. Might be worth a shot for somebody with a busted floppy drive. I of course cannot guarantee that this will not fry your motherboard.
    -Timothy A. Seufert

    (This article was written in 2000, for a Beige G3 Desktop to ATX conversion article posted in 2003, see the links below.)


    Previous ATX Conversion Articles:


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