Ken’s Rare iMac G3 Prototype Won’t Power On – Can You Help Fix it?

My good friend Ken (of the Computer Clan YouTube channel) invited me over to take a look at something very special. A prototype of the first Apple iMac G3 model from 1998. He featured this unique clear-case iMac in a recent video… but there was just one issue… it didn’t work! So I was summoned to try to work my magic and fix the iMac. Watch the video below to see what happened.

Ken’s Follow-Up iMac G3 Video featuring Steve from Mac84

Judging from the title of this article and the contents of the video, you can see that we were unsuccessful in getting the machine to power on. We certainly tried everything we could think of. There was also a lot of tests that we did and filmed that didn’t make it into the final cut of the episode. However, we know there are some smart folks out there, so if you have an idea of something to try, let us know! Leave a comment on the YouTube video if you know something we don’t.

Below is a detailed list of our observations about the iMac, followed by a list of things that we tried to get the machine to work.

Notes about the prototype iMac’s unique features and items of interest:

  • CPU: Like the retail iMac, this iMac’s G3 processor is on a removable CPU card. The card is inserted into two plastic slots. One of these slots had a bit of broken plastic. There is a chance that this missing plastic could cause the CPU card not to be sitting correctly, or for a pin to be shorting the system out. But in our tests we could not confirm if this was causing an issue.

    A handwritten sticker on the CPU card reads “IBM 233 SAM 117”. This refers to the iMac’s IBM PowerPC 750 processor (which Apple dubbed the “G3”) which is clocked at 233 MHz. “SAM 117” could simply refer to “Sample 117”, or another type of identification number.

    The heatsink clip (to hold the heatsink in place) and the CPU card metal cover is missing from this prototype, but is not essential for use. The design of the CPU card cage and the holes in it differs slightly when compared to the retail unit.
  • Memory: The iMac uses PC-66 laptop style memory and has two memory slots on the CPU card, similar to the PowerBook G3.

    There was already 1 stick of memory installed on the bottom memory slot, which is located on the bottom of the CPU card. A second memory slot was blank, but this should not affect the system’s ability to boot.

    The installed stick of memory is not visible when viewing the board from the top, as the CPU card must be removed and flipped over to see this memory module.
  • Video Memory: The add-on VRAM slot is empty, but the iMac has built in VRAM memory. The slot is used only to extend the video memory. It can run fine without this slot being filled.
  • Rubber feet: The prototype iMac G3 has opaque rubber feet on the bottom of the case, whereas the retail iMac G3 have clear rubber feet. One foot on the prototype was still glued one, while another was loose, and two others were missing.
  • Battery: The battery installed in the prototype was an aftermarket Saft branded battery, which measured at a healthy 3.6 volts when tested, meaning that someone was trying to get this to work before we did.

    After our testing, this battery was later transferred to the retail iMac, as it had a dead PRAM battery.
  • Floppy Disk Drive Header: A 20-pin floppy drive port is present on the prototype logic board. This is the same internal floppy drive header used on all beige Macintosh computers. A long floppy drive cable was attached to this header. This suggests that a floppy disk drive was connected to this system during the prototype phase.

    This header suggests that a floppy disk drive was in consideration in early iMac models. Alternatively, this header could have been installed simply for internal testing purposes. Some unconfirmed reports specify that early prototypes featured a front or rear-mounted floppy disk drive.

    An unpopulated version of this header exists on early retail revision iMac G3 computers. This makes it possible for some tech savvy tinkerers to add a natively supported floppy drive to the iMac G3. However, some later iMac ROM and firmware revisions may hinder this unsupported floppy drive.

  • J3 Header and 4-pin header : A two pin header labeled “J3” is unique to this prototype. It is unknown what the purpose of this header is, although the two points for this header also exist on the retail board, although unpopulated.

    A 4-pin header exists directly to the left of the Floppy disk drive header. This could have potentially been for audio, power, or another purpose. An unpopulated version of this header exists on retail iMacs.

    In addition, the speaker / audio header on the iMac logic board has a wider plastic header. During our investigation, we noticed the audio cable was installed incorrectly (leaving two exposed pins to the left, toward the VRAM slot). This was corrected during our testing. It is unknown if this caused any problems during the previous owner’s testing.
  • Dial-Up Modem: The internal dial-up modem differs in design from the final shipping model. This may have been an off-the-shelf part (or a reused PowerBook part) used during the development of the iMac.

    The iMac was originally announced with shipping with a 33.6 K modem. However, before launch Apple announced all iMacs would instead include a 56 K modem. At this time we are unsure of the speed of the modem module in the prototype.
  • IrDA (Infrared) Functionality: Retail iMac G3 computers (revision A and B only) feature an infrared / IR window on the front left of the unit next to the speaker. Using technology similar to a TV remote control, this IrDA functionality would allow the iMac G3 to communicate wirelessly between computers and other devices, like IrDA equipped digital cameras. This communication required uninterrupted line-of-sight between devices and a relatively short distance between them (usually 3 to 6 feet).

    This prototype has the IR window present (the black plastic piece next to the speaker). However, the IrDA hardware and cable have been removed, or was never installed on this prototype. Although all the necessary ports and space are there if one wishes to install the hardware.

    IrDA may sound antiquated – but keep in mind this was long before 802.11 Wi-Fi and Bluetooth were available in retail computers.
  • Cable Routing: The placement of the power, video, and audio cables on the case of the iMac G3 differ slightly. These cables come from the case of the iMac G3 via two holes in the case and plug into the logic board tray. The order and placement of these specific cables differ when compared to the retail version.
  • ADB port & Soft Power Switch: An Apple Desktop Bus port and a soft power switch are unique to this prototype. These pieces were removed from the final retail model. Although the placeholders and holes for these components exist on the retail model logic board, they are not populated.

    Soft power is a feature that allows Apple’s machines to power on without using an on/off toggle switch, like the one on the back of a PC power supply. This allowed Macs of the late 1980’s and onwards to power on by simply pressing a key on the keyboard.

    In addition, a series of capacitors and LEDs near these items are also unique to the prototype. These were likely there to test activity of the I/O bus.
  • Hard Drive and Optical Drive: The hard drive is dated August 12, 1998, this is a few days before the Rev A. iMac went on sale. It appears to be a standard hard drive and has no development stickers.

    However, it is well within the expected dates of these parts, as they would have needed to be made by a company, shipped to Apple, and installed in the machines long before the retail iMac was boxed up and sent to stores and customers.

    The optical drive however does have stickers to indicate it is not a final drive.

The prototype iMac G3 has a more translucent case and is missing the metal shields present in the retail model. You can also see the gray floppy disk drive cable on the side of the unit.

Things we tried:

  1. Trying Retail Board in Prototype iMac Case

    Action: Plug in Retail Logic Board (RLB) into the prototype iMac case (which contains the analog board, power supply, and CRT).

    Result: The iMac did not turn on. No signs of any life.

    Assumption: The prototype iMac case may have failed components which prevent it from powering on. Some exposed copper on the analog board suggests the power supply board may have leaking capacitors or damaged components.

  2. Trying Prototype Board in Retail iMac Case

    Action: Plug in Prototype Logic Board (PLB) into the retail iMac case (which contains the analog board, power supply, and CRT).

    During this process, the following actions were also taken:
    • Plugging in an ADB keyboard and using the soft-power switch on the keyboard
    • Using the logic board’s soft power switch
    • Shorting the pins used for the soft power switch on the logic board
    • Using the USB keyboard’s power button
    • Using the front iMac’s power button
    • Installing a new PRAM (clock) battery
    • Removing the PRAM (clock) battery
    • Shorting the mysterious J3 switch
    • Pushing the CUDA switch
    • Trying to turn the machine on without a CPU card installed.
    • Removing and reseating memory into the CPU card.
    • Using the working retail CPU card on the prototype logic board.
    • Swapped the prototype and retail Power Supply Filter board (which sits below the logic board).
    • Attempted to startup the logic board with all devices disconnected (hard drive, optical drive, video, USB keyboard, speakers, etc.)

Result: The iMac did not turn on. No signs of any life.

Assumption: There is an issue with the PLB which is preventing it from initiating and turning on. This could be an electrical short, or a damaged component.

  1. Performing Voltage Tests and Multimeter Probing

Action: Testing the voltage on the prototype logic board with a multimeter while the computer was plugged into power (per the Apple Service Manual’s instructions).

Result: The iMac appeared to show 5 volts, which is the “known good” amount per Apple’s Service Manual. This indicates that the logic board is receiving power from the iMac’s power supply.

In additional tests a potential short was discovered between the ground and +5 volt rail of the iMac prototype logic board. Two different multimeters pointed out odd readings from these and other areas, suggesting that a short may be present on the board. This could potentially prevent the board from working.

This short could be caused by a failed component or a damaged area of the board. Additional testing will need to confirm the cause of this short.

Assumption: The prototype board is getting power from the iMac’s power supply, and through the power filter board. A short may be present which may be preventing the computer to function.

  1. Sanity Test with Retail iMac components

    Action: Plug in Retail Logic Board (RLB) into the retail iMac case (which contains the analog board, power supply, and CRT).

    Result: The retail iMac turned on and functioned perfectly.

    Assumption: The retail iMac logic board and the retail iMac case (and analog board, power supply, and CRT) are all working perfectly. The iMac booted up and read the hard drive and optical drive.

Article by Steven

Owner of Mac84.