PowerHack G4 - An insane iMac G3 logic board hack

[Daniël]

So recently I've been parting out two iMac G3s, a trayloader 333MHz Tangerine, and a slotloader Indigo 400MHz. The Indigo's colored shell has been transplanted to a Graphite, since I own two functional Graphites next to the dead Indigo, and I'd rather have two different colored ones that are functional than two of the same color, and the Graphite I sacrificed had some bad yellowing anyway. 

Now, to get more to the point of the thread, the reason I'm parting the Tangerine out is because it has CRT issues, and smells like it has been left in the woods for a decade or so, incredibly musty. I've decided that I'll part it out, rinse the plastic parts, and only keep the tube in it, so it can be repurposed as a decoration item, or maybe a table. 

But that left behind a functional logic board. And I got to think about what I could do with that. It then hit me: I should make it its own system in a custom case, but I also want to mod it so it isn't just another "iMac G3 in a box". So allow me to present my idea for a custom PowerPC Mac, which I am calling...

The PowerHack G4!​

Now, I'll explain the ideas I have for this Mac, and why it will be different from the normal "iMac in a box" modifications.

• The case. Now, this might seem like a fairly mundane part of the mod, but I do plan to make it a bit more unique. I'm going to look for an unbranded desktop style case, in either white or beige, to put it in, and I plan to add decals. Next to the Rainbow Apple logo and the PowerPC logo, I'd add "PowerHack G4" in the Apple Garamond font, to match the typical beige Mac of the era. And while the inside of the case will be a bit of a mess, I plan to keep the outside looking as clean and Apple-like as possible. I also want to see if I can mount two internal speakers, and power those with the board's original audio circuitry.
• The PSU. Of course, I need to modify an ATX supply to work with the iMac, including soft power on. That's been documented, so it should be easy to replicate that in my mod. But it's still an important part, without power we don't have a functioning computer  
• The CPU. My Tangerine has a IBM PPC750L G3 processor, clocked at 333MHz, which is perfect, since it runs at the right VCore voltage to replace it with a Motorola PPC7410 G4 processor, so it's upgraded to a G4 processor. I plan to jerryrig a bigger cooler with a fan on the chip, and try to clock it up to 533MHz, the highest it will go as far as I am aware. I also want to see if the L2 cache can be upgraded, and whether the bus speeds could be overclocked as well, to get the most out of it.
• ADB support. While the iMac G3 killed the ADB port on personal Macs, the solder pads for a connector are still there. I'm not 100% sure if it would work, but it's worth a shot. Plus, the monitor I will be using with it, the Multiscan 17", has an ADB port. The reason I am using this monitor is because it has the same standard Apple video connector of that time period, which the trayloaders actually use internally. I have confirmed that it works with the iMac (it doesn't ID the monitor though). I got it for free a while back, which is very nice.
• Floppy support. Just like with the ADB port, the pads for a header are still there. With this one, it's even more tricky. It might not work at all, and if it does, it might only work with 8.1 which can't be booted OOB on an iMac G3, which could proof to be a challenge. But hey, might as well try, right?
• IDE devices. Now, if I put the iMac G3 in an ATX case, I can't, of course, use the iMac's original optical disc drive. Fortunately, if you disconnect the ODD, and attach a normal IDE cable to the HDD header on the iMac G3 logic board, you can use two regular IDE devices without a problem. I tested this with the Quantum Fireball it came with, and a ZIP drive, and those both worked fine. So I could either go for an HDD and regular IDE 5.25" ODD, or just a ZIP drive instead. 
• More USB ports. Simply put, two ports don't cut it. I have some old USB 2.0 hubs which I could sacrifice for this project. All I'd do is internally mount them by soldering the two hubs to the USB pins of the logic board, and soldering USB headers for the front and rear of the case to the USB pins of the hubs' ports. The power required for the hubs can be pulled from the ATX power supply. 
• And of course, routing the other ports to the back of the case. Again, should be quite straightforward. I am removing the modem, since it's fairly pointless to keep it.

Those are the ideas that I'm definitely implementing into this unique PowerPC Mac I'll build, but there are some more out there ideas that could be implemented if I had exceptional soldering skills, the iMac schematics, and was even more insane about this mod  

• Replacing the ATI Rage chip with a GeForce 6200 PCI card. This chip is basically just a condensed PCI device, if I had the skills and knowledge, I'm sure one could tap a full PCI slot from it, and via that attach a full size GPU like the GeForce 6200 PCI card. But that's way out of my league.
• Adding a PCI slot via the Mezzanine slot. Again, same story as with the Rage chip, the Mezzanine slot is just a weird format of PCI, and could potentially be converted to a PCI slot with the skills and knowledge. Perhaps even with some PCI bridge chips, multiple slots could be achieved. But I'll leave that for someone else to figure out and do
  
• Upgrading the internal VRAM. While I'm not sure if the iMac can address more than 6MB of VRAM, if you would upgrade the internal 2x 1MB VRAM chips with 2x 2MB VRAM chips, like found on the VRAM sticks, you could theoretically get 8MB of VRAM. 
• Changing the startup chime. There's a modification for the BW PowerMac G3 to change the chime, and I'd like to see if that potentially could be achieved with the iMac G3. I'd love to have the Quadra 700-style chime on the PowerHack G4.

So those are my ideas for this rather crazy modification. There's no point to it other than me having some fun with some leftover Mac parts, and making a neat proof of concept. If you have any ideas for what I could add, let me know!

 

[max1zzz]

 have some experience int his field What I can tell you is this:

A floppy drive will work fine in your imac, you just have to solder the 20 pin connector on to the board, however there is no driver for it in the iMac's firmware. The driver dose exist in the Mac OS Rom file providing the file is version 1.1.2 or lower (1.1.5 might work, I remember trying it but can't remember if it worked) This means you will be limited to Mac OS 8.6 as Mac OS Rom 1.1.2 won't work with anything newer.

As far as I know the ADB port works fine, But I have never tried this personally

I'm most interested int he Mezzanine slot PCI mod, it should be possible but who knows what the pinnout of that connector is (But you have got me intrested enough I am going to do some digging and see If i can find a schematic for the trayloading iMac logicboard...)

Also, have you thought of adding a serial port? The irda port on the iMAc is just a normal serial port iirc

 

[CC_333]

And the now-vacant modem slot can potentially be converted to a second serial port, which would be super handy if you were to use something that required two ports.c

c

 

[Trash80toHP_Mini]

I'll be watching with great interest. iMacs aren't my thing, but it should be a lot of fun to see how it progresses. If you arrange the funky collection of hardware artfully, a clear case side and neon to show it off will be most appropriate, especially if your design aesthetic is "fugly as possible."

 

[HeatedCane]

So how is your project going in very curious!?