The Ugly Details

There remain a few mods to make this puppy work.

1. Power On

The Mac puts out a high "power on" signal to the power supply, but PCs put out an active low. This means our Dell PSU is expecting a low, when we have a high. In other words, we have to invert the signal. The prototype was a BC108 (general purpose NPN) driven by pin 12 of the video connector through a 2.2K  resistor. 

If it works with one silicon junction above ground (0.6V) then it's probably expecting a TTL low. That means you can also drive it with a gate if you can find room. 

Close-up of mod showing the transistor glued on top of a tant and soldered to the digital groundplane. A surface mount 2K2 drives the base. The tranny is a ZTX450 but any small signal NPN such as a 2N3904 will do. The grey wire goes off to the power supply. Yellow Kynar wire goes off to pin 12 of the video connector.

If you don't want to do this you can always put a mechanical switch from the power supply ON signal down to ground, but it's groovier to use the Mac keyboard to turn on and off. Here's the schematic. 

2. Trickle

The Mac can't tell the power supply to switch on unless it has a little tiny bit of power driving the standby circuitry. The purple wire is the power supply standby 5V. This has to go on pin 3 of that header next to the lith. There wasn't a pin header in mine, but then I couldn't find a female so just soldered the wire anyway...

3. Test

The whole rig has to be tested before plugging in to the motherboard. Ground the power supply wire to turn it on and carefully check the voltage on every single pin.

Check the standby wire for 5V trickle and connect that to the motherboard.

4. Memory

Whatever your iMac came with, it probably isn't enough for OSX. Each slot of the later iMacs takes 512Meg, which is a good minimum for OSX, anything above that is a welcome bonus. The earlier machines take 256M per slot of PC100 (total 512M), later 512M per slot of PC100 or PC133 for a total of 1Gig. For now, I stuck in 2 x 256M PC133s.

5. Hard Drive

Again, whatever your iMac came with, it probably isn't enough for OSX. Some of these machines had 6Gig drives, which won't even take OSX, so an upgrade here is a requirement. This puppy now has a 40Gig drive.

6. CPU Clock

This board came with 400MHz, and overclocked reliably to 500, which will do for now. 

iMac clock jumpers coming soon!

7. LED

The floppy connector was a convenient place to pick up 5V and drive an LED on the front panel. This LED is a high output Aqua (505nm) and even with a 1K0 ballast is plenty bright enough.

8. Mac Badge

The finishing touch was an old stripy Mac logo, those were the days...

Pic coming soon!