Conversion of an iMac Rev. B into an ATX case

If you intend to try all or part fo the modifications described in this article, you do it at your own risk. There is a serious danger of injuries, electrical shocks and a risk of losing your warranty. I decline any responsability.

Published under the

GNU Free Documentation License

Story

It started as it did with lots of iMacs: During startup, as soon as it gave a signal to the monitor, the iMac powered off. I did some research in the internet, it must be a problem with the analog board, more precisely with the flyback transformer. I connected an external monitor to the iMac, which works without any problems. But beacause I didn't like to have the broken iMac wasting space under my desk, I decided to take it apart. I like the design of the old iMac, but form follows function, the iMac screen was useless, so why have it around any more? I didn't want to spend 100 Euro or so for a new analog board or a flyback transformer, not said that I didn't know which part to order, so I went to the second hand and bought an old ATX PC for 20 Euro.

I could find some information about conversions of Macs into ATX cases at xlr8yourmac. There is a report of converting iMacs into rack mount cases, but the guy wants 40 US$ for the whole documentation, and all I was interested in is the conversion of the iMac power supply to an ATX power supply. So I tried it myself.

Thanks to Tom, he mailed me a link to a french site where a similar conversion is described: "FYI... Looks like this fellow figured out the switch-on/off issue."

Result

It all ended in an ugly PC case. The iMac is booting, but if I plug in the AC power, it immediately starts the boot process, i.e. you hear the startup chime. So it is not possible to leave it plugged in. As you can imagine, the sutdown stops with the frozen "rotating sun" and I have to switch off the box manually. Well anyway, it is working and not enough, it saves power. A plugged iMac needs about 15 Watts! At the time the mainboard is laying around somewhere in the case...
The only part I fried (but it is still working) is the power filter board (the small thing where you connect the power coming from the analog board). I use the ATX connectors for the harddrive and CD-RW, so maybe the HD power from the mainboard is dead...

Prerequisites

You should be prepared to fry your mainboard in case it is not working, and some soldering practice is required.
You will need the following parts:

You won't need any more:

The translucent case is also optional, I intend to make a lamp out of it. Some guys made fish tanks of Mac plus, this might be an option ;-)

Procedure

First I downloaded the specs for ATX cases and power supplies and the iMac Service manual, it is helpful for taking apart the iMac, but as I did this before it didn't helpme a lot. Then I made some multimeter measurements, and in the end I just played "yes or no"... Follow these instructions:

Power cable

You can find the layout of the ATX plug below. It is taken from the above mentioned specs. The "signal" of the iMac plug are taken from the power supply board.

Pin Signal Color Pin Signal Color
1 +3.3VDC Orange

11
[11]

+3.3VDC
[+3.3 V default sense]		 Orange
[Brown]
2 +3.3VDC Orange 12 -12VDC Blue
3 COM Black 13 COM Black
4 +5VDC Red 14 PS_ON# Green
5 COM Black 15 COM Black
6 +5VDC Red 16 COM Black
7 COM Black 17 COM Black
8 PWR_OK Gray 18 -5VDC White
9 +5VSB Purple 19 +5VDC Red
10 +12VDC Yellow 20 +5VDC Red

iMac Power Connector

Pin # signal color   Pin # signal color
1 GND black   2 GND black
3 GND black   4 GND black
5 GND black   6 GND black
7 GND black   8 3.3VR green
9 5VR gray   10 5V red
11 5V red   12 5V red
13 5V red   14 5VS white
15 12V yellow   16 12V yellow
17 -12V dark blue   18 3.3V orange
19 3.3V orange   20 3.3V orange
21 3.3V orange   22 3.3VS light blue
23 PFW brown   24 T5V purple

These are the connections I made. I am not sure about all the GND,3.3V, 5V and 12V connections, I just soldered like colors, where a wire was missing I soldered two to one.

ATX iMac   ATX iMac
pin # pin #   pin # pin#
1 18+19   11  
2 (two cables) 20+21   12 17
3 (two cables) 1   13 2
4 10   14 8
5 3   15 5
6 11   16 6
7 4   17 7
8     18  
9 24   19 12
10 15+16   20 13

Now you end up with some cables not connected (iMac pins # 9, 14, 22, 23/ ATX pins # 8, 11, 18). Don't ask me where to put them, I just left them dead-ended.

Hard drive and CD

Connect the hard drive with the IDE cable to the mainboard. You can take power directly from the ATX supply.

Connect the new CD-ROM using the 2.5->3.5" converter cable according to the following photos:

Watch out that the red wire is on the right side looking from the front to the former back of your iMac. Two pin rows have to be left unconnected on this side. On the left side one row remains unconnected.

I had to snip off the power part because I didn't use it. Don't know if it is working anyway. I recommend using the power from the ATX supply.

There you are: Connect the other cables as usual and put the whole stuff in the case somehow (I had no time to screw it in nicely so far) and power up!

CPU temperature and cooling

A good ariflow for cooling should be guaranteed when making modifications to the original set. So I did some measurements, as I intend to overclock the CPU some time.
I remember the CPU temperature in the old iMac was about 60°C. I wondered what it was now and downloaded CacheControl X. With extensive Photoshop filtering, it rises to almos 80°! Then I took the small CPU fan of the old PC and mounted it on top of the G3. The temperature went down to 40 - 44 °C.

 

tinux_at_ mail.com / last update: January 4, 2003 / hits since update: