I have an early revison iMac. I know it is Bondi colored. I think it might be revision B. I just have the motherboard (logic board?) with the CPU and RAM and VRAM on it. The motherboard then goes to the A/V board which deals with the monitor and audio connections. My friend modified an ATX power supply and the Mac turns on, but there is no boot chime. The LED on the motherboard near the CUDA button turns on. The hard drive spins up, the CD-ROM turns on, but the monitor connected to the 'Video port' (that says NO EXTERNAL MONIOR) acts like there is nothing connected. The Mac seems to want to start up, but there is no video on screen or a boot chime. I'm using headphones since the origial iMac speakers are not with me at this time. There are two sticks of RAM in the iMac, removing one or the other doesn't make a difference. There is also a stick of VRAM (I think) when I take that out the Mac doesn't try to bootup at all. I do NOT have a Keyboard or Mouse plugged in. It's odd but maybe this is causing the problem. The hard drive I used is a IDE 6gb, which I formated in DiscUtility as Mac OS Exteneded (Journaled) from which I installed Mac OS 9.2.1 on. Does this configuration sound ok?
I have found online that other people have successfuly done this, I just wonder what is wrong with mine. My guess is the video memoery...any thoughts?
Thanks in advance! 
The one ram chip off on its own is video memory, its a 4MB chip. I did this swap last year, mine works fine. ( www.mlode.com/~stuka/imac-atx )
As for why yours wont boot, are you sure the PSU is wired up right? And, are you using the original power on button, or did you make one yourself? What kind of monitor is it, and what kind of adapter are you using to connect it to the apple style connector? Not having a monitor hooked up wont stop it from booting. Also, have you tried hitting the cuda switch? They can be fussy at times, mine has done this a few times, hit the cuda switch and all is well.
I'm pretty sure the power supply is setup correctly since the machine does turn on. I'm not using any power button for the mac, as soon as I plug it in the power supply turns on, which makes the Mac turn on. I have tried 3 different monitors. An old Apple one with the original apple style connector, a VGA monitor with an adapter, and a VGA monitor with an adapter with dip switches. I have tried pressing the Cuda switch and still nothing happens. I can try again but still for some reason I hear no startup chime, which usually is the 'everything is ok!' sound. Thanks anyway
He has most likely wired up one of the PSU wires incorrectly. I remember when making my adapter, I missed out one of the sense I thought it did not need and it would start up but not chime.
What parts of the original iMac are you using?, afaik you won’t hear a chime from the audio out socket on the logic board unless you either have the headphone board connected or you connect pins 8 & 12 of the audio connector (J18) to ground (these are the headphone sense lines which disable the audio out and builtin speakers whenever headphones are connected). When I built mine, the only parts I had were the Logic board, CPU card and RAM so I had no option but to connect the monitor directly to the video connector. All the pinouts I have are on my website http://www.so-weird.co.uk
Thanks a lot! Maybe the Power Supply might be a bit hooked up wrong
I'll look at your page and see, thanks a lot!
Well I don't know exactly what do do, let me show you a diagram of how I have it hooked up and tell you what I have plugged into were.
*iMac Audio (ribbin like cable) plugged into A/V bridge
*iMac video cable plugged (ribbin like cable) into A/V bridge
*Apple MultiSync 1705 Monitor plugged into A/V bridge
*USB Apple Keyboard plugged into USB (1) Port
*Speakers plugged into 'Sound Out port'
*ATX like cord plugged into iMac
*Power cord plugged into PC ATX Power Supply
*Hard Disk Drive with OS 9 - Plugged into IDE and iMac's HD Power cable
*NO CD-ROM connected
*RAM plugged in in both slots
*Video Memory (VRAM) plugged in
Here is a diagram (a very messy but efective one) made in Photoshop. This might help you guys see what is hooked up wrong.
Diagram:
http://homepage.mac.com/ibook238/imac-power.jpg
Thanks a lot for all of your help, I hope to get this working eventually :?
Do you get a chime if you try to boot with the monitor not connected the iMac?
Oh, and you have to have the sound board (power on board - the thing with 2 headphone jacks and a powerbutton) connected to the iMac motherboard to get any sound from the iMac, even if you use the speaker jack on the mobo.
Ok thanks, I have to dig that out from the box of parts I have. Thanks!
With the original speakers in and the little sound board connected, I hear a startup chime! Yay! Still no video on the screen though. The Mac makes the startup chime if the monitor is plugged in or not. The hard drive spins up too.
I noticed on the PC's Power Supply it says "DC Output 120w" Is that enough?
Not enough power for video? Also an Apple USB keyboard or mouse DO NOT get any power. 
(Even when a powered USB hub). Maybe the USB is dead?
Or maybe 120w is not enough for this iMac. If that's the case I have another power supply I can rip apart... :o
Sorry for all of these posts, Safari is being very picky, it doesn't think I'm logged in or not, anyways....I guess I need more power
A 120W should be fine with a single CDROM and a single HD. The monitor does not matter as far as power consumption goes. I still think you have something with the PSU wired wrong.
Me too. That brown wire has to go somewhere...
Are you sure that monitor works with modern macs? The more modern macs miss out sense lines that older apple monitors expect.
Secondly, brown does not have to be connected if I remember correctly..
Thirdly, like said before 120w is fine.
Finally, have you wired up the video correctly. A short cable should go to the motherboard to a video board...
The video cable goes from the motherboard to the A/V bridge, the Mac monitor then plugs into that. I have tried 3 screens. One old Apple 14'', One Gateway 15" with multiple VGA adapters, and the Apple 1705 (VGA) with converter, all screens work fine. I'll post pictures in a bit.
I've seen Macs chime but no picture when the PRAM battery is flat. When I've had that in the past, pressing the reset button after the first chime boots up fine with a picture. I've also had similar problems using VGA adaptors with beige G3s whereby the dip switches need to be set differently
So thats why I had to always press the reset after first boot up. I don 't think its because of the pram though, as mine was not flat and it would boot up first time with the iMac monitor.
I think it was something to do with the deguass coil in the monitor as when the monitor was prewarmed it would boot up first time...