I had this problem a long time ago with my imac g3 333Mhz, it would chime, and try to power up, and in a few seconds, shut off, in the end, i needed to replace the board that drives the CRT, try running the computer with the internal display disconnected, i believe these iMacs have a VGA out port on the back as well. This board is hard to get to, but after a little disassembling, i got it, if you decide to try this, take a picture of all of the connections, since it was really hard to remember where every thing was connected.
"Sounds like a power supply problem. I see you've tried a DVD and external drive with no luck. But it may be worth pursuing it a little further...
First, have you zapped the PRAM (boot while holding down command, option, P, and R; keep holding them down for 3 chimes)? Can you boot into Single User Mode without it shutting down? Hold down command and S while you boot. You could also try booting into FireWire Target Mode (hold down the T key while booting) just to see if it stays on; Target Mode bypasses all the operating system stuff, so it's a good test of something like the power supply.
If it shuts off in SUM or Target Mode, then it's likely hardware. Especially since it followed a power outage, which could have affected the power supply.
If it does run in SUM/Target Mode, then maybe you should try resetting the NVRAM: boot into Open Firmware by holding down command, option, O (that's an oh, not a zero), and F while booting; you'll get a white screen with black text. Type ""reset-nvram"" and Return. Type ""reset-all"" and Return, whereupon the computer will reboot itself.
If you do get it going again, make sure you have the most recent firmware. If you reply back, we can tell you how to check it.
I've had some G3 iMacs do this in the past, but I can't remember the resolution; I think we just got rid of the computers, as they were pretty old by then and not worth fixing."
and 2. Zap the PRAM(Restart, while screen is blank, hold down option, command, p and r until two start up chimes).
*try 3. Put a system install disk in the slot loading cd-player as you hold down the option key, it might show a. icon of your HD and b. the system install disk. If it does click on the sys. install disk, and find the disk utilities to try to repair permissions etc.
*try 4. Safe start, hold down the shift key when restarting. This turns off all extensions.
*asterisks means not sure if you INSTALLED SYSTEM has these options.
*Disk warrior and/Tech Tools are good disks to try to start-up on."
Quite often the Flyback Transformer shorts out. This will cause the problem. The answers above should all be tried first. I finally disconnected the video and used an external monitor.
"First of all I'll explain all the actions that I perform on the iMac 6,1 that does the same as starting up, the hard drive stops making noises and shut down after the loading circle disappears.
1. I try to install Leopard, Snow Leopard, Mountain Lion, Maverick, and Yosemite (several versions of each one). All the installation process failed, it always turns off, sometimes before the installation process or after when it reboots (I used DVD and USB).
2. I install a hacked version of Snow Leopard, it boots but it fails when you want to shut it down, it enters a weird state of sleep mode.
3. I check the HDD with the Disk Utility in the hacked OS. The disk is fine, no error found.
4. I upgraded Snow Leopard from the installed OS, everything worked fine, until reboot. The iMac turns off again like before.
5. I reinstall the hacked OS into a USB hard drive, it boots like the internal HDD, and the weird sleep mode...
6. I downloaded the AppleHardwareTest, I can't make boot at startup, and I don't know how to boot from the OS.
Any idea about a hardware test once the OS is loaded (Snow Leopard OS).
Their is one other thing that you can try to do, if you put the imac on a table, screen facing down on a towel or something, you should open the bay where the memory goes, on the bottom left side, look inside the imac with a flashlight, you might see a lila/red colored button, thats the PMU reset button, press it when the unit is OF the power, another thing that crosses my mind and what you can try is resetting the SMC of your imac. Unplug the power, keyboard and mous, press the power button for about 5 seconds, then plug everything back in and see if the imac will boot, if it doesnt, its more likely to have a power supply/analog board faillure due to age. Good luck with your lovely imac :)
