iMac G4 700mhz "iLamp" won't boot, no chime, no display, fan works

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Many things may account for your present issue, but it is likely that you failed to reconnect the display cable to the logic board, or—and this is a fairly common problem when inexperienced personnel open and then reassemble a machine the iMac flat panel product family—you may have pulled the video socket to which the cable harness attaches to the logic board partially or completely off the board.

These machines also require the replacement of thermal pads and the application of thermal paste to the assembly, and the precise tightening of the bottom assembly to the case with 17 inch pounds of torque. Too much, and you will sheer the bolts which hold the assembly together. Too little, and the cooling mechanism will not draw heat off properly, leading to resets, restarts and to damage of the processor.

If the thermal pads were present and you did not remove them, and if you left the old thermal paste in place and did not clean it off and fail to put more on, then those factors alone would not keep the machine from booting up. Nor would the improper tightening of the assembly, which is an integral part of the cooling system, prevent it from starting up.

If you hear fans and your hard disk spinning, can operate the optical drive by ejecting it, but have no video, it must involve the display harness or damage to the board. Either something has not been properly reconnected, or electrostatic or physical damage has caused the logic board to fail. An interconnect issue seems more likely.

The thermal pad kit 076-0925 is used if these must be replaced, but your machine is old enough to have achieved vintage status, and replacements may be difficult to find, as would the 661-2589 replacement logic board itself, which ships from Apple with both a thermal pad kit and a tube of thermal paste.

Thermal pads are located on the bottom of the assembly against which the logic board is mounted. If you haven't removed the logic board from the base, then they are present and need not be replaced. If you have removed the board, you may find that they adhered to the board itself, rather than the case bottom. Thermal paste belongs at the end of the thermal pipe through which one of the assembly bolts passes. If you are replacing it, you should first carefully scrap off the old, hardened paste with a 'black stick' or similar tool, from both mating surfaces. You should not use liquid cleaner, abrasives or a sharp tool which might slip and damage adjacent portions of the logic board.

Although these assemblies are large and 'clunkly' by contrast with the electronic gear you've worked on, their design—including the integrated heat dissipation system, the short cable assemblies and tight space inside the base—make them a challenge to repair without tools, instructions and experience specifically with them.

You may have simply crashed the PMU, and need to reset it. On the underside of the base—accessible when you remove the cover and located near the lower edge of the AirPort card connector—is a PMU switch. With the unit not plugged in, push that button once. Then plug in the AC cord and press the startup button.

If you disassemble the unit, you need to determine if the 3.6 volt lithium battery is still good. Hint: after this long in service, it likely is not, and should be replaced. Be sure that the power cable which feeds the hard disk and optical drive are properly plugged in. In simplistic terms, that may explain why they do not but the fan does.

I have the exact same issue with my G4 iMac. last night, I replaced the stock hard drive with a 250GB Western Digital IDE hard drive.

When I first put the machine back together and started it - with the Leopard install disk in the optical drive - it booted nicely, recognized the new hard drive. I partitioned it, and started the installation process. About 20 minutes into the installation, the screen froze. Mouse won't move, keyboard would not respond - caps lock led would not work.

I had to hard restart the machine. Since then, when I press the power button, all I get is a blank screen with the DVD spinning.

If the mouse is connected to one of the rear USB ports, its red light works. If I connect it to the ports on the keyboard, it does not.

Looks like I did miss the step of re-applying the thermal paste when I put the machine back together. I wonder what the extent of damage to my machine is. Is it recoverable by opening up the machine and applying thermal paste.

I am also wondering if the power supply is capable of driving the new hard drive. Its current ratings for 12V supply are slightly greater than the stock drive (Seagate).

Thanks for the pointers so far. Eagerly waiting for more input.

Have you resolved this issue? I ALSO replaced my hard drive last night, with a bigger HD I already had installed leopard on from another machine. Started right up and worked awesome. Used it about 10 minutes, shut the computer down to see if it would start up any faster the second time. Started up and gave me a "no bootable folder" blinking error. Shut down, started up again and it is exactly like yours. Blank screen, cooling fan powers on but not the HD, no sounds... I need help. I was just trying to make my imac better, not kill it.

I got my iMac checked out at the local Mac Store. They did a diagnostics and found that the logic board is faulty. It will take $430 or so (parts + labor) to get it up and running.

At this point, I am not willing to put that kind of money in an old computer. I am looking to see if I can get hold of a working G4 iMac logic board for cheap. If I can, I'll try to fix it, otherwise, it is going back to its box.

I am wondering if I could use one of those tiny Nvidia ION based motherboards to do some mod and use the rest of the parts.