iMac G4 troubles

[AL1630]

I scored a non functional iMac G4 1.25GHz on Craigslist a while ago and I've been trying to make it boot. At first it did absolutely nothing when the power button was pressed. So, here's what I've done with it.
1. Replace 160watt PSU with a known good one
Ok, now the fan and hard drive spin but it won't start.
2. Replace logic board
Back to no fan spin.
3. Replace 160w PSU with a 180w - Maybe the new board requires a different one.
The fan spins again, but nothing happens.

I've also tried to plug it into a monitor, but no luck. I'm almost ready to just give up on it. Is there anything else that could be wrong with it?

 

[Project Alice]

If you replaced the logic board and it still doesn't work:
Is it the same type of logicboard?
What about the PRAM battery? Is it the same one? If so does it work?
RAM could be another factor here. Try it with some known good RAM that is compatible with an iMac G4. My iMac G4 1Ghz has proven just as picky as a G5 with ram. Out of all the DDR sticks I have, that work in other macs will make my G4 either not POST, or just flat out ignore the stick. Only 2 sticks out of a pile of like 10 sticks worked in my G4.
Is it the same drives it came with? Problems with IDE drives will make a computer refuse to boot. Try it with a known good IDE drive, as well a known good IDE cable. Or even just disconnect the drives all together and see if you get it to POST.

I'm unsure about the PSU differences, but that could definitively be the problem as well. I would find one that matches the original. Maybe someone else who has more experience in dead iMac G4s can tell you.

 

[AL1630]

Mine has an extra stick of RAM installed in the bottom. Is the internal stick required for the computer to POST?

I've hooked the hard drive up to a Power Mac and it booted from it, so I'm pretty sure it's working right.

 

[Project Alice]

Mine has an extra stick of RAM installed in the bottom. Is the internal stick required for the computer to POST?

I've hooked the hard drive up to a Power Mac and it booted from it, so I'm pretty sure it's working right.

As far as I know it doesn't matter which ram stick is installed as long as it has ram in it. The other one just happens to be a laptop style sodimm module instead.

 

[CooperBox]

Good advise here from PAlice.
This is how I would have initially gone about troubleshooting your iMac.
i) Remove the base access panel (Phillips #0 screws), check for presence of Ram module and note the details (for your model you should have PC-2700 200-pin SO-DIMM). Remove, and re-install ensuring correct engagement.

ii) Remove the bottom plate housing (4 screws Torx 15) exercising care when lowering the housing not to strain/damage the wires still attached to the logic board. Remove the inner RAM module and note the details (for your model you should have PC-2700 184-pin SDRAM). Re-install ensuring correct engagement.

iii) Whilst the base is still open, measure the voltage of the PRAM battery. If less than 3.6v replace with new battery.
Re-assemble the bottom housing, ensuring that no internal wires are trapped/being pinched whilst progressively tightening the 4 screws in order of #1,3,2,4.

iv) Before re-installing the base panel, and without the power cord connected, perform a PMU reset - next to the memory slot, there is a tiny rectangular button under a protective plastic covering. Press this just once!

After ensuring that serviceable and correct type RAM modules were installed, and new PRAM battery installed, if the iMac still failed to boot correctly, I would then change the hard drive and the IDA cable.

The logic boards are not interchangeable between all G4 iMac models, and I suspect the same applies to the power supplies for the 15", 17" and 20" models.

When you ask, "Is the internal stick required for the computer to POST", are you referring to the Apple so-called 'factory-installed' internal stick? If so I believe the answer must be 'yes'.

 

[MBAir2010]

where did you get the logic board? my G4 igloo needs a video card/logic board but these things are so obsolete the project is on the back burner, and Craigslist for 40 bucks. CL where i live means if the item is not free, it's not selling!

 

[AL1630]

I checked the ram and PRAM battery, and the battery tested ok, I put in a battery from another mac to double check as well. I haven't tried a PMU reset yet though.

 

[AL1630]

Update - I reset the PMU, reseated the RAM, and replaced the PRAM battery with one that measured 3.6v (The one in there measured 0.3v), but still no luck. I guess next I'll pull a drive out of a Power Mac and see if anything changes.

Edit: I tried putting in a known good hard drive and cable from my Power Mac, and I can't even feel the drives spin up. Is that likely to be a power supply issue or bad power cable?

 

[Project Alice]

Update - I reset the PMU, reseated the RAM, and replaced the PRAM battery with one that measured 3.6v (The one in there measured 0.3v), but still no luck. I guess next I'll pull a drive out of a Power Mac and see if anything changes.

Edit: I tried putting in a known good hard drive and cable from my Power Mac, and I can't even feel the drives spin up. Is that likely to be a power supply issue or bad power cable?

Is this the PSU that was the wrong wattage? I'd say that's probably what the problem is. If it was just the power cable to the drive, the mac would still power on and not be aware of the unpowered drive.

 

[AL1630]

After testing the first time, I swapped in the 160w PSU, and it still didn't work. The fan spins, but the screen is black.