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Power Mac G4 not starting up

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Panzerfaust101

macrumors newbie
Original poster
Dec 17, 2011
21
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I've got a Power Mac G4 that a friend gave to me and I really want to get it up and working again. When I connected the cable to the power supply and pressed the power button, the power button flashed, but nothing in the computer started. The power supply and the button are all connected to their correct ports on the motherboard and I've tested the power supply with results of a 5.0V, 5.8V and a 25.8V on 3 of the connections (I've worked with a couple of PC towers before so I know how the innards of a tower works). Any help on getting it started? Thanks
 
If it's an MDD you can likely resolve this by removing the logic board battery for 10-15 min and then putting it back and resetting the PMU.
 
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@skinniezinho: It's either a MDD or a FX 800 (what's the difference and how can you tell?), but it is the model with the dual 1.42GHz CPUs and the model says m8570 (which I think applies for both models). Not sure if I can reset the PRAM because nothing happens when I press the power button (except the power button lights up)
@zen.state: Trying it now, although my G4 might be a FX 800. If that's the case, what do I do?
 
http://www.outofspec.com/frankenmac/wire.shtml - see Mirrored Drive Door.

Diagram is flipped but wires colors are matching. Test voltages to see if PSU is OK. Judging by symptoms it isn't. OFC PRAM reset is impossible at this moment - computer must start first.
Reseat graphics card and CPU if you see any signs of messing with them by previous owner.

FW800 or 400 the're the same models technically.
 
There is no FX 800 only FW800 which stands for Firewire 800 equipped powermac G4, and they came in two variants as far as I can remember, dual 1.25GHz and dual 1.42GHz. Bus speed is 167MHz and uses PC2700 DDR ram. max ram should be 2GB.

First try what zen has recommended.
 
Tried taking out the battery and putting it back in then PMU reset with still no response. Tested the battery and it's still working with a p.d. of 3.6V. Is it a logic board problem if both the battery and power supply work then or is there some other method to fix it?
 
There should be an LED that turns on, on the logic board. It can be seen near the RAM slots, I can not pin point the exact location since I am out in the field. But if the logic board receives power from the power supply it should turn on. Are the fans turning on?
 
@Nameci: Can't seem to find the LED, but the light in the power button does turn on, so the logic board should be receiving some power and no fans are turning on.
@Thundersnake: All the connections of the PSU were completely off from the diagram (e.g. one was -5V when it should have been positive and one was -25V when it should have been 5V). The power supply in the Mac seems to be an original part (Made by Samsung, S/N: RR3123A2P0WA and Apple P/N: 614-0224) so I don't know if something's happened to the ports that's screwed the connections up (is it possible?) because the Apple P/N matches with the PSUs that were put in MDDs. Would this actually just provide enough power to let the light in the power button flash but provide not enough for the computer to start or should the light have not started in the first place? Should I try to source another PSU from the internet or is there a method to deal with this?

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Does anyone know about a PCU which would work with my G4 that's easier to find? Doesn't have to fit inside the case, but has the same connections and voltage so I can see if that's the only issue (and possibly use if I can run it outside of the case)
 
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Are you certain that the orientation of your testing corresponds to that of the diagram? Note that the diagram indicates that the clip is on the right. No, I really don't think it's possible for things to be off in the way that you suggest.

Yes, there are cheaper options. It's possible to modify a new ATX supply if it turns out that yours is bad, but I don't think we've determined that yet.
 
@Thundersnake: Positive that the orientation was correct. Doubled checked by doing the tests the other way (clip on the left) with different but still inaccurate results. I might go around some Apple stores and fixing places to see if they have a similar PCU and see if it works with another unit. In case it does, where can I get a replacement (preferably cheaper than getting an original Apple part)? Any other tests or advice?
EDIT: When I tested the PCU, the unit was not on (no fans were running). Do I have to turn it on to get a more accurate result or does it make no difference? If so, how do you turn it on?
 
Pull the connection from the motherboard and test again. If you're still getting 25v on what should be 5v, then something really weird is wrong with your PSU. If it reads normally this time, then electricity is flowing somewhere on the motherboard that it shouldn't. Even in the first scenario, the motherboard has likely been damaged.

One more time, though, just for the sake of clarity for future readers: The diagram in my link depicts the connector as you're looking down on it while it's attached to the motherboard with the retention clip on the right hand side, and the only purple wire positioned in the bottom left corner. Is the the way you tested it?
 
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On the motherboard? I've been testing the socket from the PSU to the motherboard so when I test the power supply, the two are NOT connected. Is that right?
Found where I've been going wrong - I thought the diagram was for when you pulled it out and were looking directly at the sockets rather than when it was connected to the motherboard. I'll post results of testing the correct way in just a moment
 
What kind of video card do you have? Is it a flashed card that should have pins 3 & 11 taped? ADC monitors are powered through the AGP connector at 25 volts. If your readings are correct, something could possibly be happening there. Pull out the video card and see that nothing is shorting contacts in the slot or on the card itself. If it's a taped card, make sure the tape is still intact.

Edit: I posted this before I saw your last post. First things first. Let's see what the PSU test tells us this time.
 
@Thundersnake: Found the issue. Some of the sockets of the power supply give a p.d. of the correct value (e.g. 25V for the first test and 5V for the second test) but all the rest had a p.d. of 0V (all other tests) so the power supply is a bit screwed up. Do you know any PSU that would work for it that is cheaper (and preferably more modern) than the stock one?
 
Bummer. Sorry about the bad news. The pinouts are different, so modifications will need to be made, but you can use an ATX supply. It doesn't look that difficult, but I've never done it and I don't know what models would be more desirable for fit. Maybe someone else will know. In the meantime, take a look at this link: http://www.applefritter.com/node/23857

And google MDD ATX PSU mod.

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Also, if you look around, you can get an entire working MDD for $60 or so. This will give you plenty of spare parts.
 
Lol it's fine - I'm just glad it's not a motherboard problem ^.^
Hopefully I can source one and modify it accordingly and get this old Mac up and running again. Thanks everyone ^.^
 
Just a thought, but the PSUs in the PC mini towers are smaller than a normal ATX.
 
zen.state said:
If it's an MDD you can likely resolve this by removing the logic board battery for 10-15 min and then putting it back and resetting the PMU.
Click to expand...
I have the same problem with mine. Today I installed a new optical drive. During that period I removed the battery. Once the drive was installed, I replaced the battery and reset the PMU. No change.
 
visvaldis said:
I have the same problem with mine. Today I installed a new optical drive. During that period I removed the battery. Once the drive was installed, I replaced the battery and reset the PMU. No change.
Click to expand...

Try again with the tower unplugged while the battery is out. This exact method worked for adcx64 (a member here) just a couple days ago.
 
Yep, my MDD would turn on for a few seconds and then turn back off. Leaving it unplugged and the battery removed for 10 min solved the problem.
 
Just come back to say that my G4 is now working properly and will hopefully get a hard drive with OS X installed and be up and running. Thanks guys ^.^
 
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