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| PowerMac G4 MDD won't turn on after clean shutdown. |
Posted by: Loganius on 2026-05-30 11:48:53
I have a mid 2002 1.25GHz PowerMac G4 MDD which works perfectly, except for one thing. If I cleanly shut down the G4 in Mac OS, I can't turn it back on without physically cycling the power to it (as in, unplug it and plug it back in). It's had this issue since the day I got it (I bought it used about a year and a half ago). This doesn't occur if I force shutdown the computer by holding down the power button, however. I've tried resetting the PRAM and the PMU; neither helped. Any ideas as to what could be causing this, and how to fix it?
For what it's worth, it still has the original power supply, although I did swap the fans for some Noctuas. All the caps on the motherboard look fine (I checked them today), and if I remember correctly all the caps in the power supply looked fine when I was doing the swap, although it has been over a year since then.
Thanks for your time, and please be kind. This is my first time posting on 68kmla. |
Posted by: croissantking on 2026-05-30 12:13:23
I would recap the PSU as a matter of course. They are extremely unreliable in this model. |
Posted by: Loganius on 2026-05-30 12:18:41
I would recap the PSU as a matter of course. They are extremely unreliable in this model. I've heard that (quite possibly from messages you've left in other threads lol). I'd rather not if I can help it since I have very little experience soldering, and am a little nervous to crack open the power supply (again). Besides, I don't even know what caps I would need.
Is there any way I can tell if the PSU is the issue before I go recap it? If not, can you give/link me a list of the caps I will need so I can at least come prepared? And also, do you have any advice to help me avoid, say, shocking myself or ripping off the pads? I really don't want to ruin my PowerMac 😅. |
Posted by: croissantking on 2026-05-30 12:20:10
I can’t provide you with safety instructions sorry youre on your own
But I can share a mouser cart with the parts needed if you want |
Posted by: Loganius on 2026-05-30 12:21:20
But I can share a mouser cart with the parts needed if you want That would be helpful, thanks. |
Posted by: Byte Knight on 2026-05-31 06:17:47
Welcome! I have the exact same issue with my MDD. It's only been a minor annoyance so I haven't bothered looking into it. Let us know if recapping the power supply fixes the issue. |
Posted by: croissantking on 2026-05-31 09:03:23
That would be helpful, thanks.
Sharing my Mouser cart here.
Diagram attached. |
Posted by: mikes-macs on 2026-05-31 09:40:41
Let us know if recapping the power supply fixes the issue. My Dual 1.42 G4 had this same problem too. Eventually, it would not turn on at all no matter how many times I unplugged it. I paid a local TV repairman (yes they still exsist )to recap the power supply. Has been working correctly ever since then.
If you don't trust yourself to repair it then hire someone as I did. Try to have someone local do it if possible so you don't have to ship it off and pray that they don't disappear from the planet and keep it for themselves. Even if you do have to ship it and pay a lot for repairs it's better than destroying the Mac or burning down the house. |
Posted by: croissantking on 2026-05-31 12:34:37
pray that they don't disappear from the planet and keep it for themselves Is there a black market for failed MDD PSUs haha |
Posted by: Loganius on 2026-06-02 08:56:37
Alright. I ended up putting together my own capacitor list based on the values & original models listed on Caps Wiki (mostly I picked capacitors with longer rated lives than croissantking because I never want to do this again), but they have been ordered. I'll let y'all know how it goes once they arrive. Wish me luck! 😅
(I did remember to pick caps with the same capacitance, diameter, lead spacing, and same or higher voltage rating). |
Posted by: Loganius on 2026-06-06 10:08:38
Well, the capacitors arrived. I have no idea how to get the board out so I can start replacing the capacitors. There's this mysterious tan stuff all over the board, one of the screws holding the board in is hidden under this... green thing, and even if I manage to get the board free, I don't know how I'm going to get it out when the AC plug is soldered to the board (via jumper wires) and attached to the case.
Does anyone have a guide I can look at?
In other news, I noticed some capacitors (appears to be C21 and C29) appear to be bulging, so it seems increasingly likely it's a capacitor problem.
Update: I managed to get the green thing unstuck from the case and out of the way just enough to get that last screw. Still stuck on the AC plug tho. |
Posted by: s_pupp on 2026-06-06 12:00:24
I admire your persistence with this project. I've recapped/repaired a variety of Mac PSU's, but after opening up my MDD G4's PSU and looking inside, I said "Nope," closed it back up again, and sent it away for repair. I've since obtained a couple of MDD G4 PSU's, and plan to get to them when I'm in the right frame of mind to give it a go.
I'm concerned that Noctua fans may not provide enough static pressure and airflow to meet the PSU's cooling needs - I'd appreciate the input of someone with experience refurbishing these power supplies |
Posted by: Loganius on 2026-06-06 12:06:57
I admire your persistence with this project. I've recapped/repaired a variety of Mac PSU's, but after opening up my MDD G4's PSU and looking inside, I said "Nope," closed it back up again, and sent it away for repair. I've since obtained a couple of MDD G4 PSU's, and plan to get to them when I'm in the right frame of mind to give it a go. I really don't want to pay for it to be professionally repaired.
As for the Noctuas, well, in retrospect I should've bought higher flow rate fans, but so far it's been a year and the power supply still works, and the symptoms of failure I mentioned at the start haven't gotten any worse.
I also don't have the original fans anymore 🫠. |
Posted by: croissantking on 2026-06-06 12:40:35
Well, the capacitors arrived. I have no idea how to get the board out so I can start replacing the capacitors. There's this mysterious tan stuff all over the board, one of the screws holding the board in is hidden under this... green thing, and even if I manage to get the board free, I don't know how I'm going to get it out when the AC plug is soldered to the board (via jumper wires) and attached to the case.
Does anyone have a guide I can look at?
In other news, I noticed some capacitors (appears to be C21 and C29) appear to be bulging, so it seems increasingly likely it's a capacitor problem.
Update: I managed to get the green thing unstuck from the case and out of the way just enough to get that last screw. Still stuck on the AC plug tho.
Just pick off or cut the green stuff (as you did) and the tan stuff.
The blue and brown wires will need to be desoldered from the PCB unfortunately. |
Posted by: Loganius on 2026-06-06 21:18:47
After a very eventful (and stressful) 10 hour power supply recapping, I am happy to report that, not only does my PowerMac G4 live, it now turns on properly after a clean shutdown. 😀
I ended up just replacing the 12 "shotgun" caps plus the 3 100uf 450V filter caps.
Just one last question: are you expected/able to change the title to include "Solved" after the issue has been resolved on 68kmla?
Thank you all so, so much for your help. In particular, that schematic croissantking sent was so useful. I referenced it constantly. |
Posted by: croissantking on 2026-06-07 05:29:47
After a very eventful (and stressful) 10 hour power supply recapping, I am happy to report that, not only does my PowerMac G4 live, it now turns on properly after a clean shutdown. 😀
I ended up just replacing the 12 "shotgun" caps plus the 3 100uf 450V filter caps.
Just one last question: are you expected/able to change the title to include "Solved" after the issue has been resolved on 68kmla?
Thank you all so, so much for your help. In particular, that schematic croissantking sent was so useful. I referenced it constantly.
Good work and you deserve a rest after that! |
Posted by: Loganius on 2026-06-07 10:37:16
...even if I manage to get the board free, I don't know how I'm going to get it out when the AC plug is soldered to the board (via jumper wires) and attached to the case. For anyone reading this thread in the future, what ended up working for me was to take out the fans, slide the board backwards (away from the AC plug), kinda shimmy it out from under the plug, then flip it over and desolder the brown and blue wires (as croissantking suggested). |
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