Non-working iMac G4 power supply - possible to repair?

Discussion in 'PowerPC Macs' started by 1984power, Nov 4, 2017.

  1. 1984power macrumors newbie

    Aug 28, 2017
    Greetings everyone!

    I found a 17" 800Mhz G4 iMac for a great price.
    It was listed as non-working though, since no picture displayed on the screen when it was turned on.

    With these symptoms in mind I originally thought the LCD or some of its components were to blame.
    However, upon connecting the power cable for the first time I immediately felt air coming from the fan. Pressing the power button did nothing.

    This led me to believe energy management of some sort was at fault - I hadn't even pressed the power button yet.
    After a PMU reset the fan would not turn on anymore upon connecting the power cable.
    From this point on, after pressing the power button all I get are some motherboard LED flashes and a few noises (seemed like the optical drive trying to start).

    I followed the troubleshooting procure on Apple's Service Manual and, lo and behold, one of the power supply plug pins has no voltage.
    The service manual says to replace the power supply, but I'm wondering if I can repair it :)
    No component is visibly wrong/burnt... Is the only way out a new one?
    PSU photos below.

    (I have tried to put in a new motherboard battery, but no difference, not even after resetting PMU. Although the first time I reset the PMU I pressed the button more than once, I hope that did not damage anything...)

    Attached Files:

  2. bobesch macrumors 65816


    Oct 21, 2015
    Kiel, Germany
    Repair of the power-unit is possible. There's a company in the south of Germany who does it for about 80€ but it seems to be a major procedure that's beyond my knowledge ...
    I guess, it's cheaper to look for a working unit for spare parts ...
  3. weckart macrumors 601

    Nov 7, 2004
    There is no quick or cheap way to do this. For PSUs you need to have the requisite techical knowledge to proceed. After that, you are dealing with old parts. Even if replacement components are available, you may have to buy in bulk or pay through the nose unless you have the right contacts.

    Either way, cannibalising other units seems to be the way to go. From what I have seen on forums like Vogons, where repairs were made on vintage PSUs, this involved specialist knowledge, a fair amount of time to determine the fault and money.
  4. gsahli macrumors 6502a


    Jun 1, 2007
  5. AphoticD macrumors 68000


    Feb 17, 2017
    The OP could also consider offering the failed PSU to them for a slight discount. They might do in house reconditioning of failed units, who knows..
  6. Interceptor2 macrumors newbie

    Aug 9, 2010
    Gloucester, England
    I've repaired a number of similar power supplies, usually replacing the main controller chip does the trick but a replacement can sometimes be difficult to find. The best way is just to drop the cash and buy a replacement.
  7. ctmpkmlec4 macrumors 6502

    Jul 4, 2014
    Lyons, KS
    If you're in the US, I might have a power supply you can have for the cost of shipping. I'll have to see if I can find it. The drawback is I can't guarantee it's good... no way to test it.
  8. 1984power thread starter macrumors newbie

    Aug 28, 2017
    I'll be on the lookout for these on eBay and local sites...
    Thanks to everyone for your replies.

    @ctmpkmlec4: I have sent you a private message.
  9. 1984power thread starter macrumors newbie

    Aug 28, 2017
    Greetings everyone!

    A while back I found the YouTube video of the "hair dryer trick" for these iMacs and someone in the comments section claimed to have solved his PSU problem by replacing capacitors C216, C215 and C212.
    I did just that but nothing changed... so I took my PSU to a local electronics repair shop.
    They told me there was nothing wrong with it parts-wise, but they couldn't turn it on without the iMac.
    I actually never thought of bringing them the whole computer... my fault. I tested it again but nothing changed.

    During the last few days I have been tinkering with the iMac again to see if I can fix it. I soldered a loose video socket pin back to the motherboard and straightened some video and blind-mate connector pins that had gotten twisted.

    Here's where I'm at currently:

    - the fan spins up, the motherboard LED comes on, the hard drive spins up, as well as the optical drive when a disc is in it (sometimes they don't though);
    - the motherboard LED and the fan sometimes flicker/have interrupted operation (maybe current fluctuation?);
    - the computer turns on the moment the power cord is connected (sometimes it does not turn on at all and I have to reinsert the power cable) and pressing the power button does nothing at all times;
    - nothing on the display, no backlight;
    - no current on USB ports (keyboard caps lock key doesn't light up);
    - no voltage on the same PSU pin as before.

    I'm guessing the chip on the power supply is malfunctioning, since the repair shop told me it was good parts-wise. I won't be losing any more time with this one though.

    Should I get a new power supply or is it possible this iMac already has other issues?

    (I'm attaching a picture that highlights the PSU pin with no current.)

    Attached Files:

  10. CooperBox macrumors 65816


    Nov 28, 2010
    France - between Ricard & Absinthe
    I love working on these iMacs, but must confess with the problems it's throwing up I'd be lost for a solution, except looking around for a doner unit. Don't know where you are located, but I've found fully serviceable units often for less than 30dollars, and once for as low as 15 at a charity store - advertised as 'not working'. Once back home found that the 'tester' hadn't pushed the power lead all the way in. That's the sort of fix I love!
  11. 1984power thread starter macrumors newbie

    Aug 28, 2017
    Thanks for your reply, @CooperBox.
    You're right but unfortunately I don't think I'll be able to get ahold of another one of these for parts very soon... where I'm located broken ones don't show up often. I've also double checked my cord is all the way in :D

    I'm now considering researching more about the PSU's chips, following @Interceptor2's suggestion earlier in this thread.

    What makes me hesitate about paying for a replacement is the fact my power button does not operate correctly and the USB ports have no current... I don't want to end up with a new power supply and a bad logic board.
  12. 1984power thread starter macrumors newbie

    Aug 28, 2017
    I'm afraid I forgot to ask one more thing in my last post: is an iMac G4's screen supposed to flip backwards?
    I know you can adjust the screen's angle, but mine can flip backwards until it's facing the opposite side.
  13. 1984power thread starter macrumors newbie

    Aug 28, 2017
    I got a miniVGA adapter for the iMac... even through the VGA out, no image/signal ever appears.

    If anyone has a functioning power supply for one of these iMacs that you don't need please let me know.
    By the way, is an iMac G4's screen (swivel arm) supposed to flip backwards? I hope that's not my problem in the end.

    Thanks for the help.
  14. CooperBox macrumors 65816


    Nov 28, 2010
    France - between Ricard & Absinthe
    Sorry I can't help with a power supply.
    From memory the G4 iMac arm should move from the vertical, down forward to the horizontal position. It shouldn't flip backwards. Also unless I'm mistaken, the arm should swivel 180deg thro its axis, i.e. looking from the front you should be able to turn the screen 90deg left or 90deg right.
  15. 1984power thread starter macrumors newbie

    Aug 28, 2017
    Thank you for your reply @CooperBox.
    It's not the arm joint next to the hemisphere: I'm talking about the joint that connects to the screen. The other one seems normal.

    Here's a picture. It's not great but it's possible to make out the display's Apple logo on the bottom.

  16. CooperBox macrumors 65816


    Nov 28, 2010
    France - between Ricard & Absinthe
    No, never seen anything like that. All mine are rock solid at that particular joint.

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15 November 4, 2017