imac g4 snowball power issues

[tony359]

Hello all,

I have acquired an iMac G4 snowball which did not power up. I took it apart and inspected the PSU. The second half of the PSU was getting high voltage from the other half as expected but I did not read anything out of it. I found a small solder ball on the PCB which seemed to be shorting the output: once removed I powered up and it worked!

I thought it was ok, so a few days later, after swapping the flat battery, I took it in my living room, plugged it in and... dead again.

However after a few days being unplugged I plugged it back again and it was alive again. It survived several power cycles and restarts.

After a night unplugged, it was dead again.

There is a small button under the MB which I understand resets the PRAM but that did not do anything. I believe the PSU only outputs voltage when it's connected to the motherboard. Can anybody tell me how that works exactly so I can go ahead with some diagnosis? I found online some reference about some test points at the back of the motherboard but they are not present on my unit.

Any help appreciated!
Thanks
Tony

 

[tony359]

Hi - does anybody have any info that could help me on this please? Schematics or just a brief direction on how the PSU powers up - am I expecting 12V all the time or does it need the motherboard to actually output voltages?

Thanks!

 

[elf69]

It sounds like a dry joint somewhere.

We had a few G4 imac machine come to my old work with PSU issues.

There is a mac parts specialist called "thebookyard" they may be able help with a spare PSU.

 

[tony359]

Thanks! Before I swap parts I would like to diagnose a bit more - i.e. if I know the PSU always output 12V whether the motherboard is plugged in or not, then I can probably say it's the PSU. And I wouldn't mind trying to repair the PSU if something is wrong - as you say, it sounds like a dry joint or something potentially easy. But first I would like to understand if and when that thing outputs a voltage!

Comparing to an ATX PSU, if you don't know the PSU is idle until PW_ON is shorted to ground, you may as well think that the PSU itself is dead!

 

[elf69]

For ATX psu you can buy a tester.

Shame no device to do same on this psu.

Maybe some other bright spark in here may know if an ATX tester could be modded to test an apple psu?

 

[tony359]

To test an ATX psu I can just short the green wire to ground and then use my multimeter 🙂

this PSU only outputs 12V if memory serves. What would be useful to understand is: is 12V ALWAYS supposed to be present at the terminal, even without the motherboard attached?
if not, what is the action that wakes up the PSU?

 

[tony359]

Just for the records:

Yes, the PSU will output 12V regardless whether it's plugged into the MB or not. I found that when the PSU was switched on, it would output about 6V and then immediately stop and slowly drop to 0V. Once in a while it would work ok and output the proper stable 12V.

In my case it was a bad capacitor on the DC-DC board - the second half of the assembly. It read ok and was not shorted but clearly was not happy when being run at 400V.

I hope this helps whoever may search for this in the future.