Apple G4 Power Supply Woes?

Predius asks: "I, like many others based on forum hopping, have an Apple G4 desktop that has decided it no longer wishes to power up. I've replaced PRAM batteries, verified power supply voltages, pressed the CUDA switch, and done combinations of the above. Trying to start it just results in a blip of the powersupply fan and the power led on the motherboard flashes once. Every now and then it will actually boot, and when it does it's 100% rock solid, will reboot fine, etc. If I power it off, I get to play games again trying to power it up. Does anyone know how to manually start an Apple Gigabyte or later rev power supply? Standard ATX power supplies can be fired up by shorting the green PWR ON lead to ground, but Apple decided to be different. I await the collective's wisdom."

Comments

Re:Humidity

[Anonymous Coward]

A quick blip of the status light on a Macintosh can idicate a power short. It could be a wire in a supply lead has been exposed and is intermittently contacting the metal chasis, or the power supply has an internal problem.

Inrush current

[morcheeba]

Sounds like this may be a inrush current problem... maybe something is taking extra juice to start up, or the supply can't deliver the initial kick. A multichanel scope would help a lot to see that all voltages are getting to where they need to be in a reasonable time. Otherwise, try removing everything that you can and see if it comes on (drives, ALL ram, usb, pci cards, modem, etc.) - maybe eliminate it that way. Remember, a bad card (one that's drawing too much current) may not cause the system to fail until another good card is inserted and pushes the power supply over the threshold.

corrupt PMU?

[Lizard_King]

Have you tried resetting the PMU?

Pop that sucker open (unplug first!) and there should be a little black button on the lower front of the logic board. "Reset pmu" will be printed on the board somewhere near the button. Push it for one second and only press it once. Let the box sit for roughly 5 minutes unplugged and then turn it on.

Aggravating problem

[Mulligan]

Especially if you are out of warranty. Apple describes it in support article 95039. The most frustrating part is that replacing the power supply is the last step -- they try to replace the CPU before checking the power supply. This means that you have to commit to purchasing a CPU if it is the problem child. My CPU (500Mhz G4) was ~USD900 last time I checked -- more than the value of the the computer at the time. All this is after you pay their diagnostic fee.

Don't get me wrong, Apple Care is wonderful, but once it expires you can be in for some expensive repairs...

Why haven't you tried replacing the powersupply?

[stienman]

"I've replaced PRAM batteries, verified power supply voltages, pressed the CUDA switch, and done combinations of the above."

Look, there are many ways a powersupply can fail and still appear to have the correct output voltages. For instance, the powersupply has a power good signal going to the motherboard. The PS determines whether it's within its specifications, and when it is (some mS after actual turn on) it asserts the signal, and the motherboard comes out of reset. It could be that either the powersupply is not asserting the signal correctly, or that it is still out of spec.

But it could be a dozen other problems. Until you swap it for a known good PS, you cannot know for sure that the PS is good. (other than, perhaps, putting it in another good system and powering it up there)

Here is the pinout of a dual G4 powersupply. I suspect that the pinout is substantially similar. You may be able to force the PS on, but you'd have to find and assert the power good signal (I doubt that the supply doesn't have one - chances are they made this pinout with a multimeter, and there's no easy way to find the power good without a scope or analyzer)

But I suspect that you'll spend less money and time by simply getting a new PS and replacing it.

Please note that powersupplies contain many parts that fail over time. Aside from the obvious fan, the electrolytic caps eventually dry out, causing out of spec voltages, out of spec ripple, and out of spec regulation. You won't be able to see the ripple on the average multimeter.

Also note that computer hardware troubleshooting is a black art, and should only be practised by mages trained in this specialty.

-Adam

Power issue from motherboard

[esammer]

I have a G4 Dual 400 (mirrored drive door) and had a similar problem. After numerous games, and since it was still under warranty, I brought it in. Four visits to my local Apple store later, it works. They replaced the power supply, memory, ram, processor, and finally after I demanded it, the motherboard (or "main logic board" in Apple lingo. It was the motherboard.

Turns out there's a known Apple TechNote on the mirrored drive door boxes that produces a power on, but unexpected results type problem. Here's the technote (iirc) or at least one similar: http://docs.info.apple.com/article.html?artnum=950 38

I HIGHLY recommend making a list of technotes that directly refer to and explain the problem in detail. Do what you can yourself. Go to an Apple store (make it a road trip if there isn't one near you - I'm in NY, US so it wasn't a problem), walk up to the "Genius Bar" and make them work for their famed support reputation. I've owned macs since sometime prior to the Quadra and when it doubt, Apple *will* save you should you ask the right way. Be nice. Beg. Your warranty is an intangible and can be extended by the manufacturer if it is a known problem and you plead your case correctly.

This is usually related to a bit more than the power supply and is absolutely a known issue.

Good luck. Chin up... and beg if you're out of warranty.

Re:Why haven't you tried replacing the powersupply

[babbage]

:-)

You are, of course, exactly right.

Still, it's not as bad as it used to be. As far as I can tell, the only really proprietary parts on a modern Mac are the CPU, motehrboard, the power supply, and maybe the video card. In the case of the video card, it's only "proprietary" because the systems are Open Firmware based (like Sun, SGI, and maybe some others), and the video card has to be able to talk to the rest of the system -- but it uses the same AGP bus that x86 uses, and my impression is that there's no reason that the "Mac" cards wouldn't work just fine if (say) AMD ever designed an Open Firmware based x86 platform.

Beyond that though, it's all standard hardware. Same RAM, same hard drives, same PCI bus for expansion, same peripherals on the USB & Firewire ports. And so on.

It used to be much worse than that, in the days up to the blue & white G3, with the Nubus architecture, "standard but exotic" SCSI bus for hard drives, etc. Moreover, above the hardware level, they've gotten very good about using standard protocols & formats for nearly all of the system, and have offered up their new developments to be used as open standards (e.g. Rendezvous).

While it's annoying that they can get away with charging so much for the proprietary parts that they still control, at least these parts are generally pretty reliablee & most people don't have any problems. And the rest of the system is just so much nicer than anything that could be run on x86 that I for one am willing to take a chance on what Apple has to offer, knowing that if anything does go drastically wrong, the fix can be ridiculously expensive compared to the same repair on the other site.

Everything is a tradeoff, ya know?

How old is the machine?

[gozar]

There was a revision of the G4 in August, 2000. The first ones off the line had power switch problems, giving almost the same symptoms. We had them all replaced under warranty.

One work around, find an older Mac USB keyboard that has a power button on it. Plug it in and see if the machine will start then. If so, you will need to get a replacement power switch.

More info available from ZDNet.