Troubleshoot this G3 PowerMac!

January 19, 2005 4:05 AM

So, I bought my friend Tom a G3 PowerMac on eBay, which turned up, then booted to a blank screen. After a reboot, it started up, then the power supply went flash, bang and off. I've since connected my PC's power supply to the G3, tried the PRAM battery from my G5, and no luck. When the power button is pressed, the fans in the PSU twitch, and the lights on the motherboard come on, but then go out straight away. Is this a power supply compatiblity thing, or is the logic board fried?
posted by armoured-ant to Technology (10 answers total)

What condition was it said to be in, when you bought it? Did the previous owner claim that it worked fine, or did you buy it as a fixer-upper?
posted by veedubya at 4:10 AM on January 19, 2005

What you're describing, unfortunately sounds like either a processor gone bad or the logic/mother board, it's probably not power related. I would try to get a refund from the seller if possible. How was it packed during shipping?
posted by jeremias at 4:23 AM on January 19, 2005

Response by poster: It was packed quite well, big sturdy carboard box, braced with similar board and loads of bubblewrap inside. It was advertised in full working condition. The seller's been a star, though, and has offered to have it collected by courier if I can't repair it.

I'd just prefer not to have to send it back, wait for a refund, buy another one, etc., if all I need is some sort of Mac-compatible PSU or something. That said, buying replacement parts of a computer my mate just paid for is a little... silly.
posted by armoured-ant at 5:02 AM on January 19, 2005

It could be something as simple as a dead resistor, but having someone fix anything surface-mount will probably cost you a pretty penny.
posted by Civil_Disobedient at 5:24 AM on January 19, 2005

I've had what you describe happen on my PC twice. One time it was the motherboard and the other time is was the processor. Just like jeremias, said. Sounds like returning it is your best option for the money.
posted by mauriteb at 7:23 AM on January 19, 2005

Best answer: From Accelerate Your Mac: "The B&W G3 uses a power supply that's almost but not quite identical to an ATX supply. The lone difference is that the wire which supplies -5V on an ATX supply (pin 18) becomes a ground wire in the B&W G3. If you just plug in an ATX supply without any modifications, you'll short out its -5V output."

You also might need to to cut the “power good” wire on pin 8 from the connector side and splice it together with any orange wire (+3.3v), to send the message that it's OK to boot. Not sure if the G3s need that pin or not. If you don't want to modify a power supply, returning is probably your best bet.
posted by sysinfo at 7:58 AM on January 19, 2005

Sounds fried to me. Even if you fix the PSU now, something else might break down the line.
posted by omnidrew at 8:08 AM on January 19, 2005

The fact that it took out a power supply (you did confirm that the old PSU is dead, right?) would make me extremely hesitant to simply swap another one in before I understood exactly what happened. That it hasn't done so to your PC supply may simply be because it has a higher capacity.

You could try disconnecting everything from the mobo (drives, PCI cards, etc.) and see if one of them is the culprit. It probably won't boot, but it should at least chime. My old G3 had built-in video, so hooking up a monitor would allow you to see what codes came up.

But I would simply send it back. Your seller sounds like a good one, so this shouldn't be a problem. Don't forget to give them positive feedback on eBay.
posted by tommasz at 11:25 AM on January 19, 2005

Did you reset the PMU? There's a little button on the motherboard - disconnect all power (AFAIK that includes the PRAM battery too) and hold it down for 10s. Had machines (albeit G4s) that wouldn't boot but worked great after I did that. The PMU dealies can get kinda jumbly with moving about and stuff. Don't understand it myself.

You could try modding an ATX PSU.. if you just do a cut-n-swap on the cables you could always put it back if the board's dead.
posted by mrg at 6:34 PM on January 19, 2005

If you get the power supply problem fixed, my dad had a blank screen problem with a G3 and it was solved well-enough by keeping the monitor switched off (at the wall) until half-way through booting.
posted by cillit bang at 6:59 PM on January 19, 2005