Member Avatar for tvprom

I'm trying to get my hands on either a copy of the motherboard manual or some sort of information relating to a G4 Motherboard, made by Apple Computer Ince, model 820-1276-a

There seems to be some major power issues that I'm trying to resolve. With all components and power plugged in normally, the power button will light up but no fans, booting, etc.

If I unplug the one of the two main power cables going into the Motherboard, labeled P4, I can get a fan to turn on atleast and one of the led's on the motherboard will light up. The LED that lights up is labeled D55 and illuminates a solid red.

Why is it I can get some activity out of the machine wth only partial power? What does the solid red coming from D55 mean? These are questions that I'd love to have the manual for. Any inforation would be greatly appreciated.

Thanks, Brian

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Brief expert summary and next steps for the 820-1276‑A (Power Mac G4 Quicksilver) issue described by .

The small red LED marked D55 is a board‑power indicator (not a fault code). Apple’s G4 service documentation shows that a single red LED on Quicksilver/G4 boards simply means the logic board has power present — it does not by itself diagnose what is wrong. (manualzilla.com)

Why the board can show partial activity when a connector is removed: the G4 uses multiple rails and “trickle”/standby power so some circuits can remain alive while others are disabled. On Quicksilver models the service manual explains which rails the board expects and gives a power‑supply verification procedure you should follow with a multimeter (measure standby and main rails at the logic‑board connector). That pattern — LED on, fans or drives silent until a specific rail is present — is consistent with a PSU/rail problem or a logic‑board power‑control fault. (manualzilla.com)

Practical, safe troubleshooting (if PRAM/PMU and battery resets have already been tried as suggested):

  1. Visually inspect PSU and logic board for bulging/leaking electrolytic capacitors (common failure on vintage G4s). If caps look bad, the PSU or board may need recapping or replacement. (forums.macrumors.com)
  2. Follow the service manual power‑supply verification steps with a voltmeter to confirm +5VSB, +3.3V and the Quicksilver main rail (+25V) at the board connector. Only probe with power applied and the case open if comfortable and taking ESD/power safety precautions. Replace or swap the PSU if the expected rails are missing. (manualzilla.com)
  3. If the PSU reads correct rails, the logic board itself (power‑control circuitry) is suspect and a board swap or professional repair may be required. Replacement Quicksilver boards and recap/repair services are still available from salvage sellers or marketplaces if repair is chosen. (ebay.com)

Given age and parts cost, weigh repair (recap/PSU rebuild) versus used replacement board. The service manual and an iFixit teardown are the best step‑by‑step references while working on this hardware. (manualzilla.com)

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Member Avatar for Perigee

is the motherboard battery still good? Have you zapped the PRAM? I dunno what info you are looking for, but if the motherboard is bad, there really is not way to fix it...if it is still good, the PRAM may be skewed, and/or the battery may be dead...also, check to see if there is a CUDA button on the board...it''s a small round black button on a small square mount...holding that down for 30+ seconds will clear out things in the PRAM that the keyboard combo will not...note that not all motherboards have a CUDA button - if it does not have one, you don't need to be concerned about it...also, check to see if it boots with the case door open...I've seen some G4's behave as you described, but would boot if the case door was open (probably from a bad wire coming from the power supply)

Member Avatar for tvprom

Thanks for the reply, but those were troubleshooting steps that I've checked already. I'm looking to get a bit more technical than trying the typical fixes to MAC's flaws. I'm assuming I have a bigger problem than the simple answer of it being a MAC.

I was hoping that the manual would explain more about what the individual components on the board are for. Beep codes, what the LED's and their patterns represent, etc. I know this information is readily available for many PC based motherboards. Just trying to find some information on this specific MAC board.

Any help would be greatly appreciated. Thanks.

Member Avatar for mot

Hi Brian. Did you figured something out with that explained problem?
I have exactly the same problem right now. May you know more now after some time has passed.
i would apreciate some help.
Thanks. Daniel

Member Avatar for tvprom

Hi Brian. Did you figured something out with that explained problem?
I have exactly the same problem right now. May you know more now after some time has passed.
i would apreciate some help.
Thanks. Daniel

Unfortuantly, no I didn't come up with much of an answer. What I've determined is that the motherboard (or some part within it) is simply fried.

At the time of my reserach, the Mac Pro was not out yet, but for the cost it would take to dig up a G4 motherboard I could have bought a new G5 for about $300-$500 more. So as I'm sure you can assume, I let the G4 sit in the graveyard.

Luckily for us, that was a spare machine that we are fine without, though it sure would have been nice to have it up and running...

Thats about all I have or can remember about the situation. But I did find some bookmarks I made when I was researching it, I'll add them here and see if they help you out at all.


http://www.mac-pro.com/


http://www.icomputerfair.com/Computer/Apple-Computer-Macintosh/Components-and-Accessories_16.shtml

Member Avatar for mot

Hi Brian. So no good news for me. But thanks a lot anyway. It looks like I would need top go for a new machine too. Unfortenately there`s my office on top the machine. But i can safe the hard disks and have almost all important data I will need.
Thanks and have a good one.
Daniel

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