MDD Not Powering Up

I have an MDD G4 that has really been giving me trouble, and I'm not sure where else to look on it.

Let me say first of all that I can pretty safely rule out the PSU as being the problem. I have two MDDs and three PSUs floating around, including one that is supposedly rebuilt from Ebay. I know one PSU is dead, although my two "good" PSUs will both power my other MDD flawlessly. So, as I said, I think I can rule that out as being the problem.

On the problem MDD-it will sometimes power up and work perfectly. I've been able to get it running long enough to do a clean Leopard install on it, as well as download and install all the updates.

Occasionally, however, it will just seemingly die. Whenever this happens, the power button will glow for 1-2 seconds when pressed, but then go out. The fan on the door will sometimes "twitch", but not actually spin up. Sometimes hitting the PMU reset or pulling the battery(along with unplugging it and plugging it back it) will allow it to boot, but at other times does nothing.

I've given the whole thing a general cleaning, including cleaning the contacts on the video card and the RAM, and repasting the CPUs.

I should also add that it has 2gb of all low-density ram in it. I dropped this full set of ram in my other MDD, and it worked no problems.

And, just for general system information-this is a first generation "Firewire 400" model with dual 1ghz CPUs.

Just to be clear, I have two MDDs total-one working, and one not.

As I said, I have one power supply that I know to be dead, one freshly rebuilt one, and one known good power supply. Both the "good" PSUs give identical results in both the "good" and the "bad" MDD.

Since someone above suggested the power button as a potential failure point, I pulled the power button assembly out of my good MDD(a surprisingly un-Apple-like job that involved plastic pop rivets similar to what holds a lot of the plastic parts on in my car) and hooked it up to the dead MDD. It gave the same result-the LED glows briefly when the button is pressed, but the computer doesn't power up. Of course, I guess the possibility of a bad ribbon cable between the button and the logic board-since this is buried sort of deeply in the case, I didn't transport cables between the two computers.

I have exactly one ADC display. It's currently connected to my Quicksilver(which is in a different physical location from my MDDs). I could transport one to the other, although the USB ports are a bit "touchy" on my display and are currently working so I'm hesitant to disturb the current set-up.

I'll proceed with changing out other parts, and see if I can get somewhere.

With that said, though, I did just buy what are essentially the complete "innards" of an MDD on Ebay. I'm getting the logic board, 2gb RAM, a set of case fans, a dual 1ghz processor card and heatsink, a Radeon 9000 Pro, and both case fans(all for $35 including shipping). That should at least give me some spare parts to play with, and depending on what is ultimately wrong with the "bad" MDD, maybe I can also upgrade my "good" one from a single 1.25 to dual 1ghz.

gsahli said:

I'd try taking out the backup battery and then startup.
I had similar symptoms and had to regularly reset PRAM - but taking out the battery made it work fine (you have to have network time working, too).
Go figure.

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As I said, I have gone through various iterations of PMU resets as well as booting with the battery in and out. Sometimes this will get it to boot, but more often it won't.

OK, so it looks like you've made some progress--you've eliminated the power supply and power switch as the problem.

Next thing to test is the cpu daughtercard. Install the daughtercard from the problem MDD in your "known good" MDD. If it boots, then the cpu can be eliminated as the problem. If it doesn't boot, then you have your answer--you need a new daughtercard, which it sounds like you have already ordered.

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Will do as you suggest, although I've had a family emergency come up and it will most likely be the middle of next week before I can play with any of this stuff again. My spare will also hopefully have arrived by then, so at least I'll some additional parts to include in the switch-around sequence also.

Thanks again for all the help here.

So, I went through part-by-part with my good machine, and had no issues getting the good machine to boot even with the RAM, graphics card, and processor daughtercard from the "bad" machine.

So, with that in mind, I went ahead and swapped logic boards for the one I got in the above-mentioned parts lot. With the new logic board installed, the system booted right up. With that in mind, I guess the issue was a bad logic board.

Thanks for the help with troubleshooting this!