[sm8000]

Disclaimer: I'm not a Mac guy, so I might need my hand held and to be walked through any suggestions.

I am helping my sister sell her old G4 FW800 MDD with dual 1.42s. Last night she came over, got her files, then wiped and reinstalled 10.4 fresh onto it. Everything was fine.

I should have left well enough alone but that CPU fan noise was bugging me. I opened it up, unscrewed the heatsink, cleaned off the old gunk (didn't touch the CPUs) and applied some Arctic Silver 5.

Ever since then, it wouldn't boot. At first it would sound the startup chime, but no image would appear on screen. Eventually the system would turn off.

I then tried Cmd+Opt+P+R, but no luck. I also tried Ctrl+Alt+Shift, no effect.

After this it started booting up, no startup sound, fans kick into overdrive until I shut it off. This morning I located the PMU button, hit it and tried booting again, same thing.

Help!

[King Bob On The Cob]

Take the heatsink off again and make sure the thermal paste didn't get anywhere it wasn't supposed to get.

Also, you're talking about a 7(!!!) year old machine. They're touchy. Check all the memory sticks and the power connections to the motherboard. If it doesn't chime, it's because it is failing before it does a self check, so it's something pretty early in the boot process. Try discharging the Power Management Unit by pressing the CUDA button:

Power Mac G4 Power Supplies

[Eriamjh]

You could try reseating the Cpus. Also look for any cables that may have come unplugged.

[Doc HM]

Did you touch the cable to the drive(s) at all? These Macs can be touchy about booting when the boot drive is attached to an IDE cable different from the one factory.

It's easy to use the second IDE connector in the main drive bay instead of the first one

[seanc]

Indeed my MDD is very touchy.
Won't start up with a monitor connected, it won't shutdown, reboots with fans on full blast, won't sleep either.

[olePigeon]

I have a QuickSilver that was tempermental. All I did was move the computer and it started freezing during boot. I took the heatsink off, reseated the CPU, still wouldn't boot.

So I sandpapered the heatsink, put on some generic thermal grease, put it back together, and it just started to work again. I have no idea.

The best suggestion is to reseat the CPU, it may randomly decide to work again. Also, make sure the fan next to the CPU is working. It really needs the fan blowing or it'll overheat.

[sm8000]

Originally Posted by King Bob On The Cob 

Take the heatsink off again and make sure the thermal paste didn't get anywhere it wasn't supposed to get.

Also, you're talking about a 7(!!!) year old machine. They're touchy. Check all the memory sticks and the power connections to the motherboard. If it doesn't chime, it's because it is failing before it does a self check, so it's something pretty early in the boot process. Try discharging the Power Management Unit by pressing the CUDA button:

Power Mac G4 Power Supplies

Looks to me like all the AS5 is on the CPU cores only.

I've seen different descriptions of how to use the PMU button, what steps would you suggest in this scenario?

[sm8000]

Originally Posted by Eriamjh 

You could try reseating the Cpus. Also look for any cables that may have come unplugged.

Newbie question, how to I reseat them? I really don't know how to get the dual CPUs out of their little daughterboard, if you can, nor do I know how to remove that board. It does wiggle in place a little bit, and if I push down it does bend down a bit, maybe it should be more snug to the logic board?

[sm8000]

Doc HM, didn't replace or reconnect any cables at all, left them all as-is.

olePigeon, the CPU fan is definitely working. Also there is a red LED between the RAM and video card that comes on when the system is powered on. See above, how do I reseat the CPUs?