iMac G4 Display Problems

[cblackburn]

Hey all,

I have just bought an iMac G4 1GHz USB 2.0. When I run the system for more than about 15 mins the screen dies and goes a glorious multi coloured pattern that I cannot recover from. This is persistant from then on through reboots until I Zap the PRAM.

Any ideas on what is causing this?

Chris

 

[mkrishnan]

Mmmmm, defective hardware? But every time you zap the PRAM, it works for fifteen minutes? That's kind of weird. Try doing a more exhaustive test, out of curiosity...

1) Zap all the non-volatile RAM using the Open Firmware method (below)
2) Reset the PMU
3) Run the Apple hardware test off the install/restore disc.

But it sounds most like defective hardware (the "logic board" or motherboard).

At the boot prompt, instead of Cmd-Opt-P-R, instead type Cmd-Opt-O-F

Hopefully you should see a simple grey text interface for Open Firmware. Type the following three lines, each followed by <return>, each exactly as shown.

reset-nvram
set-defaults
reset-all

The system should then boot up again.

 

[cblackburn]

Hmm, interesting second symptom I just noticed. When the system fails the air that is being blasted from the top of the case is cold, not just unhot but not warm at all.

Surely some heat should be generated?

Chris

 

[mkrishnan]

Maybe an errant penguin has turned the inside into an igloo?

 

[cblackburn]

Ok, after some poking about with the service manual I have found what I think *might* be the problem.

The CPU is cooled via a heat extraction pipe to the top of the case where the Heatsink is. Whenever the case is take apart the thermal seal on this heatpipe is broken and stops working.

So my theory is that some idiot has had it apart and slapped it back together without resealing it properly. Which is bad becasue it means I have to get my hands on a Torx screwdriver set

Chris