iMac G4 Technical Issues - Black Screen/Keyboard unpowered

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Level 1

4 points

iMac G4 Technical Issues - Black Screen/Keyboard unpowered

Hi there,

I'm trying to troubleshoot an old imac g4 ppc 20" usb2.0 model.

I purchased a slightly decrepit version of this machine from ebay, still working - and had to make a few modifications to get it up to spec. After DVD RW replacement, new logic board battery, thermal paste replacement, and it seemed to be in working order. I was able to install 10.4 Shuriken and made sure that it was working as expected. However, the machine since crashed while trying to load a website and we've had a weird issue which has given me some problems.

After the crash, the iMac booted up again and played the chime but just showed a black screen, my housemate attempted to do a Parameter-RAM reset from the keyboard, and it allowed the screen to turn on and hang on the startup screen. I since picked up an OS 10.5 installer to try and do a fresh OS install, but we've not been able to repeat the above step as the keyboard is not receiving any power.

It's an odd issue, as the USB ports are certainly functional - when a mouse is plugged in, the laser lights up. But keyboards do not seem to work (tapping the caps button on an old apple keyboard doesn't allow it to light, or does the usb hub seem to power an attached mouse). This has prevented us from being able use the shortcut that would allow the machine to boot from disk, perform another Parameter-RAM reset, or otherwise. We've get the OS 10.5 installer into the DVD drive, but it required opening up the machine to manually insert the DVD into the drive, however, without being able to use the keyboard shortcut to boot from the disk - we're locked out there too.

We thought potentially that this might be a PMU issue - and have attempted to reset using the switch, but this hasn't had any effect.

Using a multi-meter, I've checked the PMU battery test point, and it appears to be receiving the required 3.5V.

Next step we might try is to buy a powered USB hub to see if that will resolve this problem - but I wasn't able to find anything referencing this particular problem online or in the imac service source manual, so we're flying in the dark a little. Does anyone have any thoughts on what might be causing this problem, or any thoughts on how we might remediate it?

Thank you!

Earlier Mac models

Posted on Dec 7, 2024 2:42 PM

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Answer 1

Allan Jones

Level 9

73,028 points

Dec 11, 2024 9:42 AM in response to richsc21

Wow! Quite a project!

I'm digging back into a cobweb-covered memory vault—it's been a long time since I bashed on any PowerPC Macs.

1—PowerPC Macs did not like burned system installer disks or USB installers. The latter is surprising because some late PPC Macs were supposed to be USB bootable. Didn't always work that way. Do you have original, Apple-pressed Cds/DVDs that are retail install disks?

2—Related to Item 1, many PowerPC Macs used model-specific system files. Full retail install disks had a full set that powered any Mac that supported that macOS level. They installed a system config specific to the Mac. Grey, system install/restore disk usually had only those for the Mac sub-model with which they shipped. They were anything BUT universal.

3— There were important torque specifications for reinstalling the heat dispersal system. Did you use those? Failing to torque could fry the LB in within a few minutes.

4— Was the replacement optical drive booting OK from the installer disk OK? Some are not Mac-bootable. Is it internal?

5—We thought potentially that this might be a PMU issue - and have attempted to reset using the switch, but this hasn't had any effect.

How did you do that? On Macs with "New World ROM" systems, basically anything with a USB port, the PMU reset was a single, momentary button press. Some people used the "press and hold" method that reset the CUDA of Old World Macs and ended up with severe issues. I used to have "fix that" instructions bookmarked for how to recover from an over-enthusiastic PMU rest, but that was several computers ago. I'll look, but it's not on either of my current Macs.

6—Next step we might try is to buy a powered USB hub to see if that will resolve this problem

What are you planning to connect to the powered hub?

I seem to have pulled a muscle in my brain tripping down Memory Lane. I'll go off and noodle on this some more. I’m not the only PPC survivor here. Hopefully, Jeff will see this.

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Answer 2

HWTech

Level 9

62,114 points

Dec 16, 2024 2:07 PM in response to richsc21

@Allan Jones has a pretty good memory.

I would suggest removing the PRAM battery entirely to see what happens. It should boot fine without the PRAM Battery.

I would also suggest disconnecting internal items just in case one of them is causing a problem. I would even disconnect the built-in Display cable. You can use the fan as a guide as to whether the system if powering on...I believe the fan will run all the time, but my memory may be a bit off, plus you can try a PRAM Reset in this configuration. If you don't hear the chimes, then your Logic Board is most likely bad (keep the speakers connected). You can also listen for the startup chime and use an external display (be careful with some newer displays as this old Mac may not be compatible).

Keep in mind your "new" Logic Board may not be working. Since you bought a working iMac, it means you now have two Logic Boards and the system does not work. Either you have two bad MLBs, or you probably have a bad power supply. Make sure the power supply connectors are fully seated (was there just one main connector?)

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