The PowerMac G4 Cube is a compact personal computer from Apple Inc. It runs on either a 450 or 500 MHz processor with a selection of graphics cards like ATI Rage 128 Pro with 16 MB of SDRAM, nVidia GeForce2 MX with 32 MB of SDRAM, or ATI Radeon with 32 MB of DDR SDRAM.
Cube won't turn on
Your computer will not boot.
If your computer doesn't boot and the problem is the RAM/memory, you won't be able to hear a startup sound, and the power-on light will flash in a sequence of two, three, or four times. The number of times that the light flashes tells you what particular problem with memory you have.
NOTE: A flash is considered 1/4 seconds.
Two Flashes: No Memory is Installed or Detected.
If you already installed memory, then the problem might be with it not being secured into its slot. In this case you should make sure that it is securely placed into the slots specified for RAM. If no memory is installed, then you should install them into their respective slots. If all goes well it should boot up again, but if the light still continues to flash, we suggest you contact Apple Support for assistance.
Three Flashes: Incompatible Memory Types are Installed.
If three flashes occur, that means that the memory that you installed is not supported by the computer system. You should only install memory that is supported by logic board. We recommend that you remove the current memory and install ones that are supported.
Four Flashes: No Memory Banks Passed Memory Testing.
Either the memory or the memory slots may need to be replaced with new ones. You are going to have to check the pins on the logic board in order to decide if it needs replacing. You don't need to completely remove the memory slots, you just want to be able to see if there are any bent pins. If there are bent pins, you will have to contact Apple for replacement parts and repair services. But if you don't find any bent pins, all you need to do is reset the logic board. This can be done by removing the video card from the logic board and pushing the reset button (S1) on the board.
Refer to this repair guide for RAM replacement/ installation:
Bad Power Adapter
Before actually concluding that the reason your computer won't turn on is the power adapter, make sure the power cord is plugged into a working wall outlet and a properly connected to the adapter. If the computer still doesn't turn on and you have a spare power adapter that you know works, try swapping it to see if it'll turn on. If it turns on then you'll need to replace the old power adapter. If it still doesn't turn on it may be the logic board.
Depending on power adapter fault, a Cube may power-on successfully, but its touch switch will not illuminate properly and machine may behave erratically.
Bad Logic Board
If none of the solutions listed above can get your Cube to turn on then you'll need to find a replacement logic board for it. The Cube runs on a PowerPC 7400 Processor so a compatible logic board is required. There are also upgradeable processors like the AGP G4 Daughter Card, but whether you get the same logic board or a upgraded one, make sure you read the specifications and compatibility of the board so you know you are getting the correct one.
The Cube Doesn't Stay On
The computer's power button lights up when touched, but goes out as soon as you remove your finger, and the computer does not turn on.
Sometimes the DC-to-DC converter card may move slightly out of its connector, causing the computer not to turn on when the power button is touched. To fix this problem, you will just need to press the card firmly back into its slot.
My Screen Isn't Working Properly
The computer screen doesn't display anything when turned on or has very distorted graphics.
Bad Display Cable
The display cable for the screen has exposed pins on the connector so a bad or bent pin can cause display problems when turned on. Check to see if there are any bad pins in the connector. If there are bent pins carefully try to straighten them out and then reconnect the screen and test if the problem still occurs. If the connector doesn't have any bent pins or the problem still occurs, try testing the screen on another computer. This will help conclude if the problem is actually the cable or not.
Bad Graphics Card
The main component that deals with the display is the graphics card. If the screen works perfectly fine on a different computer, then there is a problem with the graphics card. Check the video card's display connector for any bent pins or damage to the plastic housing. If you find any bent pins, DO NOT try to straighten them. You are going to have to replace the graphics card if you have found bent pins and or if the plastic housing is damaged. If replacing the graphics card does not solve the problem, you will have to replace the logic board.
Here is a complete guide on how to remove and replace the graphics card for the G4 Cube.
My Mouse Doesn't Work Correctly
If the pointer doesn't move with the mouse or if it doesn't move at all when you start up the computer.
With mouse problems, an easy way to fix it is to disconnect and reconnect the mouse from the USB port. Note, if the mouse is connected to the USB port on the keyboard, just connect it to a port on the desktop instead. If the mouse works, then just replace the keyboard. Otherwise if the problem still occurs, test out the USB ports on the desktop one by one with the mouse or a different device. If the USB ports are fine, then you will have to replace your mouse. If not, then the USB ports need to be replaced instead.
None of the Above
Nothing listed above matches the symptoms of your Cube or you have tried some solutions, but the problem is still occurring.
If nothing that you've done to try and fix the problem works, then there is usually only one main solution, which is to replace the logic board entirely. Since the logic board contains multiple components that don't all rely on each other, they can all malfunction separately and not affect the functionality of other components. If you have tried replacing a broken component like the graphics card, memory or display and the original problem is still present, it is highly possible that the logic board needs to be replaced.