Powers up (can hear hi-voltage), No Sound, No Picture

@daveyk021 since you don't get any sound or picture, this appears to be an issue with the power supply (PAV board which stands for Power, Analog, Video). Be forewarned, it's a bit of a PITA to work on ;-). Of course, it's a CRT so you want to be extra careful since you will need to discharge the CRT and not getting zapped on it. It'll bite pretty good, so unless you are familiar with CRT's, I'd get somebody to help with that first. Here is a good guide for the replacement iMac G3 Model M4984 Power Supply Replacement and here is a decent document G3 Disassembly

Now, to work on the CRT use the Apple safety guide ESD Safety Guide After you get to your power supply, take some really good pictures and post those with your Question. That way we can see what you see.

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No power. System is completely dead. No LED, no fan, no hard drive power, and the screen is completely black.

Verify the power outlet is good. Try a known-good power cord to determine if the existing power cord could be the problem.

Possible bad USB keyboard. Verify the keyboard cable is connected properly to the computer.Disconnect the keyboard and power on the system via the button on the front of the computer. Do you have power to the system now?

• Yes: The keyboard is bad. Replace the keyboard.
• No: Go to next step.

Reset the CUDA chip and try again. If the problem persists, reset the logic board. (See “The CUDA Chip” and “Restarting the Logic Board” earlier in this chapter for instructions.) Do you have power to the system now?

• Yes: Run diagnostics.
• No: Go to next step.

Possible bad logic board or power/headphone board. Check for trickle power on the logic board. Is there +5V power?

• Yes: Replace the logic board. If problem persists, replace the power/headphone board.
• No: Check for trickle power at the power supply to power filter cable. Is there +5V power?
  • Yes: Replace the logic board. If problem persists, replace the power filter board.
  • No: Go to next step.

Possible bad power supply to power filter board cable. Check for trickle power at the power supply board. Is there +5V power?

• Yes: Replace the power supply to power filter cable.
• No: Go to next step.

Possible bad power supply board. Replace the power supply board

The Cuda Chip

The Cuda is a microcontroller chip. Its function is to:

• Turn system power on and off.
• Manage system resets from various commands.
• Maintain parameter RAM (PRAM).
• Manage the real-time clock.

Many system problems can be resolved by resetting the Cuda chip. Press the Cuda reset button on the logic board to reset the Cuda chip. (The Cuda reset button is located on the back edge of the logic board between the modem port and the processor module.