Testing the Power Analog Board Fuse

First remove the bottom housing, front outer bezel, and the top rear housing to access the power/analog board. The fuse is located near the AC plug.

Warning: You must unplug the iMac to do a continuity test at the power supply board fuse.

You will need an ohm meter to do a continuity test on the power supply board fuse. Refer to the picture below to located the necessary test point referenced in the following procedure.

Unplug the computer to take this reading. Using an ohm meter, touch the red and black probes to the two metal points at location F901 and check for continuity (reading of "0"). If the ohm meter registers "0", the fuse is good. If the meter registers "infinite", the power/analog/video board should be replaced. A bad fuse is indicative to a power/analog/video board problem. Note: To take this reading, it's easiest to insert the red probe of the meter through the opening on the back side panel.

Imac 400 Fuse
Testing Fuse 901 for Continuity

The PMU Chip

The PMU (Power Management Unit) is a microcontroller chip that controls all power functions for the computer. The PMU is a computer within a computer. It has memory, software, firmware, I/O, two crystals, and a CPU. Its function is to:

  • Tell the computer to turn on, turn off, sleep, wake, idle, etc.
  • Manage system resets from various commands.
  • Maintain parameter RAM (PRAM).
  • Manage the real-time clock.

Important: Be very careful when handling the main logic board. Remove the battery when handling the logic board so the PMU is not affected. The PMU is very sensitive and touching the circuitry on the logic board can cause the PMU to crash. If the PMU crashes, the battery life goes from about five years to about two days if the PMU is not reset. Refer to the next topic, "Resetting the PMU on the Logic Board" for the procedure. Many system problems can be resolved by resetting the PMU chip. The PMU reset button is located on the right side of the board, refer to the logic board diagram below.

A1181 Logic Board Fuses
iMac (350 MHz) Logic Board Diagram

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