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Component Isolation

One of the major difficulties in troubleshooting computer systems is pinpointing the specific cause of a hardware issue.

In Lesson 2, you learned about diagnostic tools and references that can assist you in determining whether an issue is caused by hardware or software. In addition, earlier in this lesson we reviewed the general principles of troubleshooting that Apple advocates.

With that knowledge under your belt, you should be at a stage where you can tell whether you're seeing a software issue or a hardware issue. You should also be prepared to systematically address a troubleshooting issue. We will now look at a method that can aid you in identifying faulty hardware components. This procedure is called component isolation—a technique with which you can accurately and decisively determine the source of hardware issues.

Here's how it works: Using a minimal system, you start up a computer and observe its behavior. Armed with an understanding of the normal power flow sequence (discussed later in this lesson), the symptoms you observe may direct you to add or replace components in a specific sequence until you can determine the hardware component that is causing the issue.

You should not confuse this procedure with randomly swapping modules until a system finally works. As you will see, component isolation works in a much more systematic manner.

You should use component isolation:

  • When you are attempting to isolate intermittent, hard-to-find hardware issues
  • When other approaches have not worked, and you need to make sure that the system hardware is working correctly

How Does Component Isolation Work With Diagnostics?

The last lesson introduced you to a variety of Apple and third-party diagnostic tools. All of these diagnostics can give you indications of defective hardware components, but no diagnostic software is accurate every single time. That's why experienced technicians use multiple diagnostics to verify a particular finding.

Component isolation offers a fail-safe method of confirming that hardware components are functional. It should be considered as a companion technique to diagnostic software.

Understanding the Power Flow Process

The basis for the component-isolation troubleshooting technique is an understanding of power flow within computers.

When a computer starts up, many different activities occur. All of these activities rely on the correct flow of power within the system.

Let's look at a Power Mac G4 (QuickSilver) as it starts up. The following steps are a very simplified description of a complex process. Nevertheless, these simplified steps will assist you in understanding component isolation.

When you press the power button on a Power Mac G4 (QuickSilver):

  1. Power flows through the power cord to the power supply. If the power cord or power button is defective, the system will not start up.
  2. The power supply feeds power to the main logic board. If the power supply or the connection from the power supply to the logic board is defective, the system will not start up.
  3. The logic board in this Mac model feeds power to a CPU card. If the logic board or the CPU card is defective, the computer will not start up.
  4. The logic board feeds power to the RAM as well. If the RAM is defective, the computer will not start up. Instead, you will hear an error sound for defective RAM.
  5. The logic board sends a startup sound or signal to the speaker assembly in the front panel board if the power-on self test (POST) is successful. If this startup sound occurs, you know that the components in this power chain are working correctly. If you don't hear a startup sound, the speaker could be disconnected or defective, or the speaker volume may be turned down or muted altogether.

Starting With a Minimal System

In the description of power flow, we made no mention of hard disk drives. This was intentional, because when setting up a minimal system for the component-isolation technique, you start with only the components necessary to hear a startup sound or see a flashing question mark on the monitor.

You do not need a hard disk drive when testing power flow in a minimal system. The POST does not rely on any components of the Mac OS residing on the hard disk. Likewise, if you have a working power button on the Mac itself, you do not even need a keyboard.

A minimal system is exactly that. For a Power Mac G4 (QuickSilver), for example, the minimal system consists of the AC power supply (including, of course, a power cable), logic board, front panel board, speaker assembly, and CPU with heat sink. All other devices should be disconnected, although it's not necessary to physically remove them from the computer unless they're in the way.

Minimal System Chart

The following diagram is a component-isolation job aid for the Power Mac G4 (QuickSilver and AGP Graphics), showing which components on the logic board must be disconnected and which components must remain connected.

Note that some of the required minimum components are not on the logic board. For example, the CPU and speaker assembly are on the logic board, as you can see below, but the front panel board (which controls the power button) is not.

Component-Isolation Procedure

Students in the AppleCare Technician Training (ATT) program are asked to perform the following procedure to reduce a Power Mac G4 (QuickSilver and AGP Graphics) to its minimal configuration for testing. If you are studying this on your own, the preceding diagram gives you the information you need to complete the procedure, but you should be cautious about going ahead without skilled supervision.

  1. Reduce the machine to the minimal configuration.
  2. Press the power button. You should hear a startup sound. A startup sound means the minimal configuration is working. If you get no startup sound, the logic board is probably corrupted or another module in the minimal configuration is faulty.
  3. If you do not get any sound from the minimal system, verify that the power supply is working according to the instructions in the service manual. (For Power Mac G4 systems, you can find this procedure in the Troubleshooting section under “Power Supply Verification.”) If the power supply does not check out as specified in that procedure, replace the power supply with a known-good component.
  4. If you still do not get any sound from the minimal system, reset the PMU and restart. If the PMU chip reset has no effect, perform a logic board reset by removing all power from the logic board for at least 15 minutes.
  5. If you still do not hear a startup sound, remove the RAM DIMM, reset the PMU and logic board, and restart the system. If you get an error sound signifying a memory error, replace the memory with known-good RAM and restart the system.
  6. If none of these steps has corrected the startup issue, install the video card and connect a known-good monitor. Reset the PMU and main logic board, and restart the system. If you get a flashing question mark but no sound, the issue is probably with the speaker assembly or the front panel board.
  7. If you do not hear an error sound or see a flashing question mark, then replace the CPU with a known-good component, reset the PMU and main logic board, and restart the system.
  8. If the system still does not start up, replace the main logic board with a known-good component. Reset the PMU and logic board, and restart the system.
  9. If you hear the startup sound, install the video card (if it is not already installed) and attach a known-good monitor to it. Restart the system and look for a flashing question mark. If you see that image, the video card is working correctly.
  10. By this stage you should have achieved minimal configuration. (You don't have to reset the PMU and logic board once you have achieved minimal configuration unless another service issue appears.) Reattach the internal hard disk drive to the connector on the main logic board. Restart the system. You should see a normal startup screen for the Mac OS installed on that system.
  11. Continue to add any additional components and peripherals one at a time—with power off, of course. If a service issue appears while adding other components, you should go back to the last stage before the service issue appeared, reset the PMU and logic board, and recheck. By this point, you will have a good idea what the service issue is.

Component-Isolation Exercise

  1. What are the five components of a minimal system for a Power Mac G4 (QuickSilver)?
  2. When you first start up your minimal system, you do not get any sound. What component should you check first?
  3. If your minimal system is starting up correctly, what component do you add first?
  4. You get no startup sound from the system after swapping the main logic board. What components are likely at fault?
  5. Why is it important to check cables?

Component-Isolation Exercise Answer Key

  1. AC power supply, logic board, front panel board, speaker assembly, and CPU with heat sink
  2. The power supply
  3. The video card
  4. The speaker assembly and the front panel board
  5. A bad connection due to a defective cable acts just like a bad component.
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