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Apple imac 20 inch early 2008 User Manual

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    							iMac (20-inch Early 2008)  Take Apart — Chassis 2 21
    Replace the hard drive fan.11 . 
    Replace the h12.  ard driv e.
    Replace the optical drive flex cable if removed.
    13 .   
    						
    							iMac (20-inch Early 2008)  Take Apart — Chassis 222
    Replace the optical driv14. e.
    Replace the logic board.
    15 .  
    Replace the right speaker16.  .
    Replace the display panel.
    17.  
    Replace the front bezel.18.  
    Clean and replace the glass panel19 . . Note: This step is very important. Follow the procedures 
    to ensure the glass is clean and free of particles.
    Replace the access door.
    20.   
    						
    							© 2008 Apple Inc. All rights reserved.
     Service Source 
    Troubleshooting
    iMac (20-inch Early 2008) 
    						
    							iMac (20-inch Early 2008) Troubleshooting — General Information 224
     
     
    General Information
    Serial Number
    The serial number for this iMac model is located on the bottom of the computer stand. 
    						
    							iMac (20-inch Early 2008) Troubleshooting — General Information 225
    Power On Self Test (POST)
    Intel-based Mac computers such as the iMac rely on a combination of tones and blinking LEDs to 
    display Power On Self Test (POST ) error codes.If the computer detects out-of-specification or no SDRAM or the RAM installed does not 
    • 
    meet the appropriate specifications, the screen will remain black but the computer will beep  
    This error condition may be due to physically damaged RAM, installing the incorrect type of 
    RAM, or not having RAM installed.
    Some RAM may appear to pass the Power-On-Self-Test (POST ) but still cannot be used by 
    • 
    the operating system. In this case, the computer will display a gray screen, sound three tones 
    and repeat the tones until the computer is turned off. 
    The solution to both of these situations is to first re-seat the memory and test the computer 
    • 
    again. If the memory fails the POST test again, try memory that has been verified to work 
    correctly on another system (i.e., “known-good”) or order new memory.
    Firmware Updates
    Firmware is the name given to software that is written into memory circuits such as flash 
    memory, that will hold the software code indefinitely, even when power is removed from the 
    hardware. Firmware on Intel Mac computers is designed to be updated if necessary through a 
    software update.
    Information about firmware versions for Intel Macs can be found in Knowledge Base article 
    303880 Mac OS X: Firmware Updates for Intel-based Macs.
    DDR Memory
    The iMac computer has two SDRAM slots in the bottom of the computer. The iMac ships from the 
    factory with at least 1 or 2 GB MB of DDR2 SDRAM installed, depending on the configuration.
    The maximum amount of RAM you can install in the iMac is 4 GB total. You can use small-outline 
    dual inline memory modules (SO-DIMMS) of 512 MB, 1 GB, or 2 GB that meet all of these criteria:PC2-5300
    • 
    unbuffered• 
    nonparity• 
    200-pin• 
    800 MHz DDR2 SDRAM• 
    Note:  DIMMs with any of the following features are NOT supported in this iMac:registers or buffers
    • 
    PLLs• 
    ECC• 
    parity• 
    EDO RAM•  
    						
    							iMac (20-inch Early 2008) Troubleshooting — General Information 226
    How to Reset the System Management Controller (SMC)
    The System Management Controller (SMC) is a chip on the logic board that controls all power 
    functions for your computer. If your computer is experiencing any power issue, resetting the SMC 
    may resolve it. The SMC controls several functions, including:Telling the computer when to turn on, turn off, sleep, wake, idle, and so forth.
    • 
    Handling system resets from various commands.• 
    Controlling the fans.• 
    Note that resetting the SMC does not reset the PRAM. Resetting the SMC will not resolve issues 
    in which your computer is unresponsive—in these situations, restarting your computer will 
    generally suffice. If your computer isn’t responding, perform these steps one at a time, in this 
    order, until the issue has been resolved:
    Force Quit (Option-Command-Escape).
    1. 
    Restart (Control-Command-Power).2.  
    Force Shut Down (press the power button for 10 seconds).3. 
    Resetting the SMC can resolve some computer issues such as not starting up, not displaying 
    video, sleep issues, fan noise issues, and so forth. If your computer still exhibits these types of 
    issues after you’ve restarted the computer, try resetting the SMC.
    To reset the SMC on an iMac
    From the Apple menu, choose Shut Down (or if the computer is not responding, hold the 1. 
    power button for approximately ten seconds until it powers off ).
    Unplug all cables from the computer, including the power cord.
    2.  
    Wait at least 15 seconds. The SMC reset occurs automatically once the iMac has been 3. 
    unplugged from the AC power source for several seconds.
    Plug the power cord back in, making sure the power button is not being pressed at the time. 
    4. 
    Reconnect your keyboard and mouse to the computer.5. 
    Press the power button on the back to start up your computer.6. 
    Note: The iMac does not have a button on the main logic board for resetting the SMC, which 
    eliminates the need to remove the computer’s cover to reset it (as required for some other 
    models). 
    						
    							iMac (20-inch Early 2008) Troubleshooting — General Information 227
    Diagnostic LEDs
    The iMac has four built-in diagnostic LEDs on the main logic board that can help you to 
    troubleshoot the computer. The four LEDs are located to the right of the memory slot, under the 
    hard drive data cable. Note: The cable has been removed in this photo.
    LED #1 
    Indicates that the trickle voltage from the power supply has been detected by the main • 
    logic board. This LED will remain ON whenever the iMac is connected to a working AC power 
    source. The LED will remain on even when the computer has been shut down or put to sleep. 
    The LED will turn off only if the AC power source is disconnected or the power supply is 
    faulty.
    LED #2 Indicates that the main logic board has detected proper power from the power supply when 
    • 
    the computer is turned on. This LED will be ON when the computer is turned on and the 
    power supply is working correctly.
    LED #3 Indicates that the computer and the video card are communicating. This LED will be ON 
    • 
    when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON 
    and you heard the startup sound, but LED 3 is OFF, then the video card might be installed 
    incorrectly or need replacement. 
    LED #4 Indicates that the computer and the LCD display panel are communicating. This LED will be 
    • 
    ON when the computer is turned on and video signal is being generated. If the LED is ON 
    and there is no image on the LCD display panel, the LCD display panel or inverter might be 
    installed incorrectly or need replacement.  
    						
    							iMac (20-inch Early 2008) Troubleshooting — General Information 228
    Apple Hardware Test (AHT) Error Codes
    The error codes listed below are the most likely codes that will be generated if a sensor cable is 
    disconnected, dislodged, or if the system has a faulty cable or sensor. Clicking on the link in the 
    chart will show you where the sensor and/or cable connector is located. Or, reference the next 
    two logic board topics which show a picture of the logic board and the sensor locations.
    AHT ErrorSuspected partSuggested ActionNotes
    4SNS/1/40000000:Tp0P
    Power supply sensor 
    pin/power supply Verify that the single 
    pin connector on the 
    top edge of the power 
    supply is present and 
    securely plugged in. If 
    connector is damaged 
    replace the DC Power 
    Supply cable.If this sensor 
    signal wire is 
    disconnected 
    or faulty the 
    CPU fan will run 
    at full speed.
    4SNS/1/40000000:T00P
    ODD temperature 
    sensor on ODD 
    damaged or cable 
    disconnected from MLB Verify that the sensor 
    cable is securely 
    connected to the 
    MLB and the sensor is 
    properly attached to 
    the ODD mechanism. 
    Replace the sensor if it 
    is damaged.If this sensor is 
    disconnected 
    from the MLB 
    or it is faulty 
    the ODD fan 
    will run at full 
    speed.
    4SNS/1/40000000:TA0P
    Ambient temperature 
    sensor damaged or 
    cable disconnected 
    from MLB Verify that the sensor 
    cable is securely 
    connected to the MLB 
    and the sensor attached 
    to the left speaker 
    housing properly 
    attached. Replace the 
    sensor if it is damaged.
    4SNS/1/40000000:TL0P
    LCD temperature 
    sensor on LCD 
    damaged or cable 
    disconnected from MLB Verify that the LCD 
    sensor cable is securely 
    connected to the 
    MLB and the sensor is 
    attached to the LCD. 
    Replace the sensor if it 
    is damaged.If the sensor is 
    disconnected 
    the CPU fan 
    will run at full 
    speed.
    4SNS/1/40000000:TG0H
    GPU temperature 
    sensor (located on 
    back side of MLB/video 
    card) is disconnected or 
    faulty. Verify that the two 
    pin sensor cable is 
    securely connected to 
    the MLB. The connector 
    is located on the back 
    side of the MLB and 
    requires removal of 
    the MLB to access it. If 
    it continues to fail the 
    diagnostic replace the 
    video card.If the sensor is 
    disconnected 
    the CPU fan 
    will run at full 
    speed. 
    						
    							iMac (20-inch Early 2008) Troubleshooting — General Information 229
    AHT ErrorSuspected partSuggested ActionNotes
    4SNS/1/40000000:TC0H
    CPU temperature 
    sensor (located on 
    back side of MLB) is 
    disconnected or faulty. Verify that the two 
    pin sensor cable is 
    securely connected to 
    the MLB. The connector 
    is located on the back 
    side of the MLB and 
    requires removal of 
    the MLB to access it. If 
    it continues to fail the 
    diagnostic replace the 
    logic board.If the sensor is 
    disconnected 
    the CPU fan 
    will run at full 
    speed.
    4MOT/1/40000002:ODD
    Optical drive blower/
    fan Verify that the ODD 
    blower/fan cable is 
    securely connected to 
    the MLB and that there 
    are no obstructions 
    that would prevent 
    the blower/fan from 
    rotating. Replace the 
    optical blower/fan if the 
    error continues.
    4MOT/1/40000002:HDD
    Hard drive blower/fan Verify that the hard 
    drive blower/fan cable 
    is securely connected to 
    the MLB and that there 
    are no obstructions 
    that would prevent 
    the blower/fan from 
    rotating. Replace the 
    hard drive blower/fan if 
    the error continues.
    4MOT/1/40000002:CPU
    CPU blower/fan Verify that the CPU 
    blower/fan cable is 
    securely connected to 
    the MLB and that there 
    are no obstructions 
    that would prevent 
    the blower/fan from 
    rotating. Replace the 
    CPU blower/fan if the 
    error continues. 
    						
    							iMac (20-inch Early 2008) Troubleshooting — General Information 230
    Logic Board Sensor and Fan Connector Locations, Top Side
    This is a photo of the top side of the iMac (20-inch Early 2008) logic board showing the sensor 
    and fan connector locations. 
    						
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