Apple imac usb 2 0 User Manual

6 - iMac (USB 2.0) Troubleshooting
 General Information
Resetting the PMU on the Logic Board
Resetting the PMU (Power Management Unit) on the logic board can resol\
ve many system 
problems. Whenever you have a unit that fails to power up, you should fo\
llow this procedure 
before replacing any modules.
1. Disconnect the power cord.
2. Remove the user access panel from the bottom of the computer.
3. Press the PMU reset switch (located under the protective cover, next to the test points) once on the...

iMac (USB 2.0) Troubleshooting - 7
 General Information
Diagnostic Test Points
The iMac has four test points and one ground pad on the bottom of the logic board. These test points are 
accessible under the customer access panel via small cutouts in the plastic shie\
lding (see below).
These test points provide an easy troubleshooting tool for common problems. To use each test point, put the 
positive probe of your voltmeter on the test point, and the negative probe on the ground pad. 

8 - iMac (USB 2.0) Troubleshooting
 General Information
Logic Board Battery
Important: Apple highly recommends removing the battery when handling the logic boa\
rd. Make 
sure to use proper ESD protection when handling modules.
The battery on the logic board controls the stored system settings, such\
 as date and time. It is 
only necessary to test the battery when you can’t power on the comput\
er, or the date and time are 
reset every time the AC power is removed. 
The battery is also used to power...

iMac (USB 2.0) Troubleshooting - 9
 General Information 4. Connect the power cord and power up the system again. 
Warning:
 Whenever the bottom housing is opened for service, you must do two things:
1. You must clean and reapply thermal paste to the thermal pipe surface.\
2. You must tighten the four torx bolts on the base unit to a minimum of\
 17 in.– lbs. Failure to follow 
either of these steps could cause the computer to overheat nd damage int\
ernal components. 
Refer to the next topic, “Thermal...

10 - iMac (USB 2.0) Troubleshooting
 General Information
Thermal Paste Application
The following instructions explain how to apply thermal paste to the thermal pipe in the iMac (USB 2.0) 
computer. Failure to follow these instructions could cause the computer to overhe\
at and damage internal 
components.
Tools
This procedure requires the following tools:
• Plastic stylus or plastic spatula to remove the old thermal paste
• Plastic stylus or plastic spatula to spread the thermal paste
• Thermal paste...

iMac (USB 2.0) Troubleshooting - 11
 General Information
Procedure
1. Thoroughly clean the original thermal film from the four mating surfaces circled below. Use a plastic 
stylus to scrape the surfaces clean.  Note:  Do not use an abrasive material or liquid cleaner.
2. Squeeze a drop of thermal paste onto the thermal pipe locations circled below. 
3. Close the bottom housing.  Warning:  The bottom housing has four torx screws that must be tightened to at least 17 in.-lbs. If you do 
not have a torque...

12 - iMac (USB 2.0) Troubleshooting
 General Information
Thermal Pad Replacement Instructions
Whenever the logic board is separated from the bottom housing, you must install new thermal pads to 
three surfaces on the bottom housing. The thermal pads help cool components on the logic board. Failure 
to apply these pads whenever the logic board is separated from the bottom housing could cause these 
parts to overheat. Short term separation, where the thermal pads are not handled excessively 
(an...

iMac (USB 2.0) Troubleshooting - 13
 Symptom Charts
Symptom Charts
How to Use the Symptom Charts
The Symptom Charts included in this chapter will help you diagnose specific symptoms related to the 
product. Because cures are listed on the charts in the order of most likely solution, try the cures in the order 
presented. Verify whether or not the product continues to exhibit the symptom. If the symptom persists, try 
the next cure. 
Note:   If you have replaced a module, reinstall the original module...

14 - iMac (USB 2.0) Troubleshooting
No Power Symptom Charts
No Power 
No fan, no hard drive noise, and the screen is black.
1. Verify the power outlet is good. Plug a different device into the socket to ensure there is power, or plug 
computer into another outlet.
2. Check the power cord. Use a known good power cord.
3. Check connection of the power cord on both ends. Verify that the plug is securely plugged into both the  A/C outlet and back of the computer.
4. Remove keyboard, mouse, and other...

iMac (USB 2.0) Troubleshooting - 15
No Power Symptom Charts 11. Remove the bottom housing.   Check that al
l the cables on the logic board are securely connected. Pay 
special attention to the video connector. Power on the unit again. If you still have no power, go on to the 
next step.
Warning:   Whenever the bottom housing is opened for service, you must  do two things:
• You must clean the original thermal film from the surfaces joining the t\
hermal interface layer and  reapply thermal paste to the...