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 
bottom side of the logic board and then proceed to step 3. Do  NOT press the PMU reset switch a 
second time because it could crash the PMU chip.
4 WAIT ten seconds before connecting the power cord and powering the compu\
ter on. If the  computer powers on, go to the next step. If the computer does not power \
on, there is 
something else wrong with the computer, refer to the symptom/cure, “N\
o Power” in this 
chapter.
5. Run Apple Service Diagnostic or Apple Hardware Test for the iMac (USB 2\
.0) models and 
return the computer to the customer.
Note: This entire procedure resets the computer’s PRAM. Be sure to reset t\
he computer’s time, 
date and other system parameter settings before returning the computer t\
o the customer. 
						

							
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 the PMU chip (because the PMU chip ke\
eps time and must 
always be running) when the computer is unplugged from the wall (AC po\
wer). The PMU is 
very 
sensitive and touching any circuitry that is connected to the PMU can ca\
use it to crash. If the 
PMU crashes, the battery life goes from about five years to about two da\
ys if the PMU is not 
reset. Once the battery goes dead, the PMU will reset the time and date \
every time the AC power is 
removed. To fix this situation, replace the battery and reset the PMU (\
refer to “Resetting the 
PMU on the Logic Board” mentioned earlier in this chapter).
If the computer has a “No Power” situation, check the battery befo\
re replacing modules. 
Testing the Battery
1. Remove the user access door on the bottom of the computer.
2. Locate the battery test point.
3. With the computer powered off, measure the battery voltage. Does the battery measure at least +3.5v? 
If no, replace the battery and reset the PMU. If the battery measures +3.5v or higher, reinstall the 
battery and reset the PMU as above.  
						

							
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 Paste Application” for detailed \
information. 
						

							
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 (922-4757)
Paste Location 
						

							
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 driver, you will have to make sure these screws are tightened by hand FIRMLY, BUT 
NOT FORCIBLY. Or, purchase the service tool (076-0899) in order to ensure the thermal pipe is firmly 
mated with the top base. If the bottom housing is not securely attached to the base in this fashion, the 
CPU may overheat and become damaged. 
						

							
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 
exception would be if you are simply testing the logic board and only detach it for a few minutes), does not 
require replacement.
Procedure
1. Remove the old thermal pads (circled below) from the bottom housing. Thoroughly clean the bottom  housing surfaces after removing the pads.  Note: If you don’t see all three thermal pads on the bottom 
housing, check the bottom side of the logic board. 
2. Using the thermal pad kit, remove the clear protective backing from the new thermal pads. 
3. Place the new thermal pads in the bottom housing.  Note: Avoid unnecessary contact with either side 
of the thermal pad as dirt and body oils reduce the thermal pad's conductivity.
4. Press down on the blue protective backing to make sure each thermal pad has even contact with the  bottom housing. There should be no air pockets.
5. Remove the blue protective backing from the new thermal pads. 
6. Replace the logic board. Holding the logic board by the battery retainer and the internal drive cable  connector, slide the logic board back into the bottom housing (make sure the I/O port covers are on the 
logic board).  
						

							
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 before you proceed to the next cure.
Apple Hardware Test
Apple Hardware Test is a diagnostic tool for detecting problems with Apple internal hardware components 
such as logic board, memory, modem, video RAM, and Apple AirPort Card. The test does not check 
externally connected hardware components such as USB or FireWire devices; it also does not check non-
Apple devices such as third-party PCI cards. 
Note:  The most recent Apple Hardware Test software can be downloaded from the CD Images page on 
AppleCare Service Source; go to the Service Source home page and select ‘Disc Images’ from the quick-
click table.
Error Codes
If Apple Hardware Test detects a problem with a computer, it displays an error code, which is defined in the 
product's Error Code List. Consult the list for appropriate repair procedures associated with the code.
All Error Code Lists are included in the Service Diagnostics Matrix, Knowledge Base article 112125. You can 
also access this matrix by clicking the Service Diagnostics rollover link under any of the product areas on the 
Service Source home page. 
						

							
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 peripherals such as speakers.
5. Disconnect the power cord, place the computer in the service stand, and remove the user access  plate.
6. Reset PMU. Refer to “Resetting the PMU on the Logic Board” mentioned earlier in this chapter.
7. Using a voltmeter, check the voltage on the battery test point (see graphic below). If the reading is over  3.5 volts, go to the next step. If the reading is under 3.5 volts, replace the battery and test again. 
8. Plug the unit in, but do  NOT press the power button. Using a voltmeter, check the voltage from the test 
point marked “12VSLP” (see graphic above). You should get a reading of approximately 12 volts. If the 
reading is 12 volts, go to the next step. If you don’t get a 12 volt reading, go to step 11.
9. Press the power switch on the computer. Using a voltmeter, check the voltage on the test point marked  “12V” (see graphic above). You should get a reading of approximately 12 volts. If the reading is 12 
volts, go to the next step. If you do not get a 12 volt reading, replace the main logic board.
10. Using a voltmeter, check the voltage on the test point marked “5V” (see graphic above). You should get  a reading of approximately 5 volts. If the reading is 5 volts, go to the next step. If you do not get a 5 volt 
reading, replace the main logic board. 
						

							
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 thermal pipe.
• The bottom housing has four torx screws that must be tightened to at lea\
st 17 in.-lbs. If you do not  have a torque driver, you will have to make sure these screws are tighte\
ned by hand FIRMLY, BUT 
NOT FORCIBLY. Or, purchase the service tool (076-0899) in order to ens\
ure the thermal pipe is 
firmly mated with the top base. If the bottom housing is not securely at\
tached to the base in this 
fashion, the CPU may overheat and become damaged.
12. Check power supply output. Disconnect the power supply cable from the logic board. Measure power at  the power supply connector by touching the black probe to the ground pin, and using the red probe to 
measure power at the pins indicated in the graphic below. Did you measure +12v at each point? If yes, 
replace the logic board. If no, replace the power supply.   
13. Replace the inverter board.
14. Replace the neck assembly.