Thursday, March 10, 2016

Apple iMac 27 inch - Ram resetting procedure – System management controller reset – Battery removal – Logic board test pints - Desktop PC repair and service

Category: Desktop PC repair and service  

Contents of this article 

  • Resetting management controller 
  • Resetting RAM
  • Logic board test points 

Apple iMac 27 inch 

Resetting the System Management Controller (SMC)
The System Management Controller (SMC) is a chip on logic board that controls all power functions. If computer is experiencing any power issue, such as not starting up, not displaying video, sleep issues, battery management, or fan noise issues, resetting SMC may resolve it. To reset SMC on an iMac:
1. Shut down the computer.
2. Unplug all cables from computer, including power cord.
3. Press and hold the power button for 5 seconds.
4. Release the power button.
5. Attach the computer’s power cable.
Note: If you press the power button while inserting the power cord, the iMac will enter a mode in which the fans run at full speed. For more information, refer to kbase article
TS1433: iMac: Fans run at full speed after computer turns on
6. Press the power button to turn on the computer.
Resetting Parameter RAM (PRAM)
PRAM stores certain system and device settings in a location that Mac OS X can access quickly. Exactly which settings are stored in the computer’s PRAM varies depending on the type of computer as well as the types of devices and drives connected. To reset PRAM:
1. Shut down the computer.
2. Locate the following keys on keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in Step 4.
Note: If the keyboard does not have an Option key, use the Alt key instead.
3. Press power button.
4. Immediately press and hold Command-Option-P-R keys.
Important: You must press this key combination before the gray screen appears.
5. Hold down keys until the computer restarts, and you hear the startup chime a second time.
6. Release keys.
Removing the Battery, Measuring DC Voltage
1. Unplug the computer. Allow several minutes for power supply to discharge.
2. Remove logic board.
3. Remove coin battery for 1-2 minutes. The coin battery is located on the back side of the logic board (see graphic below).
4. Measure DC voltage on battery touching battery with red probe, and grounding with black probe. If voltage is 2.7v or less, replace battery.
5. Reinstall the battery and logic board. This will reset the logic board.
6. Power on computer.
7. If computer boots, check for and install all software and firmware updates.
Sensor Locations
Temp sensors and fan connector locations are shown in the next two graphics. Ensure cables are correctly routed and the sensors and fans are properly connected. If a sensor or fan is faulty or not connected, Apple Hardware Test will generate an error code.
Logic Board Test Points
Some test points are accessible on the logic board when the LCD panel is removed. The test points are defined below.

Logic Board Test Point
Pin 1

Pin 3

5V power for Hard Drive(s). Note: If present, the 12V power will
come directly from the Power Supply to the Hard Drive thru the
power harness cable.

Pin 4

Standby 12V power (permanent power coming from power
supply and present as long as AC cable is connected, even if
computer is off); corresponds to LED #1

Pin 6

Backlight Control pulse width modulated signal (from logic board
to LED backlight board, to adjust the backlight level setting
according to user setup)

Pin 11

12V Run-Mode power to logic board (coming from power supply,
present as long as system is on or asleep); corresponds to LED #2


Power On Request signal (from logic board to power supply
when power button is pressed)

Pin 13
Backlight Enable (signal from logic board to Backlight Controller
board, to enable backlight