Checking the Voltage Converter (720, 720C)

Checking the Voltage Converter (720, 720C)

Use the following procedure to isolate a problem with the ThinkPad 720, 720C voltage converter.
  1.  Power-off the computer.
  2.  Power-off all attached devices and disconnect them from the computer.
  3.  Unplug the AC adapter cable, then remove the battery pack  from the computer.
  4.  Carefully place the computer bottom-side up  and remove the bottom cover.
  5.  Plug the AC adapter cable into the computer.
  6.  Open the LCD half-way, then stand the computer  on its front edges so that the suspend switch is not activated.
  7.  Power-on the computer.
  8.  Check the voltages of the voltage converter on the system board.


    1.  Make sure the suspend switch (located to the left above the keyboard)  is not activated during measurement of the voltage.
    2.  Only PG is marked on the system board.  Use the following figure to locate the VA, VG, VB, VA3, and VB3  signal names.


    Signal Vdc Min. Vdc Max.
    VA +4.75 +5.25
    VG +4.75 +5.25
    VB +4.75 +5.25
    VA3 +3.42 +3.78
    VB3 +3.42 +3.78
    PWRGOOD +4.00 +5.25


  9.  Power-off the computer and unplug the AC adapter cable  from the computer.
  10.  Check for a short circuit between Ground and VA, VG, VB, VA3 or VB3. The  following table shows which FRUs can cause a short at each signal.

    Voltage converter x x x x  
    Processor card     x   x
    System board x x x x x
    Video card x   x x x
    Base memory card x        
    Keyboard control card x   x    
    Math coprocessor     x   x
    Diskette drive     x    
    Hard disk drive     x    
    LCD x        
    Indicator assembly   x x    


  11.  If there is no short circuit, replace the following  FRUs one at a time.

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