IBM-AUSTRIA - PC-HW-Support    30 Aug 1999

Checking the Voltage Converter (720, 720C)

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.

   NOTE:

   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

    

   •  If any voltage is not correct, go to step 9
   •  If all voltages are correct, the voltage converter is  operating correctly. If the power problem did no reoccur, reconnect  the devices removed in step 2 to isolate a device problem.
   •  If all the voltages are correct and the power problem still exists, replace  the following FRUs one by one.
     •  System board
     •  Processor card

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.
    •  If there is a short circuit (less than 10 ohms), remove the FRUs indicated  one at a time until the short circuit disappears. Replace the FRU  causing the short circuit.

    FRU	VA	VG	VB	VA3	VB3
    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.
    •  Voltage converter
    •  System board

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