LT1940 problem

Discussion in 'Hardware troubles and Repair support.' started by Jane, Nov 9, 2016.

  1. Jane

    Jane Member

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    Johnny,
    Did you already solve a problem with U1104 when 3.3V is present while 5V not?
    FB1 pin supplies correct 1.25V, but Vc1 pin only 0.3V
    SW1 is only 1.6V
    That LT1940 is working correctly on a different PCB.
    I disconnected R1125 (to prevent other parts to cause that problem) but still no 5V on C1164( only about 1.6V)
    I measured again on PCB and found out that disconnecting only R1125 does not disconnect LT1940 from outside completly.E.g. U1101( LM2742) still has 5V for its Vcc.
    Do you have any idea what resistors remove to disconnect LT1940 completely?
    Thank you
     
  2. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    Yeah amazing, I did not have made a drawing which shows the vcc inputs of the Lm's, only the 12v input and outputs.
    But to answer your question, this I can not give earlier than friday night.
    Only than I have the possibility to measure it from a clean motherboard.
     
  3. Jane

    Jane Member

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    Johnny, it was my fault. I did not checked that 5V, as Vin for LM2742 ,is supplied from U1109. So, R1125 does disconnect LT1940 from outside completly.
    I also found out my problem It was a capacitor connected to PG1 of LT1940.
    Thanks for your time.
     
    Last edited: Nov 10, 2016
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  4. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    Clever found, and thanks for your given solution. :)