oscilloscope troubleshooting

Discussion in 'General Electronics Chat' started by egloss25, May 28, 2014.

  1. egloss25

    Thread Starter New Member

    May 28, 2014
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    0
    hey guys,

    i am trying to measure SMD's a PCB and i am getting no wave on my oscilloscope, i have confirmed multiple times that the scope is functioning properly and i also have power going to the PCB board.

    regardless, i am still unable to get a wave or measurement, it is a subscriber module used for networking, and i am really banging my head against the wall because i cannot get a reading, when i do get a reading, it is across the diodes and i am getting a square wave, but i get a flat line across most other components.

    This is the type of wave i am getting on AC coupling across the emitter and base of a transistor, and i measured across the inductor and all i got was a screen full of noise. what is the best way to measure SMD's and get a accurate reading and normal wave on the scope?

    what settings should the scope be on? does it matter when using surface mount devices?

    i cannot release pics of the board or info about it do to company i work for.

    basic knowledge of SMD's and how to measure them with a oscilloscope it good enough for me!

    thanks!
     
    Last edited: May 28, 2014
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the waveform you have is in the power supply. if you try to get a waveform in the rf sections of the module, you probably wont get anything unless the scope you are using is an extremely high speed one. you might have to use a spectrum analyzer to troubleshoot those subscriber modules rf section.
     
  3. egloss25

    Thread Starter New Member

    May 28, 2014
    5
    0
    where the cpu is located on the board, would that be included in the RF section? there is a emi shield that is blocking the RF section. i have investigated under the shield, not i haven't even bothered to take measurements due to wanting to know about the rest of the board before i tackle that. could it be that besides the power supple part of the board (which also has the RJ45 connected to it) the rest of the board is RF?

    just trying to brainstorm! :)
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,344
    Hello,

    It looks like there is no good ground.
    When there is no good ground you will pick-up a lot of "hum", wich has the frequency of the net.

    Bertus
     
  5. egloss25

    Thread Starter New Member

    May 28, 2014
    5
    0
    where is the best place to put the ground? is there a general place that can act as a sufficient ground?
     
    Last edited: May 28, 2014
  6. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,344
    Hello,

    Most times the minus connection of the powersupply is ground.

    Do you have a picture of how you made the measurement shown in the opening post?

    Bertus
     
  7. egloss25

    Thread Starter New Member

    May 28, 2014
    5
    0
    this is the board, i am having to use a tissue to block out the company name.
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,344
    Hello,

    I have marked some points that might work as ground for the scope probe:

    [​IMG]

    Bertus
     
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