Noise Problem..

Discussion in 'General Electronics Chat' started by whoolieshop, Nov 20, 2013.

  1. whoolieshop

    Thread Starter New Member

    Jun 11, 2012
    12
    0
    I have two identical PCB's

    PCB A functions normally
    PCB B when powered up has a ton of noise on the 5volt rail and the 12 volt rail, I assume this is a ground loop problem?

    Ground is the top right pad, a 5 volt regulator is just to the left of it.

    Just to the left of that, a .01 uf cap, 33pf cap, 200k ohm resistor, then a dip 8 IC.

    The line below the +12v line is an audio line fed from a portable MP3 player / iphone. The shield from the audio cable is soldered to the ground plane on the PCB.

    [​IMG]

    Here are two pictures of the 5v line.
    PCB A is the first one with minimal noise.. This doesen't cause any problems..

    [​IMG]


    This is PCB B everything connected the same, and this one has a ton of noise on the 5v line. Any ideas?
    [​IMG]
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,042
    3,243
    Looks like something is oscillating. I would look for missing or faulty filter caps.
     
  3. whoolieshop

    Thread Starter New Member

    Jun 11, 2012
    12
    0
    Found It!

    Ended up being a faulty transistor on one of the output lines that was causing the noise. I triggered them individually with a test micro and noticed the noise only appeared when one pin was pulled high. The only thing else on that trace was a resistor and transistor, removed/replaced the transistor and the problem went away!
     
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