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diagnosing circuit boards

Discussion in 'General Electronics' started by kb, Jan 28, 2004.

  1. kb

    kb Guest

    Can someone offer me some basic guidlines on diagnosing circuit
    boards? I am in the process of working on a signal conditioning board
    with approximately 10 surface mount IC's (op amps, isolating amps,
    instrumentation amps etc). Some of these IC's are blown. I have no
    circuit diagram.

    My question is with the individual IC's. How feasible is it to test
    each IC individually (in circuit) to see if it is functioning
    properly? It would seem to me that just touching a multimeter to such
    sensitive circuits change the behavior and possibility not give a
    correct view of what is really happening (like open loop op amps and
    comparators).

    Anyway, any guidlines for diagnosing in-circuit IC's??

    thanks much - a confused beginner
     
  2. Quark Ng

    Quark Ng Guest


    Do you know what it was suppose to do if it was working? This info
    would really help.

    -Quark
     
  3. Ken Taylor

    Ken Taylor Guest

    You can generally test IC's in circuit, but without a schematic you'll be
    pushing the proverbial up-hill. I suggest tracing out the circuit, which
    will also give you an insight into how the circuit works.

    Ken
     
  4. kb

    kb Guest

    I do know what the output should be for a given set of test inputs for
    the overall board

    I'm not sure I could trace the board - it is multilayer (at least 3) -
    I guess its possible but seems very hard
     
  5. Bill Vajk

    Bill Vajk Guest

    In my personal experience a blown IC *usually* shows either an
    open or a short when an ohm meter is applied across the power
    terminals. I always use an analog meter and test both directions
    (polarities.)

    Before anyone jumps me on this, yes, there are other failure modes,
    but I wouldn't skip this simple test as it is pretty dispositive
    about strong failures and you might be able to find the fault
    that way.

    The next stage is to use a logic probe. Things start getting fun
    when you do that.
     
  6. JeffM

    JeffM Guest

    a blown IC *usually* shows either an open or a short
    and once you have determined it's a rail-to-gnd short at the board level,
    connecting a current-limited power supply to the rail
    to let the bad part get warm can sometimes help isolate it easily
    --the old wet finger test.
     
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