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Question about transistor failure modes

Discussion in 'Electronic Design' started by Wim Ton, Feb 18, 2007.

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  1. Wim Ton

    Wim Ton Guest

    Hi,

    While restoring an ancient HP5245 counter, I had to replace several
    transistors, most of them 2N708 or 2N709 and one 2N3640

    Most of them showed perfectly normal DC characteristics on the small 'Atlas'
    tester.

    How do these transistors fail and is there a way to test this easily (except
    for replacing them one by one ;-)

    TIA, Wim Ton
     
  2. Jamie

    Jamie Guest

    You should have tried to wash the board and parts first.
    if you ask me, it sounds like you had a substance that may
    have collected on the surface over the years.
    you would be surprised and how high freq things work after
    washing down a board with parts on.
     
  3. Eeyore

    Eeyore Guest

    What do you mean by normal DC characteristics ? Did they have any current gain ?

    Graham
     
  4. D from BC

    D from BC Guest

    Somewhat relevant....
    Just yesterday I was just going through the achieves and read posts on
    how reverse biasing a BE transistor junction is damaging.
    I'm working on a design where, upon unit power down, a BE junction
    gets reversed.. So the more often the unit is turned off the more
    supposed transistor damage.
    D from BC
     
  5. Guest

    There was a tale told when I was youngish - around 1978 - about fixing
    HP counters by stubbing out a lighted cigarette on the metal can of
    one of the transistors.

    Apparently the base-emitter junction of the affected part was
    regularly reverse biased into avalanche breakdown. The avalanche
    current was small, and the avalanche was brief, so the transistor
    wasn't destroyed, but the forward current gain was progressively
    degraded.

    The cigarette was supposed to get the transistor junction hot enough
    to anneal out the damage.

    Measuring forward current gain in-circuit could be a bit difficult.
     
  6. colin

    colin Guest

    Yes ive had a few designs suffer from unforseen reverse vbe.
    some can be rather sensitive, especialy RF sections that burst into
    catastrophic oscillation.

    IME 8/10 transistor failures are short circuit, shortly folowed by open
    circuit if the current is not limited, or transistor ejecting itself off the
    board.

    Colin =^.^=
     
  7. Jamie

    Jamie Guest

    Be junctions have zener effects.
    commonly around the 6 volt region.
    the Vbe in reverse that is.
     
  8. D from BC

    D from BC Guest

    I should add...
    The achieved discussions I read was about the damage in the BE reverse
    breakdown (zener ..also called avalanche IIRC) region.
    I'm guessing the transistor is ok with a BE junction reversed biased
    before that point. Any more and zener current starts kicking in and I
    suppose heat is the killer.
    IIRC somebody posted that the damage is accumulative and leads to
    eventual transistor failure.

    I might have an example of this:
    http://focus.ti.com/lit/ds/symlink/tl431.pdf
    Figure 24.
    Change the R's for say 20V output.
    Put a cap on the output..Charge up the cct..
    Then ground out Vin to imitate other loads present when the power is
    turned off.
    Doesn't it look like the transistor BE junction gets a reverse hit?
    If so, this could be a design flaw that can sneak through product
    testing..
    Might be part of mysterious transistor failure..
    D from BC
     
  9. D from BC

    D from BC Guest

    ooops spellcheck goof..."archived discussions" not achieved.
    D from BC
     
  10. Gibbo

    Gibbo Guest

    It's a pretty common input PSU circuit and a pretty common solution is
    to put a diode between be, reverse connected of course.
     
  11. Fred Bartoli

    Fred Bartoli Guest

    a écrit :
    Letting the magic smoke getting back in?
     
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