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common failure mode for digital transisors?

Discussion in 'Electronic Design' started by Maynard A. Philbrook Jr., Dec 30, 2013.

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  1. I do some repair work now and then and I got a HF transceiver
    that I once replaced a DTC114ES digital (pre biased) transistor.

    It didn't appear shorted, it just wouldn't turn on. I just got
    a call from the owner and from what he's telling me and from what
    I can gather in the schematic, it appears that maybe another
    digital transistor of the same type has failed, but only in a different
    location?

    This unit is rather old, it uses through hole digital transistors. Was
    there some fab problems back then that made those fail internally over
    time ?

    Jamie
     
  2. No, these have nothing to do with power RF actually.

    These are just switching transistors with pre-biased networks in them.

    This unit has a slew of them in various places. It's possible the ones
    that are failing could be exerting more current than others in there.

    This one that failed this time actives the early stages for the
    transmitter, as did the first one that failed but for the VOX.

    I used a IR camera on the unit the last time to measure possible
    heating after I replaced the first failed digital transistor and I saw
    nothing alarming.

    I remember years ago when I was a kid doing TV repair work for side
    cash, there was a series of TV's that were using this one particular
    transistor that always seem to fail, those that were not failed you
    could tap on them and they would fail intermittently.. I can't be sure
    but these may have been in the audio section as part of a driver. We
    stocked
    up on lots of subs for that unit :)

    Jamie
     
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