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Discussion in 'Electronic Repair' started by [email protected], Jun 9, 2007.

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  1. Guest

    No print.

    It only has about a volt drive to the HVOTs. These are FETs so that is
    obviously not enough.

    The drive is supplied by a totem pole arraingement like an audio
    output stage. Actually I wonder why they bothered to bias it, but the
    did and there are emitter resistors.

    This is where it gets wierd. On the diode scale at the gates of the
    FETs I read about 0.6 to ground. On resistance it reads like 47K ohms.
    when I lift the base of the bottom (PNP) transistor it is open
    circuit, however what perplexes me is that the base of the transistor
    reads about 1.4 to ground in circuit. How can it cause more of a short
    than it has. The set is off and powered down.

    I alsmost thought the transistor might be amplifying the current from
    my DVM, but that doesn't fly. The IC that drives this mess is an MC
    1391, and has an open collector output. The set is off, there is no
    current to turn on it's interal output transistor. I changed it anyway
    with no change in readings or symptoms.

    I can understsand that the reading at the base is caused by the 220
    ohm pullup resistor loading it to a supply that is off. But how does
    that cause a 0.6 reading at the emitter ?

    I have now changed both transistors and the IC, and there doesn't
    appear to be anything else in the circuit. What am I missing here ? I
    see no jumper wires, and it is not a two sided board, the circuit is
    simple, is this a brainfart or something ? The circuit is simply not
    that hard to figure out without a print, but who knows. I see there is
    a foil going off to who knows where, but none of this has any
    continuity to it and it seems to not be used at all.

    Perhaps I should start getting up in the wee hours and make donuts.

    Dammit, even if that PNP transistor was conducting the emitter reading
    would be higher than the base reading. Doesn't that make sense ? But
    when you disconnect the base, the emitter then reads open. How the
    hell can something do this ? The collector of the transistor is
    grounded, but even if it did some amplification,,,,,,,,,,, this kinda
    had my head spinning. First time I got screwed up readings that made
    no sense was in those old NECs with a coolant leak, and I did indeed
    check for coolant on the board, it is dry.

    The diode and resistor that provide bias for the stage might be over a
    ground trace, so I thought of the glue, but that still doesn't explain
    the lower reading at the emitter.

    If I don't get a lightbulb over my head I guess Monday I'll call my
    old shop and see if they have the print, if so maybe they'll email me
    the DWG files. I am fairly sure they have it for the DTC307 which I am
    pretty sure uses the same configuration.

    Any ideas ?

    Thanks in advance.

  2. Guest

    The parts are cheap enough, so buy some generic ones (not NEC) and
    replace them all at the same time. Do a power on test (forget your
    meter for now). I don't know what the symptoms are, but sometimes we
    can think this stuff to death when the fix is really the most obvious.
    Then, look for something odd at the imput or output (load?).

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