MODULATOR DRIVER ALARM

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    Asad Javed
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    MODULATOR DRIVER ALARM:

    A MODULATOR DRIVER alarm indicates that a fault
    exists in the bi-phase PDM drive signals. It may be
    caused by a fault on the bi-phase PDM driver PWB
    (1A2A3, 1A2A6), a fault in the detection circuitry on
    the exciter interface PWB (1A2A1), a fault on the
    50kW distribution PWBs (1A36, A140) or a fault in
    the alarm circuitry on the system controller PWB.
    Trouble shoot a MODULATOR DRIVER alarm as
    follows:
    (a)If the transmitter successfully switched to the
    standby exciter, suspect the main exciter’s bi-
    phase PDM driver PWB. See the instruction
    manual for the bi-phase PDM driver PWB.
    (b)Use an oscilloscope to view the signals at TB2-
    10 and TB2-12 on the 50kW distribution PWBs
    (1A36 and 1A40). The signals should be a
    PDM pulse train, 0 to +15 volts, 180 degrees out
    of phase with each other, with equal duty cycle.
    If signals are incorrect, go to paragraph (e).
    (c)If signals in step (b) are correct, use a
    multimeter to measure the signals at TP3, TP4
    and CR30-anode (with respect to ground) of the
    exciter interface PWB. The signal measured at
    TP3 and TP4 should be between 5.1 volts and
    the DC level measured at CR30-anode.

    (d)If signals in step (c) are correct, measure DC
    voltage at U1-14 on the exciter interface PWB.
    If approximately 0V, suspect the alarm circuitry
    on the system controller PWB (see figures SD-8
    thru SD-11). If approximately 5V, suspect the
    detection circuitry on the exciter interface PWB
    (see figures SD-13 and SD-14).
    (e)Use an oscilloscope to view the signals at TP16
    and TP17 of the bi-phase PDM driver PWB
    (1A2A3, 1A2A6). If the signals are not as
    described in paragraph (b), suspect the bi-phase
    PDM driver PWB (see bi-phase PDM driver
    PWB manual).

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