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
(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