the reference voltage at the junction of R18 and R19 by
except that comparator U2-C responds only to the output
comparator U1-D. The time constant of C9/R17 is such
from modulation detector and threshold comparator.
that during normal operation the voltage at pin 11 of U1-
During normal operation Q11 is saturated, and +24V is
D never exceeds the reference voltage at pin 10 of U1-D
(Q6 discharges C9 on each positive transition for U1-A,
thus preventing C9 from being charged continuously),
and the output from comparator U1-D (pin 13) is low.
The +24V for the rf oscillator in the exciter
Transistors Q7 and Q8 are therefore switched on and
assembly A1A2 is routed through the normal
provide indication of normal operation as follows
link or special link (fig. FO-1) as determined
(a) Shutdown relay K1 is energized by Q7 and
at the time of installation. When normal link
the +24V from power supply AlA1 is applied, through
is installed (special link removed), the + 24V
contacts 4-7 of K1 of the rf oscillator in the exciter
output from the special shutdown circuit (P1-
assembly A1A2 (fig. FO-1).
1, fig FO-10) is not used. When special link
(b) ALARM indicator DS2 is turned off.
is installed (normal link removed), the output
(c) NORM indicator DS1 is turned on by Q8.
from the normal shutdown control circuit still
(3) If the output from U1-C is low (carrier too
controls the shutdown relay K1, but the +24V
low) or the output from U1-1 does not change state (loss
for the rf oscillator in the exciter assembly
of keying), Q6 assumes a steady off state. Thus, C9 is
A1A2 is taken from P1-1 (fig FO-10).
charged continuously through R17 and, after a delay of
e. Modulation Detector. This circuit is used to
20-35 seconds, pm 11 of U1-D becomes more positive
provide a reading of the modulation depth on the control
than the reference voltage at pin 10 of U1-D. Under this
panel TEST meter. The input at P1-13, obtained from
condition, the output of U1-D changes state from low to
the power probe A1A6, is applied to comparator U2-A
high, and transistors Q7/Q8 are both switched off,
which operates as a minimum voltage detector, i.e., the
resulting in the following alarm condition.
output of U2-A assumes a steady potential equal to the
(a) Shutdown relay K1 is deenergized and the
minimum positive excursion of the input signal. The time
+24V for the rf oscillator in the exciter assembly A1A2 is
constant of U2-A is chosen so that voltage at pin 2 of U2-
A follows the keying signal. When the NDB transmitter is
effectively switched off (no rf output).
operating without modulation, the output from U2-A is a
(b) ALARM indicator DS2 is turned on.
voltage proportional to the carrier level.
(c) NORM indicator DS1 is turned off.
modulation is keyed on, the level falls abruptly, reaching
(4) NORM/BYP switch S1 applies a ground
zero for 100% modulation Comparator U2-B is used as a
(low) to either P1-2 or P1-3.
During testing or
voltage-follower, providing a high input impedance and a
troubleshooting, S1 is set to BYP position. Thus, Q7 is
low output impedance The output from U2-B is
held conducting regardless of the output level of U1-D,
connected to the voltage divider formed by set zero
ensuring that the system cannot shutdown. Transistor
The wiper of ZERO SET
Q8, however, is still controlled by the output of U1-D. At
potentiometer R1 is connected, through the TEST meter
the same time, the BYP setting of S1 removes the
M1 and MOD % CAL potentiometer R4, to a fixed dc
ground from P1-3, effectively removing C9 from the
potential. The circuit forms a bridge, balanced by
collector of Q6 and eliminating the time delay formed by
potentiometers R4 and R1. When modulation is off,
R17/C9. Instantaneous indication of the transmitter
potentiometer R1 is adjusted for zero reading on the
status is, therefore, provided at P1-5, as Q8/DS1 are
TEST meter M1. When 100% modulation is present or
switched on and off at the keying rate.
is simulated by removing the rf input at P1-13,
d. Special Shutdown Circuit. This circuit is formed
potentiometer R4 is adjusted to read full scale deflection
by U2-C, Q9 through Q11 and associated components
of TEST meter M1
The operation of the special shutdown circuit is similar to
the normal shutdown control circuit described in c above,