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TM 11-5825-271-34
ent filter configurations, set up by installing appropriate
below the threshold level set by R2. Shutdown does not
links on terminal blocks on the rf filter A1A5, cover the
take place if the carrier level falls below the threshold
complete operating frequency range of the NDB
setting of R3, however, the NORM light DS1 is turned
off, the ALARM light DS2 is turned on, and the
transmitter status monitoring contacts (K1 contacts 2-5)
become open-circuit When the special shutdown circuit
2-9. Power Probe A1A6
is used, the special link must be in, and the normal link
The power probe AlA6 detects the level of rf output and
must be out
performs the following functions.
d. The modulation detector monitors the rf level
(1) Supplies a current limit signal to the monitor
and provides the modulation reading for the metering
assembly A1A2 which keeps the rf current within safe
circuit With the modulation removed, the zero set control
limits in the event of a malfunction (e g, partial or
R1 is adjusted td provide a meter reading of 0%. The
complete short circuit) at the output of the NDB
tone level detector monitors the output of the one
oscillator located m the exciter assembly A1A2.  The
(2) Supplies the rf level signal to the monitor
output from the tone level detector is also fed to the
assembly A1A7 for modulation and carrier detection
metering circuit.
(3) Provides  forward  and  reflected  power
2-11. Metering
signals for use by the metering circuits.
(4) Provides  final  output  from  the  NDB
The TEST switch S2 and TE ST meter M1 are used to
transmitter (50 watts maximum) The output from the
monitor all pertinent parameters of the NDB transmitter
power probe AlA6 is fed to the ATU through TB1-2/3 and
Calibration controls R5 and R6 are provided to calibrate
J3 using a 50 ohm coaxial cable.
the meter for forward/reflected power and dc current
respectively. The TEST switch S2 also permits checking
of the modulation % (percentage), unregulated and
2-10. Monitor Assembly A1A7
regulated voltages, tone level, dc current and the rf drive
a. The  monitor  assembly  AlA7  contains  the
normal/special shutdown control circuits.  The rf level
from the power probe A1A6 is routed to the monitor
2-12. ATU
assembly A1A7 through R2 and R3, which set the
a. The ATU matches the antenna impedance to the
threshold levels for the modulation and carrier re-
spectively When the NDB transmitter is operating
50-ohm impedance of the NDB transmitter, automatically
properly, shutdown relay K1 is energized, normal light
compensating for variations in the antenna reactance
DS1 is on, alarm light DS2 is off, and the transmitter
caused by environmental changes The power radiated
status monitoring contacts are closed (K1 contacts 2-5)
by the antenna is dependent upon the relative values of
The shutdown circuits have a time delay which causes
the antenna loss resistance, the antenna radiation
shutdown to occur 20 to 35 seconds after the out-of-
resistance, and the ATU coil loss resistance.
b. The rf output (50 watts carrier maximum) from
tolerance condition is detected.
b. The normal shutdown control circuit deenergizes
the NDB transmitter is connected to the ATU through a
relay K1 if keying is lost, modulation falls below the
50-ohm coaxial cable Power for the ATU (+24 Vdc) is
threshold level set by R2, or if the carrier falls below the
obtained from the NDB transmitter.  The ATU has a
threshold level set by R3. When the normal shutdown
POWER ON/OFF switch S1 and a POWER light DS1
c.  The ATU power probe Al operates TEST meter
circuit deenergizes K1, the rf oscillator in the exciter
assembly A1A2 is turned off, the NORM light DS1 is
M1 to provide a reading for the forward power, reflected
turned off, the ALARM light DS2 is turned on, and the
power, and rf current. The TEST meter has a meter
transmitter status monitoring contacts (K1 contacts 2-5)
calibration control RI and an on/off switch S3. The on/off
become open circuit. When NORM/BYP switch S1 is in
switch S3, located on the side of the ATU cabinet, is
the NORM position, the BPY light DS3 is normally off.
used to switch the meter off when not required, without
When switch S1 is set to the BPY position, the normal
opening the cabinet.
shutdown circuit keeps the shutdown relay K1 in an
d. The servo probe A2 monitors the phase
energized state and prevents a shutdown, the BYP light
relationship between the current and voltage of the rf
DS3 comes on and the NORM light DS1 flashes at the
output. When an off-tune condition exists (voltage and
keying rate.
current out of phase), the servo probe A2 actuates the
c.  The special shutdown circuit is similar to the
tuning motor to raise or lower tuning slugs in the loading
normal shutdown circuit except that it causes shutdown
coils This maintains a tuned condition to compensate for
only  if  keying  is  lost  or  if  modulation  falls
changes in the antenna reactance caused by environ-

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