500 WATT RADIOBEACON TRANSMITTER
ND2000A-02x-xx0
Page 2-9
15 January 2005
ensures the leading and trailing edges of the
rectangular waveform are sharp. The mod drive
output at J2-6 is a low impedance pulse-width
modulated signal switching b15 volts dc and
ground.
2.5.9.11 Pulse-Width Fault Detector:
The
pulse-width fault detector circuit monitors the pulse-
width modulation signal for the presence of +15 V dc
pulses. The circuit contains operational amplifier
U11B, inverter U10D, relay K1 and their associated
components. A two-pole low pass filter, comprising
R68/R69 and C30/C31, continuously monitors the
average on/off ratio of the variable pulse width mod
drive signal. Normally the voltage on C31, which is
applied to U11B's non-inverting input, will not exceed
the reference threshold voltage being applied to its
inverting input from the junction of R70/R71. U11B's
output will be at near ground potential, resulting in the
U10D's output being +15 V dc. A mod drive alarm
signal will not be applied to J2-4.
If a dangerously high variable pulse width on/off ratio
should occur, the voltage on capacitor C31 will
exceed the reference threshold voltage applied to
U11B's inverting input. U11B's output will be +15 V
dc. Relay K1 will energize and remove the mod drive
output from J2-6. U10D's output will be
MOD DRIVE
ALARM
zero volts dc. A zero potential mod drive
alarm signal will be applied through J2-4 to and turn
on the lamp (A4DS3).
2.5.9.12 RF Drive Alarm Circuit:
The RF drive
alarm circuit contains operational amplifier U11A,
inverter/buffers U10B/U10C, transistor Q3 and their
associated components. When the transmitter is
turned on, the voltage at the junction of C25/R60 will
instantly rise to +15 V dc and decay slowly through
resistors R60/R61. Transistor Q3 will be forward
biased (turned on) and U11A-3's non-inverting input
will be at ground. Resistors R64/R65 establish the
threshold voltage on U11A's inverting input.
The output on U11A-1 will be low and output on
U10B-2 will be high, causing diode CR3 to be
forward biased. The input on operational amplifier
U10A-7 will be high and its output low.
The low output on U10A-6 will inhibit the mod drive.
The output of U10C-4 will be low and applied to the
RF drive alarm lamp on the exciter's front panel. The
RF drive alarm lamp will turn on. After
approximately five seconds the voltage being applied
to the base of transistor Q3 will decay through
resistors R60/R61, transistor Q3 will be reversed
biased, the output of U10B-2 will go low, diode CR3
will be reversed biased. The mod drive output on J2-
6 will no longer be inhibited. The output of U10C-4
will go high and the RF drive alarm lamp on the
exciter's front panel will turn off.
POWER ON/OFF PANEL ASSEMBLY
(A6)
(see
figure SD-1)
2.6
The power on/off panel assembly contains
the
POWER-AC LINE
and
POWER-BATTERY
switches.
Under normal operating conditions both switches will
be set to their
ON
position. AC power from the
service entrance is applied through terminals 1 and 2
on TB1 and passed to the contacts of the
POWER-AC
LINE
switch. When it is set to
ON
, 230 V ac will be
passed to power supply modules A7 and A8. The DC
power (battery) source is applied from TB2-2(-),
through fuse A6F1 and the contacts of the
POWER-
BATTERY
switch to battery panel (optional) A11. If
the optional battery panel is installed, DC power will
be inhibited by switching circuits within the subject
transmitter until AC power fails or decreases below a
low AC voltage threshold.
POWER SUPPLY MODULES
(see figure SD-10)
2.7
There are two identical power supply
modules (A7 and A8) in the transmitter. Only one of
the power supplies will be in use during operation.
Power supply A7 will be selected when the
SELECT
MAIN TX
switch is set to
A
, power supply A8 will be
selected when it is set to
B
.
2.7.1
POWER SUPPLY CONTROL PWB
(A7A1):
230 V ac from the service entrance, at J1-
1/2, is passed through the appropriate fuses
(dependent on 50 or 60 Hz) to the tapped primary
winding of power transformer T1. T1 contains two
sets of centre-tapped secondary windings. One set
provides a low level (17.5-0-17.5) ac for use as a
phasing reference and as the voltage source for a 15 V
dc regulated power supply. The other set provides a
higher level ac as the voltage source for the B- Vdc
power supply. Varistors RV1/RV2 provide transient
protection. Thyristors Q1/Q2, form a controlled full-
wave rectifier for the B- Vdc. The B- Vdc output is
maintained at the required level by controlling their
