500 WATT RADIOBEACON TRANSMITTER
ND2000A-02x-xx0
Page 2-13
15 January 2005
control signal is applied to 'remote off' input J4-1,
relay K1 will energize, the B- operating voltage will
be inhibited. A 'reset enable' (ground) control signal
will be applied to circuitry on the monitor PWB. The
STANDBY ALARM
and
SHUTDOWN-ALARM
lamps will
turn on. External shutdown and standby alarm control
signals will be generated.
2.9.2.2 A/B
Main
Select:
The A/B main select
relay (K2) is used to select which side of the
transmitter (A or B) is the 'main' side. When the
transmitter is set to local control, the relay is
controlled by the select main
XMTR
switch (S3) on the
control panel (A1). When the transmitter is set to
remote control, the relay is controlled by an external
input on the A/B main select remote line (J2-5).
Relay K2 is de-energized to select side A as the main
side.
2.9.2.3 Shutdown Relay:
During normal
operation, relay K2 will be energized. If the ground
control input on K2-13 is removed, relay K2 will de-
energize. The transmitter will turn off. A B- control
signal will be applied through J5-12 causing the
SHUTDOWN-ALARM
lamp to turn on and an external
shutdown alarm control signal (ground) will be
generated at J4-5.
2.9.2.4 Standby
Relay:
During normal operation,
relay K3 will be energized. If the ground on K3-13 is
removed, K3 will de-energize. The B- voltage will be
applied through J3-5. The
STANDBY-ALARM
lamp
shall turn on and an external standby-alarm control
signal (ground) will be generated and passed to
standby alarm (remote) on J4-3. The transmitter's
selected main side will shut down and the standby
side will turn on.
2.9.3
M15 VOLT DC
REGULATOR:
+24 V dc is applied through J6-11
or J6-4 (from either the
A
or
B
side of the transmitter)
and passed through its associated diode to +15 V dc
regulator U12-3. +24 V dc is applied to J6-12 for
external use. The reg15 V dc is applied to
J8-9 for external use and circuits within the
transmitter for internal use. TP13 provides a
convenient location to measure the +15 V dc.
2.9.4
MONITOR -15 VOLTS DC CIRCUIT:
The B- voltage is applied across resistor R74 and
zener diode CR16. The two components provide -15
VDC for the transmitter. TP9 provides a convenient
location to measure the -15 V dc. Transistors
Q10/Q11 plus their associated components provide a
sharp on/off transition for the -15 V dc. If the B-
input is less negative than -30 V dc, zener diode CR13
will be reverse biased and turned off. Q10 will be
reverse biased and turned off. When Q10 is turned
off, Q11 will be forward biased and turned on. -15 V
dc will be clamped to ground. When the B- Vdc is
turned off, the transmitter will reset (return to the
main side) in a nominal five seconds.
2.9.5
MOD THRESHOLD CIRCUIT:
The
fwd pwr input on J6-1 a dc voltage representing the
forward power of the transmitter with a super imposed
ac voltage proportional to the modulation depth, is
applied through loading resistors R1/R2 and buffer
amplifier U1A to the
MOD THRSH
potentiometer.
MOD THRSH
potentiometer is normally set for the
desired minimum modulation level (normally to
detect a -4.0 dB drop on the intended modulation
depth). The detected dc voltage (level of dc voltage
will depend on setting of
MOD THRSH
potentiometer),
from the wiper of R7, is applied through Q1/Q2 to the
junction of C7/R26.
2.9.5.1
Amplifiers U3B through U3D form a
buffered 'OR' circuit for the keyed tone input thus
allowing provision for a keyed tone input to JFET Q3
from side
A
or side
B
. JFET Q3 and the 'OR' circuit
form a synchronous detector which will only detect
signals that are in phase with the keyed tone inputs to
U3B or U3C. The detected dc voltage is applied to
the non-inverting input of comparator U4A where it is
compared to a bias voltage, established by resistors
R28/31/33, being applied to the inverting input of
U4A. When the inverting input is more positive than
the bias voltage on the non-inverting input of
operation amplifier U4A, U4A's output will be
ground. The output of U4A will be a switched
voltage that goes to ground each time the detected DC
input (modulation) becomes more positive than the
inverting input.
