Nautel FM10, Technical Instructions

Page: 103 / 200

10 000 WATT FM BROADCAST TRANSMITTER
FM10
Page 6-2
15 November 1999
6.3.3 CIRCUIT BREAKER PANEL: See
figure SD-3. The circuit breaker panel (A4) is the
transmitter's B+ volts reservoir distribution point.
Interconnecting strap A4E1 supplies B+ volts to
switching power supplies A, B and C (A6 thru A8)
via circuit breakers A4CB2 thru A4CB4.
Interconnecting strap A4E2 supplies B+ volts to
switching power supplies D, E and F (A9 thru A11)
and IPA switching power supply (A12) via circuit
breakers A4CB5 thru A4CB8. A diode (A4CR1 thru
A4CR7) is installed across each DC circuit breaker
to protect the monitoring circuitry when a circuit
breaker is tripped off. The panel also contains
MAIN
POWER
circuit breaker A4CB1.
AC ON
lamp
A4DS1 turns on when
MAIN POWER
circuit breaker
is closed.
6.3.4 LOW VOLTAGE POWER SUPPLY
ASSEMBLY (A26): See figures SD-2 thru SD-5.
The low voltage power supply assembly provides the
transmitter's unregulated 24 VDC and regulated 24
VDC, ±15 VDC and +5 VDC supplies. Fuses F1
thru F3 fuse the low voltage 3-phase (18.9 VAC line-
to-line) secondary of A1T1 applied to TB1-1, TB1-2
and TB1-3. Bridge rectifier U1 and associated
components full wave rectify the low voltage 3-phase
to create a nominal 24 VDC supply for application to
the low voltage supply PWB (A1) at P1-12. Fuses
F4 and F5 fuse the centre tapped 36 VAC (18-0-18)
secondary applied to TB1-4 and TB1-6. This voltage
is used by the low voltage power supply PWB to
create the –15 VDC supply for the transmitter. An
RF On control signal originates on the
control/monitor PWB and is used to enable the low
voltage supplies. Test points TP1 thru TP4 provide
convenient DC voltage measurement points. A
sample of each AC phase is applied to 3-phase
monitor PWB A26A2 for loss of phase monitoring
purposes (refer to paragraph 6.3.4.2 for a more
detailed description).
6.3.4.1 Low Voltage Power Supply PWB
(A26A1): See figure SD-4. The low voltage power
supply PWB provides 24, ±15 and +5 VDC supplies
for the transmitter's low level circuitry (control,
monitoring and cooling fans).
6.3.4.1.1 -15V Regulator: Diodes CR4, CR5 and
their associated components rectify and smooth the
18-0-18 VAC centre-tapped supply for application to
voltage regulator U2. The output voltage of U2 (-15
VDC) is determined by the values of resistors R11,
R12 and R14.
6.3.4.1.2 +24V Fan Supply: FET source followers
Q3, Q4 and their associated components comprise a
circuit which controls application of the
+24V
(Unreg) input to the transmitter's cooling fans.
Normally, zener diode CR10 provides a nominal 30
VDC bias voltage to the gates of Q3 and Q4 to
ensure a nominal 24BFV is applied to the source of
Q3 and Q4 when they are biased on. When 5VDC is
applied to the
RF On input (J2-3), Q1 will turn on
and cause a near ground potential to be applied to the
base of transistor Q2. Q2 will be turned off, enabling
the 30 VDC bias voltage to be applied to the gates of
FETs Q3 and Q4. 24 VDC will be applied to the fan
supply outputs,
+24V (IPA) , +24V (A/B) , +24V
(C/D) , +24V (E/F) , +24V (PS) (A) , +24V (PS) (B)
and +24V (PS) (C) thru fuses F1 to F7 to operate the
transmitter's cooling fans. When 5 VDC is not
applied to the
RF On input (J2-3), transistor Q1 will
be turned off and allow 24 VDC to be applied, thru
resistor R5 to the base of transistor Q2. Transistor
Q2 will be turned on and cause a near ground
potential to be applied to the gates of FETs Q3 and
Q4. Q3 and Q4 will be biased off and 24 VDC will
not be applied to the fan supply outputs, disabling the
transmitter's cooling fans.
6.3.4.1.3 +15V Regulator: The +24V (Unreg)
input (J1-12) is regulated by source follower FET
Q5. Normally, zener diode CR10 provides a nominal
30 VDC bias voltage to the gate of FET Q5 to ensure
a nominal 24 VDC will be applied to the source of
Q5 when it is biased on. This 24 VDC is applied to
the
+15V
and
+5V Regulator
circuits and is used as
the transmitters 24V supply. Zener diode CR11
clamps the voltage between FET Q5's gate and
source leads to a maximum of 12 VDC. Voltage
regulator U4 and its associated components create the
+15 VDC supply for the transmitter. The output
voltage of U4 (+15 VDC) is determined by the values
of resistors R18, R19 and R20. Positive temperature
co-efficient (PTC) thermistor RT1 will switch from
its normal low impedance (short circuit) to a high
impedance (open circuit) if the
+15V Regulator
circuit is shorted to ground or excessive current is
drawn.
6.3.4.1.4 +5V Regulator: Voltage regulator U3 and
its associated components create the +5 VDC supply
for the transmitter. The output voltage of U3 (+5
VDC) is determined by the values of resistors R15,
R16 and R17. Positive temperature co-efficient