350-Watt Digital VHF LB Transmitter
Chapter 4, Circuit Descriptions
DT325B, Rev. 0
4-9
LEDs. This +12 VDC is supplied to the
tray when the Driver/Amplifier is turned
on.
4.2.9 (A10) +28 VDC Switching
Power Supply Assembly
(A10) The +28 VDC Switching Power
Supply uses the 220VAC input through
the 15A circuit breaker CB1 and
provides the +28 VDC needed for
operation of the tray. Two varistors,
VR3 and VR4, connect to each leg of the
220 VAC input for overvoltage and surge
protection.
The +28 VDC output of the switching
power supply assembly connects to (A8)
the Current Metering Board at TB1-1 thru
TB1-4.
4.2.10 (A8) Current Metering Board
(1304362; Appendix D)
The Current Metering Board distributes
the voltages through fuses to the
Amplifier Devices on the Filter/Amplifier
Board, the VHF Low Band Amplifier
Board, and the three Final High Band
Amplifier Boards. The Fuses F1, F2 & F3
are 20 Amp, F4 is 5 Amp, F6 is 2 Amp
and F7 is a 1 Amp Fuse. The F5 fuse is
not used in this configuration. There are
two Spare Fuses, one 1 Amp and one 20
Amp, located on the top, right rear of the
Tray.
Fuse F1 protects (A3-A1) the Low Band
Output Amplifier Pallet, Fuse F2 protects
(A3-A2) the Low Band Output Amplifier
Pallet, Fuse F3 protects (A3-A3) the Low
Band Output Amplifier Pallet, Fuse F4
protects (A2-A1) the VHF Low Band
Amplifier Board, and Fuse F6 protects
(A1-A12 the Filter/Amplifier Board. Fuse
F7 su28 VDC to J8 Pin 2 on the
AGC Control Board, where it is connected
to the Regulator IC U7 that takes the
+28 VDC and provides a +12 VDC
output. The +12 VDC is used for
operation of the AGC Control Board and
also the +12 VDC is connected through
the Current Metering Board, by the
jumper from TB1-5 to TB1-6, to the
Phase Shifter Board, the Filter/Amplifier
Board and the Overdrive Protection
Board. The Current Metering Board also
supplies sample outputs of the operating
currents of the amplifier devices in the
Tray to the front panel Current Meter.
The Meter in the (I
1
) position reads the
current for the (A3-A1) P400 Low Band
Output Amplifier Board, (I
2
) for the (A3-
A2) P400 Low Band Output Amplifier
Board and (I
3
) for the (A3-A3) P400 Low
Band Output Amplifier Board. The (I
D
)
position reads the current for the (A2-A1)
VHF Low Band Amplifier Board. To read
the desired current, Switch S2 to the
proper position while checking that S1 is
in the Current position. These current
readings can be used when setting up the
Idling Currents, no RF Drive applied, for
the devices. Refer to the final Test Data
Sheet for your transmitter for the
operational and Idling current readings.
(
NOTE:
All front panel current readings
must be multiplied by 2 to give the actual
value.)
4.2.11 Operation of the VHF Low
Band Amplifier Tray
220 VAC is applied through Jack J4 to
Terminal Block TB1 in the Tray. When
(CB1) the 15 Amp rear panel mounted
AC Circuit Breaker is switched On, the
220 VAC is distributed from TB1 to (A11
& A12) two cooling Fans, which will
operate, and to (A10) the Switching
Power Supply. There are two Surge
Suppressors, VR1 and VR2, mounted on
TB1 that provide protection from
transients or surges on the input AC Line.
There are two other Surge Suppressors,
VR3 and VR4, mounted at the input to
the switching power supply from each AC
Line to ground, which provide protection
from transients or surges on the AC Line.
The switching power Supply only
operates when the Power Supply Enable
Control Line, Jack (J3 Pins 9 & 10),
located on the rear of the Tray, are
shorted. The Enable is generated by the
exciter/driver chassis assembly when the
