User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-9
voltage dropping resistor R16 and Zener
Diode VR2.
The +12 VDC input to the board is
applied through Jack J4 Pin 3 and is
isolated from any RF, which may occur
on the +12 VDC Line, through the use of
the RF choke L2 and the filter capacitor
C10.
4.1.5 (A8) ALC Board, NTSC (1265-
1305; Appendix D)
The automatic level control (ALC) board
provides the ALC and amplitude linearity
correction of the IF signal. The ALC
adjusts the level of the IF signal through
the board to control the output power of
the transmitter.
The visual + aural IF input (0 dBm)
signal, from the clamp/modulator board,
enters the board at the modulator IF
input jack J32. If the (Optional) receiver
tray is present, the visual + aural IF
input (0 dBm) from the receiver tray
connects to receiver IF input jack J1.
4.1.5.1 Local Modulator Select Enabled
(Normal Operation with the Modulator IF
as the Input)
The modulator IF input connects to the
relay K3 and the receiver IF input
connects to the relay K4. The two relays
are controlled by the position of the local
Modulator Select enable/disable jumper
W11 on J29. The normal operation of
the transmitter uses the Modulator IF
input at J32. The jumper W11 connected
between pins 2 and 3 is the normal
operating position for the board. This
sets the board to the local Modulator
enable position, which is both relays de-
energized and the Modulator IF Input
connected to the rest of the board. DS5
the Modulator Enable LED will be lit.
4.1.5.2 Local Modulator Select Disabled
(Operation with the ability to select either
the Modulator IF or the Receiver IF as
the Input)
If a receiver tray is part of your system,
then the relays need to be controlled by
the Modulator Select command that is
connected to J30 on the board, so that
either the receiver IF or the modulator IF
can be used by the board. The position
of the Modulator select enable/disable
jumper W11 on J29 provides for the
Modulator Select command at J30 to
control the operation of the relays. With
the jumper W11 on J29, between pins 1
and 2, the Modulator Select command at
J30 controls the operation of the relays
and with the jumper W11 on J29,
between pins 2 and 3, the modulator is
selected all of the time and DS5, the
modulator enable LED, will be lit.
4.1.5.3 Modulator IF Selected
(With the Local Modulator Select
Disabled)
The modulator is normally selected by
J11-10 and J11-28 on the rear of the VHF
exciter tray, wired to J30 on the board,
connected together. This makes J30 go
low and causes relays K3 and K4 to de-
energize. When K4 is de-energized, it
connects the receiver IF input at J1 to
50O. When K3 is de-energized, it
connects the modulator IF input at J32 to
the rest of the board and the Modulator
Enable LED DS5 will be illuminated.
4.1.5.4 Receiver IF Selected
(With the Local Modulator Select
Disabled)
The receiver is normally selected when
J11-10 and J11-28 on the rear of the VHF
exciter tray, connected to J30 on the
board, are not connected together. This
makes J30 high and causes the relays K3
and K4 to energize. When K4 is
energized, it connects the receiver IF
input at J1 to the rest of the board.
When K3 is energized, it connects the
modulator IF input at J32 to 50O and the
Modulator Enable LED DS5 will not be
illuminated.