User's Manual

2650 Watt VHF High Band Transmitter Chapter 4, Circuit Descriptions
435B, Rev. 0 4-7
Q4 drives two different output circuits.
One output is directed through voltage
dividers R14 and R15 to jack J2 that can
connect to a frequency counter. While
monitoring J2 the oscillator can be set
exactly on the operating frequency
(45.75 MHz) by adjusting C17. The
output at J2 is at a power level of
approximately -2 dBm, which is sufficient
to drive most frequency counters. The
other output of Q4 connects to prescaler
chip U1, which divides the signal by 15.
The output of U1 is applied to U2, a
programmable divider IC. U2 is
programmed through pins 11 to 20 to
divide by 61. This results in a 50-kHz
signal at pin 9 that is available as an
output at J1. The output of 50 kHz is
generally used in systems where the
visual IF carrier oven oscillator is used as
the reference for a PLL circuit. An
example of this is when the PLL circuit
uses the aural IF synthesizer board and
the aural VCO. The 50 kHz CMOS output
at jack J1 is not capable of achieving
enough drive level for a long coaxial
cable length. As a result, when a long
coaxial cable is needed, the output at
jack J5 is utilized. The push-pull
transistor stage Q5 and Q6, along with
emitter resistor R18, provide a large-load
output capability at J5.
The stages U1, U2, Q5, and Q6 are
powered by +5.1 VDC, which is obtained
using the +12 VDC line voltage, the
voltage-dropping resistor R16 and the
Zener diode VR2.
The +12 VDC power is applied to the
board through jack J4, pin 3, and is
isolated from the RF signals that may
occur in the +12 VDC line through the
use of RF choke L2 and 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 signal,
(0 dBm), from the modulator 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 the
receiver IF input jack J1. The modulator
IF input connects to relay K3 and the
receiver IF input connects to relay K4.
The two relays are controlled by the
Modulator Select command that is
connected to J30 on the board. Modulator
select enable/disable jumper W11 on J29
controls whether the Modulator Select
command at J30 controls the operation of
the relays or not. With jumper W11 on
J29, pins 1 and 2, the Modulator Select
command at J30 controls the operation of
the relays; with jumper W11 on J29, pins
2 and 3, the modulator is selected all of
the time. This is the normal position.
4.1.5.1 Modulator Selected
With the modulator selected, J9-31 &
J9-32 located on the A/V I/P & Remote
Interface Assembly are connected
together; this makes J30 low and causes
relays K3 and K4 to de-energize. When
K4 is de-energized, it connects the
receiver IF input at J1, if present, to 50O.
When K3 is de-energized, it connects the
modulator IF input at J32 to the rest of
the board; Modulator Enable LED DS5
will be illuminated.
4.1.5.2 (Optional) Receiver Selected
With the (Optional) receiver selected,
which is J9-31 & J9-32 located on the
(A12) A/V I/P & Remote Interface
Assembly (connected to J30 on the
board), not connected together, relays
K3 and K4 are energized. When K4 is
energized, it connects the receiver IF
input at J1, if present, to the rest of the
board. When K3 is energized, it connects
to the modulator IF input at J32 to 50O.
The Modulator Enable LED DS5 will be
illuminated.