Manual Chapter 3

500-Watt UHF Transmitter Chapter 3, Circuit Descriptions
425A, Rev. 0 3-7
(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
by using the +12 VDC line voltage, and
voltage-dropping resistor R16 and zener
diode VR2.
The +12 VDC power is applied to the
board through jack J4, pin 3, and is
isolated from the RF signals which may
occur in the +12 VDC line through the
use of RF choke L2 and filter capacitor
C10.
3.1.5 (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 modulator enters the
board at 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. 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.
3.1.5.1 Modulator Selected
With the modulator selected, J11-10 and
J11-28 on the rear of the UHF exciter
tray 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 50 watts. 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.
3.1.5.2 Receiver Selected
With the receiver selected, which is J11-
10 and J11-28 on the rear of the UHF
exciter tray (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 50
watts; Modulator Enable LED DS5 will be
illuminated.
3.1.5.3 Main IF Signal Path (Part 1 of 3)
The selected visual + aural IF input (0
dBm) signal is split, with one half of the
signal entering a bandpass filter that
consists of L3, L4, C4, L5, and L6. This