Assembly Instructions Chapter 4

10-kW UHF Transmitter with Chapter 4, Circuit
Feedforward Drive Descriptions
840A, Rev. 0 4-7
Q4 drives two different output circuits.
One output is directed through voltage
divider R14 and R15 to jack J2 and is fed
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 U1, a prescaler chip that
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 VCXO is
used as the reference for a PLL circuit. An
example of this is a PLL circuit used on
the aural IF synthesizer board, the aural
VCO, or the precise frequency control
tray. The 50-kHz CMOS output at jack J1
is not capable of providing 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,
provides a large-load output capability at
J5.
The stages U1, U2, Q5, and Q6 are
powered by +5.1 VDC. The +5.1 VDC 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 RF signals that may occur
in the +12 VDC line through the use of
RF choke L2 and filter capacitor C10.
4.1.1.5 (A8) ALC Board, NTSC (1265-
1305; Appendix B)
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.
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.
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.