Assembly Instructions Chapter 4

4-6 kW UHF Translator Chapter 4, Circuit Descriptions
837B, Rev. 0 4-13
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.
4.2.5 (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 translator.
The visual + aural IF input (0 dBm)
from the receiver tray connects to
receiver IF input jack J1. The visual +
aural IF input (0 dBm) signal from the
(Optional) Modulator Kit enters the
board at modulator IF input jack J32.
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. 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.
4.2.5.1 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 not be illuminated.
4.2.5.2 (Optional) Modulator Kit
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.
4.2.5.3 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
bandpass filter can be tuned with C4
and is substantially broader than the IF
signal bandwidth. It is used to slightly
steer the frequency response of the IF
to make up for any small discrepancies
in the frequency response in the stages
that precede this point. The filter also
serves the additional function of
rejecting unwanted frequencies that
may occur if the tray cover is off and
the tray is in a high RF environment.
(NOTE: If this is the case, the
translator will have to be serviced with
the tray cover off in spite of the
presence of other RF signals). The
filtered IF signal is fed through a pi-
type matching pad consisting of R2, R3,
and R4 to the pin-diode attenuator
circuit consisting of CR1, CR2, and CR3.
4.2.5.4 Input Level Detector Circuit
The other part of the split IF input is
connected through L2 and C44 to U7;
U7 is an IC amplifier that is the input to