Assembly Instructions Chapter 4

4-6 kW UHF Translator Chapter 4, Circuit Descriptions
837B, Rev. 0 4-18
the null condition would no longer occur
at U10A, pin 2. When the level drops,
the output of U10A, pin 1, will go more
positive. If jumper W3 on J6 is in the
Automatic position, it will cause the
ALC pin-diode attenuators CR1, CR2,
and CR3 to have less attenuation and
increase the IF level; this will
compensate for the decrease in the
level. If the ALC cannot increase the
input level enough to satisfy the ALC
loop, due to there not being enough
range, an ALC fault will occur. The fault
is generated because U10D, pin 12,
increases above the trip point set by
R84 and R83 until it conducts. This
causes U10D pin 14 to go high and the
red ALC Fault LED DS2 to light.
4.2.5.10 Scrambled Operation with
Encoding
For encoded, scrambled operation,
jumper W4 on J8 must be connected
between pins 2 and 3, jumper W8 on J9
must be between pins 3 and 2, jumper
W7 on J26 must be between pins 2 and
3, and jumper W5 on J21 must be
between pins 2 and 3. The IF is
connected through W4 on J8 to the
sync regeneration circuits.
If this board is operated with
scrambling, using suppressed sync, the
ALC circuit operates differently because
there is no peak of sync present on the
IF input. A timing pulse from the
scrambling encoder connects to the
board at J24. This timing pulse is
converted to sync pulses by U17A and
U17B, which control the operation of
Q8. The sync amplitude is controlled by
R149 and then applied to U15A, where
it is added to the detected IF signal to
produce a peak of sync level. The
output of U15A is peak detected by
CR26 and fed to U15B. If necessary,
intercarrier notch L39 can be placed in
the circuit by placing W6 on J22. The
intercarrier notch is adjusted to filter
any aural and 4.5-MHz intercarrier
frequencies. The peak of sync signal is
fed through R162, the ALC calibration
control, to amplifier U15C. The
amplified peak of sync output is
connected through J21, pins 2 and 3, to
U10A, where it is used as the reference
for the ALC circuit and the AGC
reference to the transmitter control
board. Voltage TP4 should be the same
in either the normal or the encoded
video mode. Monitor J9, pins 3 and 4,
with a spectrum analyzer, check that
the board is in the AGC mode, and tune
C103 to notch-out the aural IF carrier.
4.2.5.11 Fault Command
The ALC board also has circuitry for an
external mute fault input at J19, pin 6.
This is a Mute command and, in most
systems, it protects the circuits of high-
gain output amplifier devices against
VSWR faults. In this case, the action
should occur faster than just pulling the
ALC reference down. Two different
mechanisms are employed: one is a
very fast-acting circuit to increase the
attenuation of the pin-diode attenuator,
CR3, CR1, and CR2, and the second is
the reference voltage being pulled
away from the ALC amplifier device. An
external Mute is a pull-down applied to
J19, pin 6, to provide a current path
from the +12 VDC line through R78
and R139, the LED DS4 (Mute
indicator), and the LED section of
opto-isolator U11.
These actions turn on the transistor
section of U11 that applies -12 VDC
through CR21 to U10A, pin 3, and pulls
down the reference voltage. This is a
fairly slow action that is kept at this
pace by the low-pass filter function of
R81 and C61. When the transistor
section of U11 is on, -12 VDC is also
connected through CR22 to the pin-
diode attenuator circuit. This
establishes a very fast muting action,
by reverse biasing CR3, in the event of
an external VSWR fault.