Assembly Instructions Chapter 4

4-6 kW UHF Translator Chapter 4, Circuit Descriptions
837B, Rev. 0 4-17
2 on J27 and J28. Normally, the pad is
out. The IF signal is then applied to a
two-stage, frequency-response
corrector circuit that is adjusted as
needed.
Variable resistors R103 and R106 are
used to adjust the depth and gain of
the notches and variable caps C71 and
C72 are used to adjust the frequency
position of the notches. The IF signal is
amplified by U13 and U14 before it is
connected to J12, the IF output jack of
the board. R99 is an output level
adjustment that is set to provide
approximately 0 dBm of IF output at
J12. A sample of the IF is fed to J11 to
provide an IF sample point that can be
monitored without breaking the signal
path and gives an indication of the IF
signal after the linearity and the
frequency-response correction takes
place.
4.2.5.9 ALC Circuit
The other path of the corrected IF
signal is used in the ALC circuit. The IF
is wired out of the splitter through L25
and connects to op-amp U12. The
output of U12 is wired to jacks J8 and
J9 on which jumpers W4 and W8
control the normal or encoded
operation of the ALC circuitry. For
normal operation, jumper W4 on J8 is
between pins 1 and 2 and jumper W8
on J9 is between pins 1 and 2. The IF
signal is applied to transformer T5; T5
doubles the voltage swing by means of
a 1:4 impedance transformation before
it is connected to the ALC detector
circuit on the board and amplified by
U10B.
For normal operation, jumper W7 on
J26 is between pins 1 and 2 and
jumper W5 on J21 is between pins 1
and 2. The detected ALC voltage is
wired to U10A, pin 2, where it is
summed with the front panel power
control setting. The output power
adjustment for the translator is
achieved, if the (optional) remote
power raise/lower kit (1227-1039) is
purchased, by R75, a motor-driven pot
controlled by switch S1 on the board,
or screwdriver adjust pot R1 on the
front panel of the UHF exciter tray. An
external power raise/lower switch can
be used by connecting it to jack J10, at
J10-11 power raise, J10-13 power
raise/lower return, and J10-12 power
lower, on the rear of the UHF exciter
tray. S1, or the Remote switch, controls
relays K1 and K2, which control motor
M1 that moves variable resistor R75. If
the (optional) remote power
raise/lower kit is not purchased, the
ALC voltage is controlled only by
screwdriver adjust pot R1 on the front
panel of the UHF exciter tray. The ALC
voltage is set for .8 VDC at TP4 with a
0-dBm output at J12 of the board. A
sample of the ALC at J19, pin 2, is
wired to the transmitter control board
where it is used on the front panel
meter and in the AGC circuits.
This ALC voltage, and the DC level
corresponding to the IF level after
signal correction, are fed to U10A, pin
2, whose output at pin 1 connects to
the ALC pin-diode attenuator circuit. If
there is a loss of gain somewhere in an
IF circuit, the output power of the
translator will drop. The ALC circuit
senses this drop at U10A and
automatically eases the loss of the pin-
diode attenuator circuit by increasing
its gain.
The ALC action starts with the ALC
detector level monitored at TP4. The
detector output at TP4 is nominally
+.8 VDC and is applied through resistor
R77 to a summing point at op-amp
U10A, pin 2. The current available from
the ALC detector is offset, or
complemented, by current taken away
from the summing junction. In normal
operation, U10A, pin 2, is at 0 VDC
when the loop is satisfied. If the
recovered or peak-detected IF signal
level at IF input jack J7 of this board
should drop, which normally means
that the output power is decreasing,