Assembly Instructions Chapter 4

4-6 kW UHF Translator Chapter 4, Circuit Descriptions
837B, Rev. 0 4-10
prevent video transients from
overmodulating the video carrier. The
clamped video is connected to sync
clipper circuit Q4 (adjusted by R24);
Q4 limits the sync to -40 IRE units. The
corrected video connects to emitter
follower Q4 whose output is wired to
unity gain amplifier U2A and provides a
low-impedance, clamped video output
at pin 1.
4.2.2.4 Visual Modulator Circuit
The clamped video signal from U2A is
split. One part connects to a metering
circuit, consisting of U20 and
associated components, that produces
a video output sample at J8-6 and
connects through the translator control
board to the front panel meter for
monitoring. The other clamped video
path from U2A is through a sync-
stretch circuit that consists of Q5 and
Q6. The sync-stretch circuit contains
R48; R48 adjusts the sync stretch
magnitude (amount) and R45 adjusts
the cut-in. This sync-stretch
adjustment should not be used to
correct for output sync problems, but it
can be used for video input sync
problems. The output of the sync-
stretch circuit connects to pin 5, the I
input of mixer Z1.
The video signal is heterodyned in
mixer Z1 with the visual IF CW signal
(45.75 MHz). The visual IF CW signal
enters the board at jack J15 and is
connected to U9, where it is amplified
and wired to pin 1, the L input of mixer
Z1. Adjustable capacitor C78 and
resistor R53 are set up to add a small
amount of incidental carrier phase
modulation (ICPM) correction to the
output of the mixer stage to
compensate for any non-linearities
generated by the mixer.
The modulated 45.75-MHz RF output of
mixer Z1 is amplified by U5 and is fed
to double-sideband visual IF output
jack J18. The level of this output jack is
adjusted by R70. J18 is the visual IF
loop-through output jack that is
normally jumpered to J19 on the board.
If the optional visual IF loop-through kit
is purchased, the visual is connected
out of the board to external IF
processor trays.
After any external processing, the
modulated visual IF, double-sideband
signal re-enters the board through J19.
The visual IF from J19 is amplified by
U10 and U11 and routed through the
vestigial sideband filter network,
consisting of T1, FL1, and T2, and
produces a vestigial sideband visual IF
signal output. The filtered vestigial
sideband visual IF is amplified by U7
and connected to a T-type attenuator.
R62 can be adjusted to set the visual IF
gain; this is the amount of the visual IF
signal that is coupled to amplifier IC
U8. R63 and C30 are adjusted for the
best VSBF frequency response. The
amplified IF signal is fed to the input of
the diplexer circuit that consists of R76,
L13, and L12. A detected voltage
sample of the visual IF is available at
test point TP5.
4.2.2.5 41.25-MHz Aural IF Circuit
On this board, the 41.25-MHz aural IF
is created by mixing the modulated
4.5-MHz aural intercarrier signal,
produced by the aural IF synthesizer
board or from the composite 4.5-MHz
filter board, with the 45.75-MHz CW
signal produced by the 45.75-MHz IF
carrier oven oscillator board. The
modulated 4.5-MHz aural intercarrier
signal enters the board at J14 or J28
and is connected to IF relay K1. Jumper
W3 on J7 determines whether the 4.5-
MHz used by the board is internally
generated or from an external source.
With jumper W3 connected between
pins 2 and 3, the 4.5 MHz from the
aural IF synthesizer board or from the
4.5-MHz composite input is connected
to mixer Z2. If an external 4.5-MHz
signal is used, it enters the board at
J12 and is fed through gain pot R88 to
amplifier IC U13A. The amplified 4.5