User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-3
-12 VDC is connected to J1-5 and filtered
by L1, C1, and C2 before it is connected
to the rest of the board. Two
connections of the +12 VDC are to U8
and U9, which are 5-volt regulator ICs
that provide the voltage to the U10 and
U5 ICs.
4.1.2 (A5) Sync Tip Clamp/
Modulator Board (1265-1302;
Appendix D)
The sync tip clamp/modulator board is
divided into five circuits: the main video
circuit, the sync tip clamp circuit, the
visual modulator circuit, the aural IF
mixer circuit, and the diplexer circuit.
The sync tip clamp/modulator board
takes the baseband video or the 4.5-MHz
composite input that is connected to the
video input jack, either J1 or J2, which
are loop-through connected, and
produces a modulated visual IF + aural
IF output at output jack J20 on the
board. The clamp portion of the board
maintains a constant peak of sync level
over varying average picture levels
(APL). The modulator portion of the
board contains the circuitry that
generates an amplitude modulated
vestigial sideband visual IF signal output
that is made up of the baseband video
input signal (1 Vpk-pk) modulated onto
an externally generated 45.75-MHz IF
carrier frequency. The visual IF signal
and the aural IF signal are combined in
the diplexer circuit to produce the visual
IF + aural IF output that is connected to
J20, the IF output jack of the board.
4.1.2.1 Main Video Signal Path (Part 1 of
2)
The baseband video or the 4.5-MHz
composite input connects to the board at
J2. J2 is loop-through connected to J1
and terminated into 75O, if the jumper
W4 is on jack J3. With jumper W4
removed, the input can be connected to
another transmitter through J1.
Test point TP1 is provided to monitor the
level of the input. The input is fed to the
non-inverting and inverting inputs of
U1A, a differential amplifier that
minimizes any common-mode hum that
may be present on the incoming signal.
Diodes CR1 to CR4 form a voltage-limiter
network in which, if the input voltages
exceed the supply voltages for U1A, the
diodes conduct, preventing damage to
U1A. CR1 and CR3 conduct if the input
voltage exceeds the negative supply and
CR2 and CR4 conduct if the input voltage
exceeds the positive supply voltage.
The video output of U1A is connected to
J22 on the board. Normally, the video at
J22 is jumpered to J27 on the board. If
the optional 4.5-MHz composite input kit
is purchased, the 4.5-MHz composite
signal at J22 connects to the external
composite 4.5-MHz filter board and the
4.5-MHz bandpass filter board. These
two boards provide the video-only signal
to J27 and the 4.5-MHz intercarrier signal
to J28 from the 4.5-MHz composite input.
The video through the video gain pot R12
is typically adjusted for 1 Vpk-pk at TP2
and connects to amplifier U1B.
The output of U1B, if the delay equalizer
board is present in the tray, connects the
video from J6 pin 2, to the external delay
equalizer board and back to the sync tip
clamp/modulator board at J6 pin 4. If
the delay equalizer is not present, the
video connects through jumper W1 on J5,
pins 1 and 2. The delay equalizer board
plugs directly into J6 on the sync tip
clamp/modulator board. The video from
J6 pin 4 is connected through the jumper
W1 on J5 pins 2 and 3, to the amplifier
Q1. The output of Q1 connects to Q2.
The base voltage of Q2 is set by the DC
offset voltage output of U3B, which is the
sync tip clamp circuit output.
4.1.2.2 Sync Tip Clamp Circuit
The automatic sync tip clamp circuit is
made up of U4A, Q7, U3B, and
associated components. The circuit
begins with a sample of the clamped