User's Manual

2650 Watt VHF High Band Transmitter Chapter 4, Circuit Descriptions
435B, Rev. 0 4-5
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 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.
4.1.2.5 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 MHz is then connected to J7
and, if jumper W3 is between pins 1 and
2, the 4.5 MHz signal from the external
source is connected to the mixer. Mixer
Z2 heterodynes the aural-modulated,
4.5 MHz signal with the 45.75 MHz CW
signal to produce the modulated
41.25 MHz aural IF signal.
The output of the mixer is fed to a
bandpass filter that is tuned to pass only
the modulated 41.25 MHz aural IF signal
that is fed to jack J16, the 41.25 MHz
loop-through out jack of the board.
For normal operation, the 41.25 MHz
signal is jumpered by a coaxial cable
from J16 to J17 on the board. If the
(optional) aural IF loop-through kit is
purchased, the 41.25 MHz signal is
connected to the rear of the tray, to
which any processing trays can be
connected, and then back to jack J17 on
the board. The modulated 41.25 MHz
aural IF signal from J17 is connected
through amplifier ICs U15 and U16. The
amplified output is connected to the
attenuator-matching circuit that is
adjusted by R85. R85 increases or
decreases the level of the 41.25 MHz that
sets the A/V ratio for the diplexer circuit.
The diplexer circuit takes the modulated
45.75 MHz visual IF and the modulated
aural IF and combines them to produce
the 45.75 MHz + 41.25 MHz IF output.
The combined 45.75 MHz + 41.25 MHz IF
signal is amplified by U12 and connected
to combined IF output jack J20 on the
board. A sample of the combined IF
output is provided at J21 on the board. If
a NICAM input is used, it connects to J36
on the board. The level of the NICAM
signal is set by R109 before it is fed to
the diplexer circuit consisting of L28, L29,
and R115. This circuit combines the
NICAM signal with the 45.75 MHz visual
IF + 41.25 MHz aural IF signal.
4.1.2.6 Operational Voltages
The +12 VDC needed to operate the
transmitter control board enters the
board at J23, pin 3, and is filtered by
L26, L33, and C73 before it is fed to the
rest of the board.
The -12 VDC needed to operate the
board enters the board at J23, pin 5, and
is filtered by L27 and C74 before being
fed to the rest of the board.