Manual Chapter 3

500-Watt UHF Transmitter Chapter 3, Circuit Descriptions
425A, Rev. 0 3-5
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.
3.1.2.5 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.
3.1.2.6 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.
3.1.3 (A6) Delay Equalizer Board
(1227-1204; Appendix D)
The delay equalizer board provides a
delay to the video signal, correction to