User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-24
• % Aural power = 100%
• % Exciter = The level on the meter
needed to attain 100% output power
from the transmitter
• ALC = .8 VDC
• Audio = ±25 kHz with a balanced
audio input or ±75 kHz with a
composite audio input
Refer to the test data sheet for the
transmitter for the actual reading:
Remote metering samples are provided
at J1-10 for the exciter, J8-26 for the
visual, J8-27 for the aural, and J1-5 for
the reflected. U6 is a temperature
sensor IC that gives the operator the
ability to measure the temperature inside
the tray by measuring the voltage at
TP1. The sensor is set up for +10 mV
equals 1° F. (for example, 750 mV equals
75° F).
4.1.8.10 Operational Voltages
The +12 VDC needed for the operation of
the transmitter control board enters the
board at jack J4 pin 3. C28, L1, and L3
are for the filtering and isolation of the
+12 VDC before it is split and applied to
the rest of the board. The -12 VDC
needed for the operation of the board
enters the board at jack J4 pin 5. C29
and L2 are for the filtering and isolation
of the -12 VDC before it is split and
applied to the rest of the board.
Four +12 VDC outputs are fed out of the
board at J8-16, J8-17, J8-18, and J8-19
through diodes CR7, CR8, CR9, or CR10
and resistors R50, R51, R52, or R53 to
the three VHF amplifier trays for use in
their logic circuits. The resistors are for
current limiting and the diodes are to
prevent voltage feedback from the VHF
amplifier trays.
4.1.9 (A19) Visual/Aural Metering
Board (1265-1309; Appendix D)
The visual/aural metering board provides
detected outputs of the visual, aural, and
reflected output samples that are used
for monitoring on the front panel meter.
These readings are attained from
samples of the forward power and
reflected power outputs from (A16) the
output coupler assembly for the
transmitter. The board also provides
adjustments for the calibration of the
readings on the meter.
A forward power sample, visual + aural,
is applied to the SMA jack J1 on the
board. The input signal is split, with one
path connected to the front panel forward
power sample SMA jack J2 for monitoring
purposes. The other path is connected
through C1 to CR2, R4, R5, R6, C4, and
CR1, which make up a detector circuit.
The detected visual + aural signal is
amplified by U6B and its output is split.
One amplified output of U6B connects to
the aural level circuit and the other
output connects to the visual level circuit.
4.1.9.1 Aural Level Circuit
One of the detected visual + aural level
outputs of U6B connects through C6 to
the intercarrier filter circuit that consists
of R13, R14, L1, C7, and C8. C8 and L1
are typically adjusted for a maximum
aural reading. The filter notches out the
video + aural and only leaves the 4.5-
MHz difference frequency between the
visual and aural, which is a good
representation of the aural level. The
4.5-MHz signal is fed to the buffer
amplifier U6A. The output of U6A is
detected by the diode detector CR3 and
U1A and then fed through the aural
calibration control R20 to amplifier U2D.
The amplified output of U2D is split, with
the main output connected through R21
to J6 pin 1, which supplies the aural level
output to the front panel meter for
monitoring. The other output of U2D is
connected to the aural null adjust R51
and offset null adjust R48, which are
adjusted to set up the visual power
calibration output.