User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-25
4.1.9.2 Visual Level Circuit
The other detected visual + aural level
output from U6B is connected to U1C
and, if there is no scrambling, connects
directly to intercarrier notch L3, which is
adjusted to filter out the aural and the
4.5-MHz intercarrier frequencies, leaving
only a visual + sync output. The visual +
sync output is fed to a peak detector
circuit consisting of CR5 and U2A. The
signal is then fed through the visual
calibration control R28, which is adjusted
for a 100% visual reading with no aural,
to amplifier U2B. The amplified visual
peak of sync output is connected to
comparator U2C. The other input to U2C
is the level set by the aural null adjust
R51, which is adjusted for 100% visual
power after the aural is added and the
peak power is adjusted back to the
reference level and also from the offset
null adjust R48, which is adjusted for 0%
visual power with the transmitter in
Standby. The adjusted output is
amplified by U3D and connected to the
other input of U2C pin 9. The output of
U2C connects to J6 pins 2 and 3, which
supply the peak of sync visual level
output to the front panel meter for
monitoring.
If this board is operated with scrambling,
using suppressed sync, the visual level
circuit operates differently than described
above because there is no peak of sync
present on the forward sample input. For
the board to operate properly, a timing
pulse from the scrambling encoder must
connect to the board at J4. This timing
pulse is converted to sync pulses by U4A
and U4B, which control the operation of
Q2. Intercarrier notch L2 is tuned to
remove any visual + aural signal that
may remain. The sync amplitude is
controlled by the gate amplitude adjust
R25 and then applied to the minus input
of U1C. At this point, the sync is inserted
into the visual + aural signal that is
connected to the plus input of U1C,
producing a peak of sync in the signal.
The output of U1C is connected to
intercarrier notch L3, which is adjusted to
filter out the aural and the 4.5-MHz
intercarrier frequencies. The visual +
sync output is fed to a peak detector
circuit, consisting of CR5 and U2A, and
then fed through the visual calibration
control R28 to amplifier U2B. The
amplified visual peak of sync output is
connected to J6 pins 2 and 3, which
supply the peak of sync visual level
output to the front panel meter for
monitoring. R32 moves the pulse to
where the sync should be and R25 sets
the visual metering calibration with no
sync present.
4.1.9.3 Reflected Level Circuit
A reflected-power sample is applied to J3
of the visual/aural metering board and is
detected by the diode detector circuit
CR7 and U3B. The detected output is fed
through the reflected calibration pot R39,
which can be adjusted to control the gain
of U3C. The output of U3C connects to
J6 pin 7, which supplies a reflected power
level output to the transmitter control
board for VSWR cutback and also to the
front panel meter.
4.1.9.4 Voltages for Circuit Operation
The ±12 VDC is applied to the board at
J5. The +12 VDC is connected to J5 pin
3, and is isolated and filtered by L4 and
C34 before it is connected to the rest of
the board. One +12 VDC line connects
to U5, a 5-VDC regulator that provides
the voltage needed to operate U4. The
-12 VDC is applied to J5 pin 5, and is
isolated and filtered by L5 and C35
before it is connected to the rest of the
board.
4.1.10 (A14) Channel Oscillator
Assembly, Dual Oven (1145-1202;
Appendix D)
NOTE: If the precise frequency kit is
present in your transmitter, the VCXO
Assembly (1145-1206) will be used.
The channel oscillator assembly contains
(A14-A1) the channel oscillator board