User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-21
4.1.8.3 The Automatic Turning On and
Off of the Transmitter Using the Presence
of Video.
The transmitter control board also allows
the transmitter to be turned on and off
by the presence of video at the
transmitter when the transmitter is in
Auto. When a video fault occurs due to
the loss of video, J7-5 goes low. The low
is applied through the jumper W1, on
J10, to Q16, which is biased off, and to
the red Video Loss Fault LED DS9, on the
front panel, which will light. The drain of
Q16 goes high and connects to U5B pin
5, causing the output at pin 4 to go low.
The low connects to Q18, which is biased
off, and causes the drain of Q18 to go
high. The high connects to U3D pin 12,
whose output at pin 14 goes high. The
high connects to U5C pins 8 and 9,
causing its output at pin 10 to go low.
The high also connects U5A pin 1,
causing its output at pin 3 to go low.
With S2 set to Automatic, a low is applied
to U5A pin 2, and to U5D pin 13. When
U5A pin 1 is high and U5A pin 2 is low, it
causes the output at pin 3 to go low.
When U5D pin 12 is low and U5D pin 13
is low, it causes its output to go high.
When U5A pin 3 is low, it biases off Q20
and removes any pull down to the
Operate switch. A high at U5D pin 11,
biases on Q19 and applies a low enable
to the Standby switch that places the
transmitter in the Standby mode.
When the video signal is returned, J7-5
goes high. The high is applied to Q16,
which is biased on, and to the red Video
Fault LED DS9, which is extinguished.
The output of Q16 goes low and connects
to U5B pin 5. If there is no receiver ALC
fault, U5B pin 6, is also low, this causes
the output at pin 4 to go high. The high
connects to Q18, which is biased on, and
causes the drain of Q18 to go low. The
low connects to U3D pin 12, whose
output at pin 14 goes low. The low
connects to U5C pins 8 and 9, which
causes its output at pin 10 to go high.
The low also connects to U5A pin 1. With
the Auto/Manual switch S2 in Auto, a low
is applied to U5A pin 2, and to U5D pin
13. When U5A pins 1 and 2, are low, its
output at pin 3 goes high. When pin 12
of U5D is high, the output of U5D at pin
11 goes low. When U5A pin 3, is high, it
biases on Q20 and applies a pull-down
enable to the Operate switch. A low at
U5D pin 11, biases off Q19 and removes
any pull down to the Standby switch. As
a result of these actions, the transmitter
is switched to Operate.
4.1.8.4 Faults
There are four possible faults that may
occur in the transmitter and are applied
to the transmitter control board. They
are video loss fault, VSWR cutback fault,
overtemperature fault, and ALC fault.
During normal operation, no faults are
sent to the board. The receiver ALC fault
circuit will only function if a receiver tray
is part of the system. The
overtemperature fault is controlled by the
temperature of the heatsink of the
system 3 way combiner assembly.
4.1.8.5 Video Loss Fault
If a video loss occurs while the
transmitter is in Auto, the system will
switch to the Standby mode until the
video is returned. When this happens
the transmitter will immediately revert to
Operate. A video loss fault applies a low
from the ALC board to the video fault
input at J7-5 on the transmitter control
board. With jumper W1 in place on J10,
the video fault is connected to the LED
DS9 and to Q16. The red Video Loss
Fault LED DS9 on the front panel will
light. Q16 is biased off and causes its
drain to go high. The high is wired to
U5B pin 5, whose output at U5B pin 4
goes low. The low is wired to Q18, which
is biased off, and causes the drain to go
high. The high is connected to U3D pin
12, which causes its output at U3D pin
14 to go high. The high connects to U5A
pin 1. If the transmitter is in Auto, pin 2
of U5A is low. When pin 1 is high and pin
2 is low, the output of U5A goes low and
reverse biases Q20, shutting it off. The