User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-17
13. With U5A, pins 1 and 2, low, the
output at pin 3 goes high. With pin 12 of
U5D high, the output of U5D at pin 11
goes low. When U5A, pin 3, is high, it
biases on Q20, which applies a pull-down
enable to the Operate switch. A low at
U5D, pin 11, biases off Q19, which
removes any pull-down to the Standby
switch. The transmitter is switched to
Operate.
4.1.10.4 Faults
There are four possible faults that may
occur in the transmitter and are applied
to the board: video or modulation loss
fault, VSWR cutback fault,
overtemperature fault, and ALC fault.
During normal operations there are no
faults to the board. The receiver ALC
fault circuit will only function if a receiver
tray is part of the system. The
overtemperature fault is only used with
the 2-kW transmitter and is controlled by
the temperature of the reject load.
Video or Modulation Loss Fault
If a video or modulation loss occurs while
the transmitter is in Auto, the system will
go to Standby after a few seconds until
the video is returned. 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 board.
With jumper W1 in place on J10, the
video fault is connected to the red Video
Loss Fault LED DS9, causing it to light,
and to Q16. Q16 is biased off, causing 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 the output at U3D, pin
14, to go high. The high connects to U5A,
pin 1, and, if the transmitter is in Auto,
pin 2 of U5A is low. With pin 1 high and
pin 2 low, the output of U5A goes low
and reverse biases Q20, shutting it off.
The high at U5C, pins 8 and 9, causes its
output at pin 10 to go low. This low is
connected to U5D, pin 12, and, if the
transmitter is in Auto, pin 13 of U5D is
also low. The lows on pins 12 and 13
cause the output to go high and forward
bias Q19. The drain of Q19 goes low and
connects the coil in relay K1, causing it to
switch to Standby.
When the video returns, the video loss
fault is removed from the video fault
input at J7-5. With jumper W1 in place
on J10, the base of Q16 goes high. The
red Video Loss Fault LED DS9 on the
front panel will go out. Q16 is biased on,
causing the drain to go low. This low is
wired to U5B, pin 5, and U5B, pin 6, will
be low if no ALC fault occurs. The two
lows at the inputs make the output at
U5B, pin 4, go high. The high is wired to
Q18, which is biased on, and causes the
drain to go low. The low is connected to
U3D, pin 12, which causes its output at
U3D, pin 14, to go low. The low connects
to U5A, pin 1, and, if the transmitter is in
Auto, pin 2 of U5A is also low. With both
inputs low, the output of U5A at pin 3
goes high. The high forward biases Q20,
causing its drain to go low. The low
connects to the Operate coil on the K1
relay that switches the transmitter to
Operate. The low at U5C, pins 8 and 9,
causes the output at pin 10 to go high.
This high is connected to U5D, pin 12,
and, if the transmitter is in Auto, pin 13
of U5D is low. The high on pin 12 causes
the output of U5D to go low and reverse
bias Q19. The drain of Q19 goes high and
removes the low from the Standby coil in
relay K1.
Overtemperature Fault
In a transmitter of 1 kW or less, there is
no connection to the overtemperature
circuit on the board. In the 2-kW
transmitter the thermal switch on the
output dummy load connects to J8-1 on
the board. In the 100-watt transmitter
(A6) the thermal switch on (A23) the
100-watt amplifier heatsink assembly
connects to J12 on the board. If the
temperature of the thermal switch rises