Manual Chapter 3

500-Watt VHF Low Band Transmitter Chapter 3, Circuit Descriptions
325A, Rev. 0 3-19
3.1.8.4 Faults
There are four possible faults, video loss
fault, VSWR cutback fault,
overtemperature fault, and ALC fault,
which may occur in the transmitter and
are applied to the transmitter control
board. 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 only used with
2-kW transmitters and is controlled by
the temperature of the reject load.
3.1.8.5 Video Loss Fault
If a video loss occurs while the
transmitter is in Auto, the system will
change to the Standby mode until the
video is returned; at that point, it 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 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, and, 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 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 be extinguished. Q16 is
biased on, which causes its drain to go
low. The low is wired to U5B, pin 5; 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, causing
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 and causes its
drain to go low. The low connects to the
operate coil on relay K1 that switches the
transmitter to Operate. The low at U5C,
pins 8 and 9, causes its 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 this removes the low
from the standby coil in relay K1.
3.1.8.6 Overtemperature Fault
In the 500 Watt transmitter, there is no
connection to the overtemperature circuit
on the transmitter control 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, the (A6) thermal switch on
(A23) the 100-watt amplifier heatsink
assembly connects to J12 on the board.
If the temperature of the thermal switch
raises above 170° F, it closes and applies
a low to J8-1 or to J12. The low connects
to Q3, which is biased off, and to the red
Overtemperature LED DS6, which is
biased on. The drain of Q3 goes high and
connects to pins 11 and 12 of U4B. The
high at the input to U4B causes it to go
high and switches the system to
Standby; this removes the Operate
Enable commands to any external
amplifier trays.