User's Manual
300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-18
above 170° F, it closes and applies a low
to J8-1 or 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.
VSWR Cutback Fault
The reflected power sample of the RF
output of the transmitter is connected to
J2, pin 9, of the board. The sample
connects to op-amp U1B, pin 5, which
buffers the signal before it is split. One of
the reflected samples that is split is
connected to J1-5, which is wired to J10-
5 on the rear of the tray, for remote
monitoring. The other reflected sample
that is split is connected to position 3 on
the front panel meter of the tray. The
final remote reflected sample that is split
is connected to U2B, pin 5.
If the reflected sample level increases
above the level set by R22, the VSWR
cutback pot, the output of U2B at pin 7
goes high. The high is connected to Q11
through CR11, which is biased on,
making U2C, pin 10, low and causing
U2C, pin 8, to go low. This low is split
and fed out of the tray at J1-6, J1-7, J1-
8, and J1-9; these are ALC outputs to the
amplifier trays that cut back the output
power of the amplifier trays. The low
from U2C, pin 8, is also fed through
coaxial jumper W2 on J13 and J14 to
R73. R73 is a bias adjust pot that sets
the level of the pin-attenuator bias that is
available as an output at J16.
The high at U2B, pin 7, is fed to the base
of Q14 and Q13, which are forward
biased, and produces a low at the drains.
The low connects to the front panel
amber VSWR Cutback LED DS7 and
causes it to light, indicating that the tray
is in cutback, and to output jack J8-37
for a connection to a remote VSWR
cutback indicator.
Receiver ALC Fault
If a receiver tray is part of the system, a
sample of the ALC voltage from the
receiver tray is connected to J8-11 on the
board. If the receiver is operating
normally, the ALC level applied to U3C,
pin 9, remains below the trip level set by
R35; the output at pin 13 stays high. The
high is applied to the red ALC Fault LED
DS8, which is off. The high also connects
to U3A, pin 2, and to Q15. Q15 is biased
on and the drain goes low. The low
connects to U5B, pin 6. U5B normally has
a low that is connected to U5B, pin 5,
and produces a high at the output pin 4.
The high is wired to Q18, which is biased
on, and makes its drain low. The low
connects to U3D, pin 12, which, because
the level is below the preset, its output at
U3D, pin 14, goes low. A low at this point
indicates a no-fault condition. The high
connected to U3A, pin 2, causing its
output to go low. The low is connected to
Q25, which is biased off. The low is
removed from J8-12, which will not light
any remote receiver fault indicator that is
connected to it.
If the receiver should malfunction, the
ALC level applied to U3C, pin 9, goes
high; because this is above the level set
by R35, the output at pin 13 will go low.
The low is applied to the red ALC Fault
LED DS8, which lights. The low also
connects to U3A, pin 2, and to Q15. Q15
is biased off and the drain goes high. The
high connects to U5B, pin 6, which
produces a low at its output at pin 4. The
low is wired to Q18, which is biased off,
and makes its drain high. The high
connects to U3D, pin 12; because the
level is above the preset, the output at
U3D, pin 14, goes high. A high at this
point indicates a fault condition that
switches the transmitter to Standby. The
low connected to U3A, pin 2, causes its
output to go high. The high is connected
to Q25, which is biased on, causing its
drain to go low. The low is connected to