Manual Chapter 3

500-Watt UHF Transmitter Chapter 3, Circuit Descriptions
425A, Rev. 0 3-20
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.
3.1.8.7 VSWR Cutback Fault
The reflected power sample of the RF
output of the transmitter is connected to
J2, pin 9, of the transmitter control
board. The sample connects to op-amp
U1B, pin 5, which buffers the signal
before it is split. One of the split-reflected
samples connects to J1-5 on the board;
J1-5 is wired to J10-5 on the rear of the
tray for remote monitoring. Another split-
reflected sample connects to position 3
on the front panel meter for the tray. The
final split remote-reflected sample
connects 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
available as an output at J16. The high at
U2B, pin 7, is also fed to the base of Q14
and Q13, which are forward biased. This
produces a low at the drains that connect
to the front panel amber VSWR Cutback
LED DS7, causing it to light and indicate
that the tray is in cutback, and to output
jack J8-37 for the connection to a remote
VSWR cutback indicator.
3.1.8.8 Receiver ALC Fault
If a receiver tray is part of the system, a
sample of the ALC voltage from this tray
is connected to J8-11 on the transmitter
control board. If the receiver is operating
normally, the ALC level that is applied to
U3C, pin 9, remains below the trip level
set by R35; as a result, 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. In addition, U5B normally has a low
that is connected to U5B, pin 5, and
produces a high at 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, the output at U3D, pin
14, goes low. A low at this point indicates
a no-fault condition. The high that is
connected to U3A, pin 2, causes 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 malfunctions, the ALC
level applied to U3C, pin 9, goes high.
This is above the level set by R35 and
causes the output at pin 13 to 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, and
produces a low at output pin 4. The low
is wired to Q18, which is biased off, and
this makes its drain go high. The high
connects to U3D, pin 12 and, 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, and causes