User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-22
high at U5C pins 8 and 9, causes its
output at pin 10 to go low. This low is
connected to U5D pin 12. If the
transmitter is in Auto, pin 13 of U5D is
also low. The lows on pins 12 and 13
cause the output of U5D to go high and
forward bias Q19. The drain of Q19 goes
low and energizes the coil connected to
pins 1 and 6 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
energizes to the operate coil connected
to pins 3 and 4 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.
Transmitter is in Operate
4.1.8.6 Overtemperature Fault
In the 2-kW transmitter, the thermal
switch on (A8) the 3 way combiner
assembly connects to J8-1 on the board.
If the temperature of the heatsink on
which the thermal switch is mounted
rises above 175° F, the switch closes and
applies a low to J8-1. The low from J8-1
connects to the Overtemperature LED
DS6, which is biased on. The low also
connects to Q3, which is biased off,
causing the drain of Q3 to go high that
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, which removes the Operate
Enable commands to the three external
VHF amplifier trays. After the thermal
switch cools below 175° F, the
transmitter will switch back to operate.
4.1.8.7 VSWR Cutback Fault
The reflected power sample of the RF
output of the transmitter, through the
visual/aural metering board, 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 that 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 this
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 AGC outputs to the VHF
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 power adjust, bias-adjust,
pot that sets the level of the pin
attenuator bias available as an output at
J16. NOTE: This bias output is not used
in the 335B. 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