User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-19
or auto gain by its position on the jack.
Between 1 and 2 is manual gain, which
uses pot R9 to set the output level.
Between pins 2 and 3 is auto gain, which
uses an external control voltage input at
jack J4 as the level control. NOTE: The
335B transmitter operates in Manual
only.
The level set RF is pre-amplified by U1
and connected to Q1, the output
amplifier for the board. The RF output is
amplified by Q1 and connected to a
matching network with C17 adjusted to
maximize the RF signal level that is fed
to the direction coupler Z1. The RF exits
pin 4 of Z1 and connects to J2, the RF
output jack of the board (+10 dBm to
+20 dBm). Z1 provides a RF sample at
pin 3 that is split. The first split sample
connects to J8 through a voltage divider
consisting of R19 and R18 that is fed to
the front panel of the VHF exciter tray for
monitoring purposes. The second split
sample provided by Z1, pin 3, is fed
through a pad consisting of R20, R21,
and R22. The voltage is stepped up by a
1 to 4 transformer T1. The signal is then
peak detected by C32 and CR4 before
being buffered and amplified by U2A and
U2B. The peak-detected voltage at J9-1
and J9-2, which is used for metering
purposes, through the transmitter control
board, is level controlled by the pot R29
on the board.
The ±12 VDC needed for the operation of
the board is supplied by an external
power supply in the tray. The +12 VDC
enters the board at J3 pin 3, and is
filtered and isolated by L5 and C19
before being applied to the rest of the
board. The –12 VDC enters the board at
J3 pin 5, and is filtered and isolated by L6
and C35 before being applied to the rest
of the board.
4.1.8 (A17) Transmitter Control
Board (1265-1311; Appendix D)
The transmitter control board provides
system control functions and the
operational LED indications, which can be
viewed on the front panel of the
transmitter. The main control functions
are the Operate/Standby and
Auto/Manual selections. When the
transmitter is switched to Operate, the
board supplies the enables to the three
external VHF amplifier trays. The board
also performs the automatic switching of
the transmitter to Standby upon the loss
of the video input when the transmitter is
in Automatic.
The transmitter control board contains a
VSWR cutback circuit. If the VSWR of
the transmitter increases above 20%, the
VSWR cutback circuit will become active
and cut back the output level of the
transmitter, as needed, to maintain a
maximum of 20% VSWR.
An interlock (low) must be present at J8-
24 for the transmitter to be switched to
Operate. When the interlock is present,
the green Interlock LED DS5 will be lit.
4.1.8.1 Operate/Standby Switch S1
K1 is a magnetic latching relay that
controls the switching of the transmitter
from Operate and Standby. When the
Operate/Standby switch S1, on the front
panel of the tray, is moved to Operate,
the coil connected to pins 3 & 4 of relay
K1 energizes and causes the contacts to
close and apply a low to U4B-9. If the
transmitter interlock is present, and
there is no overtemperature fault, lows
will also be applied to U4B-10, 11, and
12. With all the inputs low to U4B, the
output at U4B-13 will also be low. This
low biases off Q1 that turns off the
amber Standby LED DS1 on the front
panel. Q1 off applies a high to Q2 that
turns on and lights the green Operate
LED DS2, also on the front panel. When
Q2 is biased on, it connects a low to Q12
that biases it off; which allows the ALC,
from J6 through U2C, to be applied to J1
and connect to the three external VHF
amplifier trays. The low from U4B-13 is
also applied to Q4 and Q24, which are
biased off that removes the disables from
J1-4, which connects to the remote