User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-15
-12 VDC line that is external to the UHF
upconverter board; the -12 VDC is then
applied to the rest of the board.
4.1.10 (A17) Transmitter Control
Board (1265-1311; Appendix D)
The transmitter control board provides
the system control functions and the
operational LED indications that can be
viewed on the front panel of the
transmitter. The main control functions
are for Operate/Standby and
Auto/Manual selection. When the
transmitter is switched to Operate, the
board supplies the enables to any
external amplifier trays. When the
transmitter is in Auto, the board also
performs the automatic switching of the
transmitter to Standby upon the loss of
the video input. The board contains a
VSWR cutback circuit. If the VSWR of the
transmitter increases above 20%, the
VSWR cutback circuit will operate 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 and, when the
interlock is present, the green Interlock
LED DS5 will be lit.
4.1.10.1 Operate/Standby Switch
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 switched to Operate,
one coil of relay K1 energizes; this
causes the contacts to close and apply a
low to U4B-9. If the transmitter interlock
is present, and there are no
overtemperature faults, lows will also be
applied to U4B-10, U4B-11, and U4B-12.
With all the inputs to U4B low, the output
at U4B-13 will be low. This low biases off
Q1, which turns off the amber Standby
LED DS1 on the front panel, and applies
a high to Q2. The high 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, which biases it
off, and allows the ALC to be applied to
J1, which is connected to any external
amplifier trays. The low from U4B-13 is
also applied to Q4 and Q24, which are
biased off, and removes the disables
from J1-4 and J18-1. The low from U4B-
13 also connects to Q10, which is biased
on, connects a high to Q6, Q7, Q8, and
Q9, which are also biased on, and applies
-12 VDC enables at J8-2, 3, 4, and 5,
which connect to any external amplifier
trays. The high applied to Q2 is also
connected to Q5 and Q26, which are
biased on, and applies a low enable to
J1-3, which can connect to a remote
operate indicator. The transmitter is now
in Operate.
When the Operate/Standby switch S1 is
switched to Standby, the other coil of
relay K1 energizes, which causes the
contacts to open. A high, +12 VDC, is
applied to U4B-9. The high at the input
causes the output at U4B-13 to go high.
The high biases on Q1, which applies a
low to the amber Standby LED DS1 on
the front panel and turns it on. Q1 also
applies a low to Q2, which turns off and
extinguishes the green Operate LED DS2.
In addition, Q12 is biased on and
connects to U2C, whose output goes low
and pulls the ALC voltages at J1 low,
lowering the gain of the external
amplifier trays. The high from U4B-13 is
applied to Q4 and Q24, which are biased
on, and applies disables at J1-4 and J18-
1. The high from U4B-13 also connects to
Q10, which is biased off. When Q10 is
off, it removes the high from Q6, Q7, Q8,
and Q9, which are biased off, and
removes the -12 VDC enables at J8-2, 3,
4, and 5 that connect to the external
amplifier trays. The low applied to Q2 is
also connected to Q5 and Q26, which are
biased off, and removes the remote
enable at J1-3. The transmitter is now in
Standby.
4.1.10.2 Automatic/Manual Switch
K2 is a magnetic latching relay that
switches the operation of the transmitter
to Automatic or Manual using