User's Manual

2650 Watt VHF High Band Transmitter Chapter 4, Circuit Descriptions
435B, Rev. 0 4-17
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 from the rest of the
tray by L5 and C19 before being applied
to the entire board. The –12 VDC enters
the board at J3, pin 5, and is filtered and
isolated from the rest of the tray by L6
and C35 before being applied to the
entire board.
4.1.8 (A17) Transmitter Control
Board (1265-1311; Appendix D)
The transmitter control board provides
information on system control functions
and the operational LED indications;
these can be viewed on the front panel of
the transmitter. The main control
functions are for the Operate/Standby
and Auto/Manual selections. When the
transmitter is switched to Operate, the
board supplies the enables to any
external 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
Auto.
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
and, 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,
one coil 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,
U4B-11, and U4B-12.
With all the low inputs to U4B, the output
at U4B-13 will be low. The low biases off
Q1 and this turns off the amber Standby
LED DS1 on the front panel. In addition,
this action applies a high to Q2 and 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 and
biases it off; this allows the ALC to be
applied to J1 and connect 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, and connects a high to Q6, Q7, Q8,
and Q9; these are biased on and apply
-12 VDC enables to J8-2, J8-3, J8-4, and
J8-5, which connect to any external
amplifier trays. The high applied to Q2 is
also connected to Q5 and Q26, which are
biased on, and apply a low enable to J1-
3, which connects to a remote operate
indicator. The transmitter is now in the
Operate mode.
When the Operate/Standby switch S1 is
moved to Standby, the other coil of relay
K1 energizes, causing the contacts to
open and a high (+12 VDC) to be applied
to U4B-9. The high at the input causes
the output at U4B-13 to go high. The
high biases on Q1 and applies a low to
the amber Standby LED DS1, on the
front panel, and turns on and applies a
low to Q2. This causes Q2 to turn off and
extinguishes the green Operate LED DS2.
When Q12 is biased on, the output from
U2C goes low and pulls the ALC voltages
at J1 low; this lowers 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
connects to Q10, which is biased off. The
Q10 bias off removes the high from Q6,
Q7, Q8, and Q9, which are biased off,
and removes the -12 VDC enables at J8-
2, J8-3, J8-4, and J8-5, which connect to
the external amplifier trays. The low