connector or removed from the cabinet, the load resistors will
absorb some power.
The amount absorbed is slightly higher when the modules are
disconnected and removed than when they are merely shut off.
This is because removing a module presents an open circuit to
that combiner port, as opposed to merely disabling the module,
in which the combiner sees the isolation resistors present in the
module’s output combiner.
The amount of power absorbed by any single load is highest
when a single module is removed. The total amount of power
absorbed by all the resistors is highest when half of the modules
are removed.
4.2.7.2
Aural Chain
As in the visual chain, the aural drive output is split multiple ways
and sent to parallel PA modules. The number of ports in the
combiner and divider depend on system power output and op-
tions, such as 20% aural power or parallel aural drive.
The outputs are recombined in a similar combiner. As in the
visual chain, the combiner reject loads are mounted on a heatpipe
located on the cabinet side wall.
4.2.8
Control Circuits
4.2.8.1
Inter-Cabinet Control Wiring
(See Cabinet Interconnect Wiring diagram)
The inter-cabinet wiring connects the control system to the other
cabinets in the transmitter system. Each cabinet has Monitor Data
Bus and Transmitter OFF/ON line connections. One end of the
first section of the monitor data bus connects to J4 on the monitor
board in the control cabinet, and the other end connects to J1-A
or B in the top of the next cabinet to the right of the control
cabinet.
Each subsequent section of the monitor bus connects J1-A or B
of two adjacent cabinets. The end cabinet will have either J1-A
or B connected to the monitor data bus, and the other jack
connected to a bus terminator, to prevent control signals from
reflecting back into the system from the far end.
The transmitter ON/OFF cable connects to J3 of the Main
Controller board. It contains a cable for each cabinet in the
system, and connects to J13 at top of each cabinet.
4.2.8.2
Main Controller
(See Main Controller schematic.)
The transmitter Main Controller and monitoring system are
located in the control cabinet. No microprocessor is used; the
digital circuitry is constructed entirely of discrete logic gates. A
battery back-up for the transmitter ON/OFF logic is included to
restore the transmitter to its prior operating condition after a brief
power interruption. A power down timer will automatically
reinitialize the transmitter to the OFF state if AC power is not
restored within about two hours.
The
MAIN CONTROLLER
interfaces to the remote control, front
panel controls, monitoring system, interlocks, exciter or exciter
switcher, and cabinet ON/OFF controls.
It also contains the VSWR Foldback circuit, VSWR Overload
fault logic, and RF peak detectors for sampling. These circuits
work together to protect the transmitter in the event of elevated
antenna system VSWR, in order to prevent danger to the trans-
mitter.
4.2.8.2.1
Debounce (sheet 1)
Inputs with pulsed raising or falling edges can cause problems if
the transitions are slow or if multiple transitions are present.
These signals are debounced for about 40 mS before being passed
on to the other logic. U1, the debounce IC, has six I/O debounce
buffers. The time of debounce is set by C2 to 4 or 5 cycles of the
master clock oscillator.
The remote ON/OFF commands, failsafe interlock, phase loss,
and spare inputs are all optocoupled. On the other hand, External
interlock is coupled directly into U1. Thus the voltage drop in
this loop must not be greater than 0.5 volts for a low, nor more
than 5 volts for a high. Be sure to check that the interlock satisfies
these limits before connecting it.
The +12 volts required to turn on the other interlocks is supplied
by the controller.
4.2.8.2.2
Transmitter ON/OFF (sheet 1)
The ON/OFF state of the transmitter is controlled by U5, the TX
ON/OFF latch. This latch is an R/S latch with Reset (CLEAR)
connected to the OFF logic signal, and Set (PRESET) connected
to the ON logic. The ON and OFF signals are interlocked in such
a way that an ON command cannot be asserted while the OFF
command is being asserted. The inputs come from U1, the
debounce IC.
The AC off timer, U4 and U5, reinitializes the transmitter to the
off state after an approximately 2 hour AC power failure. U4, the
timer IC, contains a 1 Hz oscillator and a binary counter. If the
counter reaches 2
13
before AC is restored, U4 pin 2 clocks U5
pin 11, the AC OFF latch, which returns the controller to the OFF
state even if the power has not yet returned.
All of the logic on sheet 1 of the schematic is battery-backed for
power failure conditions. If the battery fails, the transmitter will
come up in the OFF state when AC is restored. The battery is
monitored by the system monitor, and its voltage can be read on
the display.
The output of the TX ON/OFF latch is passed through U8 to U3,
the BAT BUFFER, and on to the cabinet on/off driver and other
logic on the board. U8 gates the TX PULSE one shot U2, which
pulses the cabinet ON command lines each time the ON push
button is pressed. This feature is used to re-enable a faulted
cabinet power supply or module when the CAB ON line is
pulsed.
4.2.8.2.3
Battery Buffer (sheet 1)
The
BATTERY BUFFER
U3 is used to turn off output drive to all
ICs that are not battery backed on power down conditions, to
reduce battery drain. U2, the PHASE STRETCHER, is a one-
shot with a three second duration, used so that short phase losses
can be logged by remote control equipment.
4-4
888-2365-001
WARNING: Disconnect primary power prior to servicing.
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