•
TB3-3: Audio (-)
•
TB3-4: Optional shield connection, capacitively coupled to
transmitter chassis ground.
Audio Input TB3 is shown on sheet 2 of the DX-25U Overall
Schematic, 839-7855-151. The following paragraphs include
additional information which may be useful in planning and
connecting the audio input.
2.12.1.1
Shield Connections
The audio input cable shield should be grounded at only one end,
either at the processor or at Audio Input terminal TB3-1. Con-
necting the shield at both ends can result in ground loop problems
and increased system noise. Audio Input terminal TB3-4 pro-
vides an AC coupled path to the transmitter cabinet. In some
installations, lower noise may be obtained by connecting the
audio cable shield to this terminal.
2.12.1.2
Polarity
If the audio processor has output terminals marked “+” and “-,”
connect one wire of the audio pair (typically red) between the
processor’s “+” output and the transmitter’s “+” input terminal,
and connect the other wire (typically black) between the “-”
terminals.
2.12.1.3
Audio Source Impedance
The transmitter uses a Bessel filter at the audio input to obtain
superior overshoot performance. Performance of this filter de-
pends on the source impedance of the processing equipment. The
load impedance of the processing equipment is not necessarily
its source impedance; for example, modern equipment may be
specified for a 600 Ohm load but have a very low source imped-
ance, 50 Ohms or less.
2.12.1.4
Selecting Source Impedance
A white Molex connector on audio input cable number 100, from
the External Interface, plugs into J1, J2, or J3 on the Analog Input
Board. Source impedance for each input is silk screened on the
board. For optimum performance and best high frequency re-
sponse, use the jack labeled with an impedance closest to the
source impedance of the processing equipment.
This connection is NOT critical; using the wrong connector may
result in some overshoot or undershoot on square waves and a
slight change in audio frequency response (typically less than 1
dB at 10 Khz). If the processing equipment source impedance
cannot be determined, try “Rs = 600 Ohms” for older transformer
equipment, and “Rs less than 50 Ohms” for newer equipment
with direct coupled operational amplifier outputs.
2.12.2
Modulation Monitor Connection
If a modulation monitor is used at the transmitter site, run a 50
Ohm coaxial cable from BNC jack J5 on the Output Monitor to
the modulation monitor input. The Output Monitor is located at
the top of the left side wall of the Center Control Compartment.
If the modulation monitor has a high input impedance rather than
an internal 50 Ohm termination, a 50 Ohm, 3 to 5 Watt termina-
tion should be placed across the Modulation Monitor input
terminals.
Do not connect the coaxial cable to the monitor until instructed
to do so during the Initial Turn On Procedure.
2.12.3
Frequency Monitor Connection.
If a frequency monitor is used at the transmitter site, a coaxial
cable should be run from BNC jack J5 on the Oscillator to the
frequency monitor input. The Oscillator is located toward the
bottom of the right side wall in the Center Control Compartment.
The RF sample at J5 is a 5 V peak unmodulated signal, at the
transmitter carrier frequency.
2.12.4
External RF Input
An external RF input from a frequency synthesizer or AM Stereo
exciter is connected to J2 on the Oscillator. Jumper P3 on the
Oscillator should be moved to the 1-2 position. Jumper P5 on the
Oscillator should be used to select the correct impedance for the
external RF source. Refer to SECTION A, Oscillator, for addi-
tional information.
2.12.5
External Interlock (Failsafe)
The External Interlock should be used for any function which
should turn the transmitter OFF by turning off the high voltage
supply. Examples include Failsafe connections and safety inter-
locks on phasor cabinets or other enclosures which could expose
personnel to RF when opened.
More than one External Interlock can be used by connecting the
normally closed interlock switches in series. All External Inter-
lock switches should be closed when the interlocked enclosure
or circuit is in the safe condition, and should open when in the
unsafe or fault condition.
External (remote) Control interface connections are made at TB1
and TB2, at the top of the right side wall in the Center Control
Compartment. Terminal information is silk screened on the back
of the Driver Compartment door.
EXTERNAL INTERLOCKS should be connected between
TB1-1 and TB1-2.
a. A CLOSED circuit between TB1-1 and TB1-2 allows the
transmitter to turn ON.
b. An OPEN circuit between TB1-1 and TB1-2 turns the
transmitter OFF by interrupting the 24 VAC coil voltage
for the External Interlock relay K3. The transmitter cannot
be turned on again as long as there is an OPEN circuit
between the External Interlock terminals. The 24 VAC is
fused at 1 Amp by F24. External interlock contacts and
wiring should be rated for 1 Ampere AC current.
c. If NO External interlocks are used, a jumper must be
connected between External Interlock terminals TB1-1
and TB1-2.
NOTE
Do not use the External Interlock for antenna pattern switching,
which requires only a brief interruption of RF output. The EX-
TERNAL PA TURN OFF connection, described under Remote
Control connections later in this section, should be used for that
purpose.
DX-25U
2-6
888-2297-002
Rev. AA: 8/9/2000
WARNING: Disconnect primary power prior to servicing.
