sub-audible signals without degrading the ON AIR sound
of the transmitter.
c. DC OFFSET: Some audio processors have a DC offset
which can shift with modulation, at either a sub-audible
or audio rate. This shifting offset voltage will generate
carrier shift, and if the “offset” shifts in a positive direc-
tion at the same time as positive modulation peak occurs,
an Overcurrent overload could occur.
WARNING
ENSURE ALL PRIMARY AC POWER IS REMOVED FROM THE
TRANSMITTER AND THAT THE GROUNDING STICK HAS BEEN
USED TO DISCHARGE ANY RESIDUAL VOLTAGE WHERE
POWER HAS BEEN APPLIED BEFORE PERFORMING THE FOL-
LOWING STEPS.
d. OPEN +230 VDC RECTIFIER FUSE: An open 250 Amp
fuse for one of the +230 VDC supply rectifiers can cause
excessive power supply ripple which can cause the trans-
mitter to trip off with an Overcurrent overload when
modulated with low frequencies. Remove the rear panel
to the PA Power Supply and check all rectifier fuses. If an
open fuse is detected, check the associated rectifier for a
short.
e. OVERLOAD ADJUSTMENT: To check the setting of the
Overcurrent overloads, refer to procedures in SECTION
V, Maintenance.
CAUTION
DO NOT MAKE ANY ADJUSTMENTS TO THE OVERCURRENT
OVERLOAD SETTINGS UNTIL ALL OF THE PRECEDING
CHECKS LISTED ABOVE HAVE BEEN PERFORMED. TRANSMIT-
TER DAMAGE COULD OCCUR IF THE OVERCURRENT OVER-
LOADS ARE IMPROPERLY SET.
6.11.2
Faults With Tone Modulation. Possible Causes:
If the transmitter is being tested with tone modulation it is
possible to generate Overcurrent overloads with high level low
frequency modulation. Second, many audio test generators will
also have a DC offset voltage in their output when they are
switched from one frequency range to another; this offset can
cause an overload. Third, if the transmitter is turned ON with a
high level, low frequency tone at the audio input, overloads may
occur due to the surge current produced as the transmitter is
ramping up to power with full modulation.
6.11.3
Overloads on Turn On. Possible Causes:
a. OVERMODULATION: If the transmitter indicates an
Overcurrent overload on turn on, the most likely cause is
low frequency, high level modulation. Lower the modu-
lation level before turning on the transmitter.
b. POWER CONTROL CIRCUIT: A problem in the power
control circuit that would release the PA OFF command
before the PA Power Supply capacitors had fully charged
could cause an Overcurrent overload on turn on. Turn the
PA OFF switch to the OFF (up) position on the Controller.
If the transmitter turns ON, refer to the turn-on sequence
in SECTION P, Controller, and SECTION J, Analog Input
Board, for additional information.
WARNING
ENSURE ALL PRIMARY AC POWER IS REMOVED FROM THE
TRANSMITTER AND THAT THE GROUNDING STICK HAS BEEN
USED TO DISCHARGE ANY RESIDUAL VOLTAGE WHERE
POWER HAS BEEN APPLIED BEFORE PERFORMING THE FOL-
LOWING STEPS.
c. PA POWER SUPPLY SHORT: If the transmitter will not
turn ON with the PA OFF switch in the OFF position, the
problem could be in the PA Power Supply. Isolate the PA
Power Supply from the RF amplifiers by removing F1
through F9 on Fuse Board A24, F1 through F8 on A25,
and F20 located on T1. Measure the resistance to ground
of the PA Power 230 VDC bus bar on the Fuse
Boards. The resistance should be approximately 500
Ohms. If it is not, troubleshoot the PA Power Supply
Discharge Circuit (crowbar). Refer to the DX-25U Over-
all Schematic, 839-7855-151, in the Drawing Package for
circuit details.
6.12
ColorStat™ panel Overvoltage Fault
The Overvoltage Fault indicates that the PA Power Supply
voltage is excessive.
6.12.1
Possible Causes
6.12.1.1
Supply Voltage Too High
NOTE
The PA Power Supply voltage will be at its highest with the PA
Power Supply ON and the power output at zero. The supply
voltage will be at its highest. The transmitter should not incur
an overload in this condition.
If the transmitter has been operating with the proper supply
voltage for some time, check to see if the power line voltage has
increased for the normal operating voltage. Measure the AC line
voltage and tap both the high and low voltage transformers for
the correct voltage. If the transmitter will remain ON long
enough to measure the PA VDC on the front panel
multimeter, compare this reading to that recorded on the factory
Test Data Sheet. If the reading is significantly higher, the PA
Power Supply transformer primary tapping must be changed. If
the transmitter will not stay on long enough for a measurement,
tap the transformer down to obtain the next lowest supply
voltage.
NOTE
The highest tap setting for T1 is 485/+4% (502VAC). If line
voltage is higher than 505VAC it will be necessary for the utility
company to lower the line voltage.
Section VI - Troubleshooting
Rev. R: 11-11-96
888-2297-002
6-9
WARNING: Disconnect primary power prior to servicing.
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