Harris DX 25U, Техническое руководство

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6.27.2.3
Bandpass Filter VSWR Fault
6.27.2.3.1
Bandpass Filter VSWR Caused By Problems In The
Output Network
If a problem occurs in the output network of the transmitter due
to a component failure, the Bandpass Filter VSWR circuit will
protect the transmitter until the component can be replaced.
a. Do NOT attempt to further raise Power.
b. Do NOT change Tuning or Loading controls
NOTE
During normal operation if a short term VSWR occurs, such as
that caused by lighting or static discharge on the antenna sys-
tem, both the Antenna and Bandpass filter indicators may flash.
The antenna VSWR circuit is set to trip slightly before the Band-
pass filter circuit. If only the Bandpass Filter indicator flashes,
this indicates a problem in the output network.
6.28
Symptom: Loss Of Positive Peak
Capability
6.28.1
Possible Causes
6.28.1.1
Power Supply Voltage Low
If the supply voltage for the RF amplifiers is lower than normal,
the positive peak capability will be reduced. Nominal PA Sup-
ply +VDC should be b225 and +235 VDC at 60 kW
output power. If the voltage is not close to the factory test data
sheet, the PA Power Supply transformer T1 must be retapped.
Refer to SECTION II, Installation, for information on selecting
the proper transformer taps.
6.28.1.2 Audio Processor Equipment Defective or Incorrectly Set
Many problems with positive peaks are due to defective or
incorrectly setup processing equipment. Check the manual for
that particular piece of equipment for setup and service infor-
mation.
6.28.1.3 Incorrect Transmitter Tuning
If the transmitter Loading and Tuning controls have not been
set properly or a change in the antenna impedance has occurred,
positive peaks can be affected. Refer to the initial turn on
procedure in SECTION II, Installation, for Tuning and Loading
adjustment procedure.
6.28.1.4 Transmitter Operated In FlexPatch™ Mode
If several PA Modules failed and other modules were substituted
using the FlexPatch™ feature, then the transmitter will have a
slightly reduced positive peak capability. The defective PA
Modules must be replaced and the transmitter returned to its
normal operating configuration before full positive peak capa-
bility will return.
6.28.1.5
Failed RF Amplifier
If an RF amplifier fails, the positive peak capability of the
transmitter will decrease. The power output will also decrease
if one of the first 23 PA Modules fail. Check the RF amplifier
fault LED’s to see if any are illuminated.
6.29
Symptom: Higher Than Normal
Audio Distortion
6.29.1
Possible Causes
6.29.1.1 Failed RF Amplifier(s)
If audible distortion is heard on the air and the problem is not
in the program content or audio processing equipment, the next
step is to determine if an RF amplifier has failed. If any RF
amplifier module between position RF33 to RF56 fails, the
carrier power will decrease. The failure of a higher step will not
be noticed as a drop in power. The audio distortion will also be
slightly higher but may not be noticeable with only one RF
amplifier failed. If an amplifier has failed, a substitute may be
used by the FlexPatch™ method. Refer to the FlexPatch™
procedure in this section.
6.29.1.2 Transmitter Mistuning
The transmitter will tune into a wide range of loads and still
produce very good audio performance. Refer to the initial turn
on procedure in SECTION II, Installation, for Tuning and
Loading procedures
6.29.1.3
Operating Into a Bandwidth Restricted Antenna
If the Distortion is poor, especially at the higher audio frequen-
cies, then the antenna impedance at the sideband frequencies
may be incorrect. Operate the transmitter into a known good
dummy load. If the high frequency distortion improves, suspect
the antenna system.
6.29.1.4 Low RF Drive Level To The PA Modules
Low RF drive levels can cause higher than normal distortion.
Typically, the RF drive to the PA Modules should be between
20 and 25 Vp-p. Refer to this section for information on meas-
uring RF drive levels.
6.29.1.5 Additional Tips For Troubleshooting Audio THD
If the distortion problem cannot be found using the above
means, an excellent way to determine if the distortion is in the
Analog Input or the analog to digital conversion process OR the
digital to analog conversion process in the Power Amplifier or
Output Network stage, is to measure the distortion out of the
Digital to Analog convertor circuit on the Analog to Digital
Converter. This sample is an actual reconstructed audio sample
from the Analog to Digital Converter. If any distortion is occur-
ring in the Analog Input Board, or in the Analog to Digital
conversion process, it will show up here. Connect a scope or a
distortion analyzer to J2 on the Analog to Digital Converter. If
the distortion is present here, troubleshoot the audio source,
Analog Input Board, or the Analog to Digital Converter. If the
distortion is not present, the distortion is occurring in the D to
A process and could be in the Modulation Encoder, RF ampli-
fiers, or output Network.
DX-25U
6-16 888-2297-002 Rev. R: 11-11-96
WARNING: Disconnect primary power prior to servicing.