UBS Axcera 325A 500-Watt VHF Low-band Television Transmitter User Manual Chapter 5

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Document Title	Chapter 5

500-Watt VHF Low Band Transmitter
Chapter 5, Detailed Alignment Procedures
Chapter 5
Detailed Alignment Procedures
The 325A transmitter was aligned at the
factory and should not require additional
alignments to achieve normal operation.
1 (+), TB1-2 (-), and TB1-3 (ground). If
stereo/composite audio is provided,
connect it to BNC jack J6, the composite
audio input jack on the remote interface
panel. Jacks J3 and J13 on the rear of the
exciter panel are loop-through connected
and the unused jack can be used as an
audio source for another transmitter by
removing jumper W1 on jack J15 on the
aural IF synthesizer.
This transmitter operates using the
baseband audio and video inputs or, if
the (optional) 4.5-MHz composite input
kit is purchased, either a single
composite video + 4.5-MHz input or
separate baseband video and audio
inputs.
Look at the front panel meter on the VHF
exciter tray. In the Video position, the
meter indicates active video from 0 to 1
Vpk-pk. The normal video input level is 1
Vpk-pk on the meter. If this reading is
not at the proper level, the overall video
level can be changed by adjusting video
level control R12 on the sync tip clamp/
modulator board.
Check that the RF output at J2 of (A9-A5)
the coupler is terminated into a dummy
load of at least 500 watts. While
performing the alignment, refer to the
Test Data Sheet for the transmitter and
compare the final readings from the
factory with the readings on each of the
trays. They should be very similar. If a
reading is off by a significant amount, the
problem is likely to be in that tray.
Switch the meter to the Audio position to
show the audio deviation (modulation
level) of the signal from 0 to 100 kHz.
The aural IF synthesizer board was
factory set for a ±25 kHz deviation with a
balanced audio input of +10 dBm. If the
reading is at not the correct level, adjust
balanced audio gain pot R13 on the aural
IF synthesizer board, as needed, to attain
the ±25 kHz deviation. The aural IF
synthesizer board was factory set for a
±75 kHz deviation with a composite
audio input of 1 Vpk-pk. If this reading is
not correct, adjust composite audio gain
pot R17 on the aural IF synthesizer
board, as needed, for the ±75 kHz
deviation.
Switch on the main AC and the VHF
exciter circuit breakers on the AC
distribution panel behind the rear cabinet
door.
5.1 (A4) VHF Low-Band Exciter Tray
(1070820; Appendix C) with
Baseband Video and Audio Inputs
The (A4) low-band VHF exciter tray
(1070820) has adjustments for video
levels, audio modulation levels, and other
related parameters.
Connect an NTSC baseband video test
signal input (1 Vpk-pk) to the transmitter
video input jack J2 on the (A12) remote
interface panel. Jacks J1 and J2 on the
VHF exciter tray are loop-through
connected and the unused jack can be
used as a video source for another
transmitter by removing jumper W4 on
jack J3 on (A5) the sync tip clamp
modulator board (1265-1302). Connect a
baseband audio input (+10 dBm) to the
balanced audio input terminal block TB1-
325A, Rev. 0
5.2 (A4) VHF low-Band Exciter Tray
(1070820; Appendix C) with the 4.5MHz Composite Input Kit
With the 4.5-MHz composite input kit,
the (A4) VHF exciter tray is able to
operate using either the separate video
and audio baseband inputs or the single
4.5-MHz composite input.
5-1
500-Watt VHF Low Band Transmitter
Chapter 5, Detailed Alignment Procedures
The 4.5-MHz composite input kit includes
a composite 4.5-MHz filter board (12271244) and a 4.5-MHz bandpass filter
board (1265-1307).
the nonlinearities of the RF amplifier
trays. The upconverter section also
includes adjustments to the local
oscillator chain tuning and the local
oscillator center frequency tuning. Both
of these were completed at the factory
and should not require adjustments at
this time.
To align the VHF exciter using baseband
video and audio, refer to the alignment
instructions described in Section 5.1 of
this chapter. Select the baseband input
operation by a applying a baseband
select, using a jumper or closed contacts,
connected between J7-6 and J7-7 on the
rear of the tray.
Move the Operate/Standby switch on the
VHF exciter tray to Standby. The setup of
the RF output includes an adjustment to
the drive level of the two VHF amplifier
trays, the adjustment of the linearity and
phase predistortion (which compensate
for any nonlinear responses of the
amplifier trays), and the gain and
phasing adjustments of the two VHF
amplifier trays.
To operate the transmitter using the 4.5MHz composite input, remove the
baseband select command from J7-6 and
J7-7 on the rear of the tray.
Connect a multiburst test signal from an
envelope delay measurement set to the
input of the rear interface panel at J2. On
(A24) the composite 4.5-MHz filter board
(1227-1244), connect an oscilloscope
between J7, the center pin, and pin 1 or
3, which are ground. Adjust C21, if
necessary, for the best frequency
response. Adjust R32 for a signal level of
1 Vpk-pk on the oscilloscope. The output,
as measured at J6 and J7 of the board,
should be video only with a minimum
4.5-MHz aural subcarrier.
Verify that all of the red LEDs on the ALC
board are extinguished. The following list
describes the meaning of each LED when
they are illuminated:
On the (A25) 4.5-MHz bandpass filter
board (1265-1307), adjust the filter with
L2, C3, L4, and C7 for a frequency
response of no greater than ±.3 dB from
4.4 to 4.6 MHz. Adjust C19 for an overall
peak-to-peak variation of less than ±.3
dB from 4.4 MHz to 4.6 MHz. Recheck
the frequency response; it may have
changed with the adjustment of the
envelope delay.
5.3 (A4) VHF Exciter Tray (1070820;
Appendix C) with either Baseband or
the 4.5-MHz Composite Input
The IF section of the (A4) VHF exciter
tray includes adjustments for automatic
level control (ALC), linearity (amplitude
predistortion), and phase (phase change
vs. level) predistortion for correction of
325A, Rev. 0
5-2
•
DS1 (Input Fault) – Indicates that an
abnormally low or no IF is present at
the input of the board
•
DS2 (ALC Fault) – Indicates that the
ALC circuit is unable to maintain the
signal level requested by the ALC
reference. This is normally due to
excessive attenuation in the linearity
signal path or the IF phase corrector
signal path or because jumper W3 on
J6 is in the Manual ALC Gain position.
•
DS3 (Video Loss) – Indicates a loss of
video at the input of the board
•
DS4 (Mute) – Indicates that a visual
Mute command is present (not used
in this configuration)
•
DS5 (Modulator Enable) – Indicates
that the modulator IF output has
been selected (this is only used if a
receiver tray is present in the
system). DS5 is always on with no
receiver.
500-Watt VHF Low Band Transmitter
Chapter 5, Detailed Alignment Procedures
The ALC is muted when the transmitter is
in Standby. To monitor the ALC, turn off
the two amplifier on/off circuit breakers
on the AC input assembly in the rear of
the cabinet and switch the transmitter to
Operate. Adjust the power adjust gain
pot on the front panel of the VHF exciter
tray to obtain +0.8 VDC on the front
panel meter in the ALC position. On the
ALC board (1265-1305), move jumper
W3 on J6 to the Manual position,
between pins 2 and 3, and adjust R87 on
the ALC board for +0.8 VDC on the front
panel meter in the ALC position. Move
jumper W3 back to Auto (between pins 1
and 2); this is the normal operating
position. The detected IF signal level at
J19-2 of the ALC board is connected to
the transmitter control board that
distributes the level to the two VHF
amplifier trays where it is used as a
reference for the automatic gain control
(AGC) in each amplifier tray.
equipment to monitor the differential
phase or intermodulation products of the
RF output signal. There are three
corrector stages on the IF phase
corrector board, each with a magnitude
and a threshold adjustment that are
adjusted, as needed, to correct for any
differential phase or intermodulation
problems. Adjust the R3 threshold for the
cut-in point of the correction and the R7
magnitude for the amount of the
correction that is needed.
Jumper W1 on J8 is set to give the
desired polarity of the correction shaped
by the threshold R11 and the magnitude
R15 adjustments. After setting the
polarity, adjust the R11 threshold for the
cut-in point of the correction and the R15
magnitude for the amount of the
correction that is needed. Finally, adjust
the R19 threshold for the cut-in point of
the correction and the R23 magnitude for
the amount of the correction that is
needed.
5.4 IF Phase Corrector Adjustment
Note: Adjusting these pots changes
all visual parameters and should be
done cautiously.
As shipped, the exciter was preset to
include linearity (gain vs. level) and
phase (phase vs. level) predistortion. The
predistortion was adjusted to
approximately compensate the
corresponding non-linear distortions of
the amplifier trays and should not require
additional adjustments.
5.5 Linearity Corrector Adjustment
The IF linearity correction function
consists of three non-linear cascaded
stages, each having adjustable
magnitude and threshold, or cut-in
points, on the ALC board. The threshold
adjustment determines at what IF signal
level the corresponding corrector stage
begins to increase gain. The magnitude
adjustment determines the amount of
gain change for the part of the signal that
exceeds the corresponding threshold
point. Refer to the VHF exciter tray
control locations drawing, ALC board
(1265-1305), to find the adjustments for
the first through third linearity corrector
stages. Because the stages are cascaded,
the order of correction is important. The
first stage should cut in near white level,
with the cut-in point of the next stage
toward black, and with the last stage
primarily stretching sync.
Locate (A9) the IF phase corrector board
(1227-1250) mounted in the VHF exciter.
The amplitude correction portion of the
board is not utilized in this configuration.
As a result, jumper W3 on J10 should be
in the Disable position, to +6.8 VDC, and
R35 and R31 should be fully counterclockwise (CCW). R68 is the range
adjustment and should be set in the
middle of the range. The phase
correction Enable/Disable jumper W2 on
J9 should be in the Enable position, to
ground.
Switch the input video test source to
select an NTSC 3.58-MHz modulated
staircase or ramp test waveform. Set up
the station demodulator and monitoring
325A, Rev. 0
5-3
500-Watt VHF Low Band Transmitter
Chapter 5, Detailed Alignment Procedures
To adjust the linearity correctors from
scratch, ensure that the transmitter is
operating at full power with the desired
A/V ratio. Check that jumper W1 on J4 of
the ALC board is enabled between pins 1
and 2. Make sure that the ALC voltage is
set to +0.8 VDC as monitored on the
front panel meter in the ALC position.
distortion as displayed on the waveform
monitor should be no more than 1 IRE.
Pot R31 on the IF phase corrector board
is used for any extra intermodulation
correction that may be needed.
Note: Any adjustments to the above
pots affects other visual parameters
and some slight adjustments of all of
the pots may be needed to meet all
specifications simultaneously.
Insert a modulated ramp video test
signal into the transmitter. Demodulate
the output signal of the transmitter and
observe the waveform on a waveform
monitor while also looking at the signal
on a spectrum analyzer. On the IF ALC
board (1265-1306), preset pots R34,
R37, and R40 (threshold) fully CCW and
the magnitude adjustments R13, R18,
and R23 fully clockwise (CW). On the IF
phase corrector board (1227-1250),
preset pots R7, R15, R23, and R35 fully
CW and R3, R11, R19, and R31 fully
CCW.
If the transmitter is being driven very
hard, it may not be possible to get
enough sync stretch while maintaining a
flat differential gain. In this case, some
video sync stretch may be used from the
sync tip clamp/modulator board; the
sync stretch adjustment is R48.
Switch the transmitter to Standby.
5.6 Phase and Gain Adjustment of
the VHF Amplifier Trays
Set the waveform monitor to differential
step filter and the volts/division scale to
.1 volts. Center the display to
approximately the blanking level.
The following procedure was completed
at the factory and should only be
followed if one of the VHF amplifier trays
is replaced.
Gradually adjust pots R3, R11, and R19
clockwise on the IF phase corrector
board, as needed, to minimize the
observed thickness of the
intermodulation as seen on the display.
Preset the phase and gain potentiometer
on each VHF amplifier tray fully CCW.
Switch the transmitter to Operate and
adjust the gain pot on each tray for 25%
Output Power. Adjust the phase control
CW on the left VHF amplifier tray. If the
% Visual Output Power goes up, continue
to adjust the phase control until either
the peak is reached or the end-of-travel
is reached. If the % Output Power goes
down, reset the phase control on the VHF
amplifier tray fully CCW and repeat the
above procedure with the phase control
of the other amplifier tray.
Adjust pots R34, R37, and R40 clockwise
on the IF ALC board, as needed, to give
correction at sync or at low luminance
levels as viewed at the left-most edge of
the waveform monitor.
The intermodulation beat products
between the colorburst and the aural
carrier at 920 kHz above visual carrier
should also be observed on the spectrum
analyzer while performing the preceding
adjustments. The frequency will vary for
different video systems. When the
adjustments are performed properly, the
intermodulation products on the
spectrum analyzer should be at least -52
dB down, with a red field input, from
peak visual carrier. The intermodulation
325A, Rev. 0
If the end-of-travel is reached on the
phase adjust, reset the phase control
CCW and add a 2-inch length of cable to
the input of the affected VHF amplifier
tray at J1. Readjust the phase of that
tray until a peak is reached or until the
end-of-travel is achieved.
5-4
500-Watt VHF Low Band Transmitter
Chapter 5, Detailed Alignment Procedures
Note: The transmitter must have 40
IRE units of sync.
If the end-of-travel is reached, repeat the
above procedure and replace the 2-inch
length of cable with a 4-inch length of
cable. Once a peak has been reached,
move the phase control that is fully CCW
up two turns and repeak using the phase
control on the other tray. This allows
both trays to have some range of
adjustment.
Adjust R28, visual calibration, on (A19)
the visual/aural metering board (12651309) for 100% on the front panel meter
in the % Visual Output position.
With the spectrum analyzer set to the
zero span mode, obtain a peak reference
on the screen. Reconnect jumper cable
W1 to J16 on (A5) the sync tip
clamp/modulator board. While in the
Visual Output Power position, adjust L3
for a minimum visual power reading.
Turn the power adjust pot on the front
panel until the original peak reference
level is attained. Peak L1 and C8 for a
maximum aural power reading and then
also adjust R20 for a 100% Aural Power
reading. Switch the transmitter to the
Visual Output Power position and adjust
R51, the aural null pot, for 100% Visual
Power.
Adjust the gain of both VHF amplifier
trays for 90% Tray Output Power.
Readjust each phase control to peak the
combined output; the phase should only
have been slightly affected. Although it
may take a few turns to notice a change,
there should be a definite peak that is
achieved while adjusting the phase of
each tray. Raise or lower the output
power of each tray to achieve 100%
Output Power. The output power of each
tray should be 90% to 100%.
5.7 Calibration of the Forward
Output Power Level of the
Transmitter
5.8 Calibration of the Reflected
Output Level of the Transmitter
Note: Only perform the following
procedure if the power calibration is
suspect.
On the meter, in the Visual Power
position, turn the power adjust pot to
20%. Check that the jumper is in Manual
on the VHF filter/amplifier board
(1064251). Reverse the cables on A9-A5,
J3 and J4, and adjust R39 on the
visual/aural metering board (1265-1309)
for a 20% reading in the Reflected Power
position. At this 20% reference power
reading, the VSWR LED mounted on the
front panel of the exciter should be
illuminated. If this LED is not lit, adjust
R22 on the transmitter control board in
the VHF exciter tray until the VSWR LED
just turns on. Turn the power adjust pot
slightly CCW and the LED should go out.
Turn the pot CW until the LED just turns
on. The reflected output power is now
calibrated.
Switch the transmitter to Standby and
preset R51, the aural null pot on the
visual/aural metering board (12651309), fully CCW. Adjust R48, the null
offset pot on the visual/aural metering
board, for 0% Visual Output. Perform the
following adjustments with no aural
present. This is accomplished by
removing jumper cable W1, the aural IF
loop-through, that is connected to J16 on
(A5) the sync tip clamp/modulator board
(1265-1302). Connect a sync and black
test signal to the video input jack of the
VHF exciter tray. Switch the transmitter
to Operate.
Set up the transmitter for the appropriate
average output power level: sync + black
0 IRE setup/wattmeter=298 watts; sync
+ black 7.5 IRE setup/wattmeter=273
watts.
325A, Rev. 0
Switch the transmitter to Standby. Move
the cables on A9-A5, J3 and J4, to their
original positions.
5-5
500-Watt VHF Low Band Transmitter
Chapter 5, Detailed Alignment Procedures
Switch the transmitter to Operate and
adjust the front panel power pot for a
100% Visual Power reading.
The input to the (A9) bandpass filter
assemblies is the output of the VHF
combiner assembly, which is the
combined output of the VHF amplifier
trays. The filter is made of aluminum
waveguide and has five resonant cavities.
The filter has five bolts for tuning
adjustments. The bandpass filter also
utilizes two integral traps at -4.5 MHz
and +9 MHz from FV. Refer to the
bandpass filter drawing for the location of
the adjustments.
5.9 (A8) 2-Way Combiner Assembly
(1198-1010 CH. 2-4 or 1222-1002
CH. 5-6; Appendix C)
There are no adjustments to (A8) the
VHF combiner assembly.
Note: The bandpass filter is factory
swept and should not be tuned
without the proper equipment. Do
not attempt to tune the filters
without a sweep generator or,
preferably, a network analyzer. If
tuning is required, consult the
Axcera Field Support Department
before attempting to make any
adjustments.
To tune the filter, connect a sweep signal
to the input of the filter and adjust the
five tuning bolts for a 6-MHz bandwidth
and a flat-frequency response across the
desired band.
Note: The bandpass ripple should be
≤0.25 dB. The 6-MHz band should
also have a minimum of 20 dB return
loss across the pass band.
5.10 (A9) Bandpass Filter
Assemblies (1076291 through
1076293 & 1077130; Appendix C)
Table 5-1 contains typical values for the
bandpass filter.
Table 5-1. Bandpass Filter Typical Values
FREQUENCY
FV-4.5
FV-0.5
FV
Fa
FV+8.08
FV-9
2FV
INSERTION LOSS (DB)
≥ 35
≤ 0.8
≤ 0.8
≥ 15
≥ 30
≥ 30
5.11 (A6 and A7) Low-Band VHF
Amplifier Tray (1198-1600; Appendix
C)
≥ 20
≥ 20
≥ 20
the AGC control board (1142-1601) in
Manual. The idling current for the
amplifier boards are adjusted with no RF
drive applied. S1 should be in the Auto
AGC position for the normal operation of
the transmitter.
The (A6 and A7) low-band VHF amplifier
tray (1198-1600) has been adjusted at
the factory to meet all specifications,
including phase adjustment to match the
multiple trays in an amplifier array when
they are combined. The tray should not
need to be adjusted to attain normal
operation. Any adjustments to the boards
in this tray should be performed in the
Manual Gain position, with S1 on (A13)
325A, Rev. 0
RETURN LOSS (DB)
Connect a dummy load with a rating of a
least 500 watts to J2, the RF output jack
of the tray.
5-6
500-Watt VHF Low Band Transmitter
Chapter 5, Detailed Alignment Procedures
5.11.1 (A5) AGC Control Board
(1142-1601; Appendix D)
5.11.4 (A2-A1) Low-Band VHF
Amplifier Board (1198-1605;
Appendix D)
Using a calibrated wattmeter, check that
the tray is operating at the rated power.
Remove the sample forward power
connection J4 from the (A13) AGC control
board (1142-1601). The output power
level should drop to 20% because of the
VSWR cutback and DS4 should be
illuminated. The front panel Module
Status LED should not be lit.
The (A2-A1) VHF low-band amplifier
board (1198-1605) has 20 dB of gain and
is biased for 3 amps of idling current, no
RF drive applied. Adjust voltage adjust
pot R10 for +24 VDC at pin 0 of the
regulator IC U1. To set the bias, remove
the RF drive from the board, measure the
voltage across R6 and R7 (two 1Ω
resistors in parallel on the high-band
driver board), and adjust R4 for 1.5 volts
(using Ohms’ Law: [E=I x R] : [E=3
amps x .5 Ω] : E=1.5 volts).
Reconnect J4 and adjust R59 to begin
cutting back on the output power level
when the reflected level increases above
20%.
Connect a spectrum analyzer to output
jack J2 on the board and adjust C15 for
peak output.
In the Power Supply Voltage position, the
front panel meter is calibrated to +48
VDC using R86 on the AGC control board.
5.11.5 (A2-A2) Overdrive Protection
Board (1198-1601; Appendix D)
5.11.2 (A1-A1) Phase Shifter Board
(1198-1602; Appendix D)
The level of the RF input and output of
the (A2-A2) overdrive protection board
(1198-1601) should be +35 dBm during
normal operation.
There are no adjustments to (A1-A1) the
phase shifter board (1198-1602). The
front panel has adjustments for phase
that are made during the amplifier array
setup procedure.
To set up the overdrive circuit, check that
the output power level of the transmitter
is at 100% and adjust R11 on the board
for a reading of .4 VDC at TP1. Increase
the output power level of the transmitter
to 110%, sync only, and adjust R12 until
the output power begins to drop off.
Return the output power level of the
transmitter to 100%.
5.11.3 (A1-A2) VHF Filter/Amplifier
Board (1198-1606; Appendix D)
The (A1-A2) VHF filter/amplifier board
(1198-1606) has approximately 15 dB of
gain. Tune the channel filter capacitors
C29 and C20 (loading), C26 and C23
(center frequency), and C24 (coupling) at
J6 on the board for the best response.
Set voltage adjust pot R19 for +24 VDC
at the anode of CR5.
5.11.6 (A2-A3) 3-Way Splitter Board
(1198-1607 or 1198-1608; Appendix
D)
The idling current, no RF drive applied, of
the device Q1 is set for 250 mA. To set
the current, remove the RF drive,
measure the voltage across R16 (a 1Ω
resistor on the filter/amplifier board) and
adjust R13 for .25 volts (using Ohms’
Law: [E=I x R] : [E=250 mA x 1 Ω] :
E=250 mV).
325A, Rev. 0
There are no tuning adjustments for (A2A3) the 3-way splitter board. The board
takes the +35 dBm input and splits it into
three equal +30 dBm inputs.
5-7
500-Watt VHF Low Band Transmitter
Chapter 5, Detailed Alignment Procedures
5.11.7 (A3-A1, A3-A2, and A3-A3)
Low-Band Amplifier Board (11981624 or 1198-1631; Appendix D)
tray that is to be calibrated. Place switch
S1 on the AGC control board in the
Manual position before beginning the
setup.
Each of the (A3-A1, A3-A2, and A3-A4)
low-band amplifier boards have 20 dB of
gain and are biased at 2 amps of idling
current when they are used as a visual,
or visual + aural, amplifier, no RF drive
applied. To set the bias for the final
amplifier board (A3-A1), remove the RF
drive and switch the front panel current
meter to the I1 position. Preset bias
adjust pot R4 CCW. Slowly tune bias
adjust pot R4 for 2 amps on the meter
for a visual, or visual + aural, amplifier.
To adjust the visual output power levels:
1. Remove the J16 cable from (A5) the
sync tip clamp/modulator board
(1265-1302) in the exciter tray. Set
Manual AGC switch S1, on the (A13)
AGC control board (1142-1601) in the
600-watt amplifier, to the Manual
position. Turn the transmitter to the
Operate position.
2. Connect a sync and black test signal
to the video input jack of the remote
interface panel.
To set the bias for the final amplifier
board (A3-A2), remove the RF drive and
switch the front panel current meter to
the I2 position. Preset bias adjust pot R4
CCW. Slowly adjust bias adjust pot R4 for
2 amps on the meter for a visual, or
visual + aural, amplifier.
3. Adjust manual gain pot R5 on the
AGC control board for:
•
•
To set the bias for the final amplifier
board (A3-A3), remove the RF drive and
switch the front panel current meter to
the I3 position. Preset bias adjust pots R4
CCW. Slowly adjust bias adjust pot R4 for
2 amps on the meter for a visual, or
visual + aural, amplifier.
Note: The transmitter must have 40
IRE units of sync.
4. Obtain a zero span reference of the
visual-only carrier on a spectrum
analyzer. Replace the J16 connector
on the sync tip clamp/modulator
board in the exciter tray. Adjust R5
on the AGC control board until the
same visual reference is obtained.
Adjust R44 on the AGC control board
for 100% Forward Power.
Connect a spectrum analyzer to output
jack J2 on each of the boards and adjust
C1 on each board for peak output.
5.11.8 (A4-A1) 3-Way Combiner
Board (1198-1625 or 1198-1626;
Appendix D)
There are no adjustments to the (A4-A1)
3-way combiner board. The three +50
dBm inputs are combined to produce the
500 watts peak of sync output at J4 of
the combiner board.
Lower the forward power reading to 80%
on the front panel meter using R5, the
manual gain adjust on the AGC control
board. Adjust R65, the AGC fault adjust
on the AGC control board, until the green
Module LED DS3 on the front panel just
begins to light. Use R5 to readjust the
forward power to 100%.
5.11.9 Calibration of the Visual Plus
Aural Output Power and VSWR
Cutback
Switch off the tray and reverse the J8
and J5 cables on the 3-way combiner
enclosure.
Check that a dummy load of at least 500
watts is connected to the output of the
325A, Rev. 0
Sync + black 0 IRE setup;
wattmeter=300 watts
Sync + black 7.5 IRE setup;
wattmeter=270 watts
5-8
500-Watt VHF Low Band Transmitter
Chapter 5, Detailed Alignment Procedures
Switch on the tray and adjust the front
panel meter, in the Reflected Output
Power position, to a 100% reading using
R53, the reflected power meter adjust on
the AGC control board. Adjust the
reflected output power to a 20% reading
using R5 on the AGC control board.
Adjust R59, the VSWR cutback adjust on
the AGC control board, until the red
VSWR Cutback LED DS4 on the front
panel lights. This sets up the VSWR
cutback circuitry.
and video inputs. The kit adds the (A24)
composite 4.5-MHz filter board (12271244; Appendix D) and the (A25) 4.5MHz bandpass filter board (1265-1307;
Appendix D) to the transmitter. When the
4.5-MHz intercarrier signal generated by
the 4.5-MHz composite input has been
selected by the 4.5-MHz composite input
kit, the 4.5-MHz generated by the aural
IF synthesizer board is not used. When
the 4.5-MHz intercarrier signal generated
by the baseband video and audio inputs
with baseband has been selected by the
4.5-MHz composite input kit, the
composite 4.5-MHz filter board and the
4.5-MHz bandpass filter board are not
used.
Readjust R5 for 100% on the meter to
achieve a 500 watts peak of sync output
+ 50 watts aural power. However, if the
system requires less output power per
amplifier tray, adjust each tray by the
same amount to give the desired total
output power.
The tray has been factory tuned and
should not need any alignments to
achieve normal operation. To align the
tray for the 4.5-MHz composite input,
apply the 4.5-MHz composite input, with
the test signals used as needed, to the
video input jack (J1 or J2 [loop-through
connections]) on the rear of the tray.
Select the 4.5-MHz composite input by
removing the baseband select from J18-6
and J18-7 on the rear of the tray.
Switch off the tray and return the J5 and
J8 cables on the 3-way combiner
assembly back to their original positions.
If the tray was originally operating below
100% output power, the AGC fault adjust
was set for 20% below the operational %
Output Power of the tray. See the Test
Data Sheet for the transmitter for the
actual readings for the tray. Place S1 on
the AGC control board in the AGC
position. This is the normal operating
position after the setup is completed.
To align the exciter using baseband video
and audio inputs, apply the baseband
video, with the test signals used as
needed, to the video input jack (J1 or J2
[loop-through connections]) and the
baseband audio to the proper baseband
audio input on the rear of the tray. For
balanced audio input, connect TB1-1(+),
TB1-2(-), and TB1-3 (GND). For
composite/stereo audio, connect the
composite audio input jack (J3 or J13
[loop-through connections]) and connect
a baseband select from J18-6 and J18-7
on the rear of the tray.
There is a spare 1-amp and 10-amp fuse
on the top, right-hand side of the tray.
These are replacements for fuses on the
current metering board.
The VHF amplifier tray is aligned,
calibrated, and ready for normal
operation.
5.12 Board Level Alignment
Procedures
5.12.2 Delay Equalizer Board (12271204; Appendix D)
5.12.1 (Optional) 4.5-MHz
Composite Input Kit
The jumper W1 on J5 of the sync tip
clamp/modulator board, if present, must
be in the Enable position between pins 2
and 3.
If the (optional) 4.5-MHz composite input
kit is purchased, the tray is capable of
operating by using either the 4.5-MHz
composite input or the baseband audio
325A, Rev. 0
5-9
500-Watt VHF Low Band Transmitter
Chapter 5, Detailed Alignment Procedures
Note: This board has been factory
tuned and should not be retuned
without the proper equipment.
5.12.4 (A25) (Optional) 4.5-MHz
Bandpass Filter Board (1265-1307;
Appendix D)
To tune this board:
This board is part of the 4.5-MHz input
kit and will only function properly with a
4.5-MHz composite input signal and the
4.5-MHz composite input selected. To
align this board:
1.
Adjust the filter with L2, C3, L4,
and C7 for a frequency response of
no greater than ±0.3 dB from 4.4
to 4.6 MHz.
1.
Connect a sinX/X test signal into
jack J1-2 on the delay equalizer
board.
2.
Monitor the video output of the
board, at the video sample jack J2,
with a video measuring set, such as
the VM700, adjusted to measure
group delay.
3.
Tune the four stages of the board
using the variable inductors (L1-L4)
and potentiometers (R7, R12, R17,
and R22) until the signal attains the
FCC group delay curve. The stages
are arranged in order of increasing
frequency. Adjust R29, as needed,
to attain the same level out of the
board as into the board.
5.12.3 (A24) Composite 4.5-MHz
Filter Board (1227-1244;
Appendix D)
This board is part of the 4.5-MHz input
kit and will only function properly with a
4.5-MHz composite input signal and the
4.5-MHz composite input selected. To
align this board:
1.
2.
Connect the test signal from an
envelope delay measurement set to
the video input of the tray at J1 or
J2.
Connect an oscilloscope to jack J7,
video out, between the J7 center
pin and pin 1 or 3 (ground). Adjust
C21, frequency response, if needed,
for the best frequency response.
Adjust R32, video gain, for a signal
level of 1 Vpk-pk on the
oscilloscope.
2.
Adjust C19 for an overall peak-topeak variation of less than ±0.3 dB
from 4.4 MHz to 4.6 MHz.
3.
Recheck the frequency response; it
may have changed with the
adjustment of the envelope delay.
If necessary, retune the board.
5.12.5 (A7) IF Carrier Oven
Oscillator Board (1191-1404;
Appendix D)
To align this board:
1.
While monitoring J3 with a
spectrum analyzer, observe the
45.75-MHz visual IF (typical +5
dBm).
2.
Connect a frequency counter to J3
and adjust C17 for 45.750000 MHz.
3.
Connect a frequency counter to J1
and check for 50 kHz, which is the
aural phase lock loop reference.
5.12.6 (A5) Sync Tip
Clamp/Modulator Board (12651302; Appendix D)
To align this board:
1.
The output at J6 and J7 on the board
should be video only, without the 4.5MHz aural subcarrier.
325A, Rev. 0
5-10
Determine if jumper W4 on jack J3
is present. Jumper W4 terminates
the video input into 75Ω. Remove
jumper W4 if a video loop-through
is required on the rear chassis at
jacks J1 and J2.
500-Watt VHF Low Band Transmitter
2.
Set the controls R20, the white clip,
R24, the sync clip, and R45, the
sync stretch cut-in, to their full
CCW position. Set R48, the sync
magnitude, fully CW and place the
jumper W7 on jack J4 to the
Clamp-Off, Disable, position.
3.
Connect a 5-step staircase video
test signal to the input of the
transmitter.
4.
Monitor TP2 with an oscilloscope.
Adjust R12, the video gain pot, for
1 Vpk-pk.
5.
Change the video input test signal
to a multiburst test pattern. While
monitoring TP2, adjust C8 and R32
for a flat-frequency response.
Chapter 5, Detailed Alignment Procedures
Change the input video test signal
back to the 5-step staircase.
6.
Monitor TP2 with an oscilloscope.
Adjust pot R41, manual offset, for a
blanking level of -0.8 VDC. The
waveform shown in Figure 5-1
should be observed. Move the
jumper W2 on J4 to the Clamp
Enable position. Adjust pot R152,
depth of modulation, for a blanking
level of -0.8 VDC.
Note: This waveform represents the
theoretical level for proper
modulation depth. Step 9 below
describes how to set the modulation
depth through the use of a television
demodulator or a zero-spanned
spectrum analyzer tuned to the
visual IF frequency.
Figure 5-1. Waveform
7.
The following test setup is for the
adjustment of the depth of
modulation and ICPM at IF:
A.
Remove the cable that is on
J18 and connect the doublesideband, 45.75-MHz visual IF
signal from J18 to a 10-dB
splitter/coupler. Connect the
coupled port of the
splitter/coupler to the RF input
of a television demodulator.
Connect the direct port to a
spectrum analyzer.
325A, Rev. 0
5-11
B.
Connect the 75Ω video output
of the demodulator to the video
input of a waveform monitor.
For ICPM measurements, also
connect the quadrature output
of the demodulator to the
horizontal input of the
waveform monitor using a 250
-kHz, low-pass filter. (An
oscilloscope can be used in
place of a waveform
monitor).
C.
Set the controls of the
demodulator to the following:
500-Watt VHF Low Band Transmitter
Detector mode – Cont
Sound trap – In
Zero carrier – On
Auto – Sync
Audio source – Split
De-emphasis – In
8.
9.
Move jumper W7 on J4 to the
Clamp Disable position. Readjust
pot R41, manual offset, for the
correct depth of modulation by
observing the demodulated
waveform on the waveform monitor
or on the spectrum analyzer set to
zero span.
Check the demodulated video for a
proper sync-to-video ratio (sync is
28.6% of the total white video
signal). If sync stretch is needed,
adjust R45, sync stretch cut-in,
until sync stretch occurs. Adjust
R48, sync stretch magnitude, for
the proper amount of stretch.
Readjust R41, manual offset, if
needed, for the correct depth of
modulation.
10.
Move jumper W7 on J4 to the
Clamp Enable position. Readjust pot
R152, depth of modulation, for the
correct depth of modulation.
11.
Set the waveform monitor to
display ICPM. Preset R53 fully CCW,
adjust C78 for the greatest effect at
white on the ICPM display, and then
adjust R53 for minimum ICPM.
12.
Recheck the depth of modulation
and, if necessary, adjust R152,
depth of modulation.
13.
On a spectrum analyzer, adjust pot
R70 for a level of approximately -10
dBm at J18.
14.
Remove the input video test signal.
Place the front panel meter in the
video position and, while monitoring
the meter, adjust pot R144, zero
adjust, for a reading of zero.
325A, Rev. 0
Chapter 5, Detailed Alignment Procedures
15.
Replace the input video test
signal (the 5-step staircase).
Turn the front panel meter to
the video position and adjust
R20 on the transmitter control
board for a reading of 1 volt
(10 on the 0 to 10 scale). This
board does not have sync
metering.
16.
Reconnect the plug to J18 and
move the spectrum analyzer test
cable to the 41.25 IF output jack
J16. Tune C59 and L17 to L20 to
maximize the 41.25-MHz aural IF
signal and minimize the out-ofband products. Adjust pot R97 for
-20 dBm at J16.
17.
Reconnect the plug to J16 and
move the spectrum analyzer test
cable to IF output jack J20. Preset
R62, the visual IF gain pot, to the
middle of the range. Insert a
multiburst test signal into the
transmitter and observe the visual
frequency response with the
spectrum analyzer set at 1
dB/division. Tune R63 and C30, the
IF frequency response adjustments,
for a flat-frequency response (±0.5
dB).
18.
While still monitoring J20 with a
spectrum analyzer, readjust R62,
visual IF gain, for a 0 dBm visual
output level. Adjust R85, A/V ratio,
for a minus 10 dB aural-to-visual
ratio or to the desired A/V ratio.
Reconnect the plug to J20.
19.
Using an input video test signal (the
5-step staircase) with 100 IRE
white level, monitor TP2 with an
oscilloscope. Set control R24, the
sync clip, just below the point
where sync clipping begins to occur.
Similarly, set R20, the white clip, to
just below the point at which the
white video begins to clip.
5-12
500-Watt VHF Low Band Transmitter
Chapter 5, Detailed Alignment Procedures
5.12.7 (A4) Aural IF Synthesizer
Board, 4.5 MHz (1265-1303;
Appendix D)
1.
Frequency/Division – 10 kHz
Resolution bandwidth – 3 kHz
Time/Division – 50 msec
Trigger – Free run
To set up the test equipment for
this board:
A.
B.
Connect the 600Ω balanced
audio output from an audio
oscillator to the balanced audio
input terminals of the tray at
TB1-1 (+), TB1-2 (-), and TB13 (ground) on the rear chassis.
Connect the combined IF
output at J21 (IF sample) on
the clamp modulator board to
the input of an IF splitter.
Connect one output of the
splitter to the video
demodulator and the other
output to the spectrum
analyzer.
C.
At the front of the
demodulator, connect a short
cable from the RF-out jack to
the IF-in jack.
D.
Connect a cable from the 600Ω
audio output jack of the
demodulator to the input of an
audio distortion analyzer.
2.
Set the output frequency of the
audio oscillator to 400 Hz and the
output level to +10 dBm.
3.
Center the aural carrier on the
spectrum analyzer with the
spectrum analyzer set to the
following:
A.
Adjust L5 for approximately
+3.5 VDC at TP2.
B.
The green LED DS1 should be
illuminated, indicating a locked
condition. If not, retune L5 for
a locked condition.
4.
Adjust R13, balanced audio gain, on
the aural IF synthesizer board for
±25-kHz deviation.
5.
Check the distortion on the aural
distortion analyzer (THD=< 0.5%).
6.
Disconnect the 600Ω balanced
audio input to the tray. Connect a
75Ω stereo audio input (400 Hz at 1
Vpk-pk) to composite audio input
jack J3 on the rear of the tray.
Follow the procedure in the stereo
generator instruction manual for
matching the level of the generator
to the exciter. Use R17 to adjust
the composite audio gain.
7.
Check the distortion level on the
distortion analyzer (THD)=< 0.5%)
5.12.8 (A8) ALC Board (1265-1305;
Appendix D) (Part 1 of 2)
Table 5-2 describes the functions of each
LED on the ALC board (A8).
Table 5-2. ALC Board LEDs
LED
DS1 (Red LED)
DS2 (Red LED)
325A, Rev. 0
FUNCTION
Indicates that an abnormally low IF signal
level is present at IF input connector J1
Indicates that the ALC circuit is unable to
maintain the level requested by the ALC
reference due to excessive attenuation in
the linearity or the IF phase corrector signal
path or because jumper W3 on J6 is in
5-13
500-Watt VHF Low Band Transmitter
DS3 (Red LED)
DS4 (Red LED)
DS5 (Green LED)
1.
5.12.9 (A9) IF Phase Corrector Board
(1227-1250; Appendix D)
A. ALC Board (1265-1305)
See Section 5.4 of this chapter for the
system alignment procedures for the IF
phase corrector board. The signal level
into the board should be approximately
the same as the output of the board.
Adjust R87, manual gain pot, to
mid-range.
B. IF Phase Corrector Board (12271250)
Move W2 on J9 to phase
correction: enable. Move W3 on
J10 to amplitude correction:
disable.
3.
manual gain
Indicates a video loss fault
Indicates that a Mute command is present
Indicates that the output from the
modulator is selected as the input to the
board
To align the ALC board, preset the
following controls on the tray:
Connect jumper W1 on J4 to
disable, between pins 2 and 3 (to
disable linearity correctors).
Connect jumper W3 on J6 to
manual, between pins 2 and 3 (for
manual gain control).
2.
Chapter 5, Detailed Alignment Procedures
The combined IF output of the sync
tip clamp modulator board is cabled
to jack J32 of the ALC board.
Remove J32 from the board, and
look to see if DS1, Input Fault, is
illuminated. Reconnect J32 and
make sure that DS1 is
extinguished.
Jumper W3 on J6 should be in the
Manual position. Monitor jack J3
with a spectrum analyzer.
4.
With a multiburst video signal
present, tune C4 for a flatfrequency response of ±0.5 dB.
5.
Before proceeding with the second
part of the ALC board alignment,
check to see that the IF phase
corrector board (1227-1250) is
functioning properly.
325A, Rev. 0
The IF input jack of the IF phase
corrector board is fed from the J3 IF O/P
jack of (A8) the ALC board.
The IF output jack of the IF phase
corrector board is fed to the J7 IF I/P
jack of the ALC board (A8).
5.12.10 (A8) ALC Board, NTSC
(1265-1305; Appendix D) (Part 2 of
2)
To align this board:
1.
Input a multiburst video test signal.
Connect a spectrum analyzer to
J11. Tune C63 for a flat-frequency
response of ±0.5 dB.
2.
Move the Operate/Standby switch
on the front panel to the Operate
position.
3.
Place jumper W3 on jack J6 in the
Manual mode and adjust R87 for
0.5 volts at TP4.
4.
Place jumper W3 on J6 in the Auto
mode and adjust the front panel
power adjust control A20 fully CW.
If the (optional) remote power
raise/lower kit is present, then
adjust switch S1 on the board to
maximum voltage at TP4. Adjust
R74, the range adjust, for 1 volt at
TP4.
5-14
500-Watt VHF Low Band Transmitter
5.
Adjust the front panel power adjust
control A20 for 0.5 VDC at TP4. If
the (optional) remote power
raise/lower kit is present, move
switch S1 on the board to midrange and then adjust (A20) the
front panel power adjust control for
0.8 VDC at TP4.
6.
Disconnect the plug that is on J12
(IF output) and monitor J12 with a
spectrum analyzer. Verify an output
of approximately 0 dBm. If
necessary, adjust R99 to increase
the output level. If less of an output
level is needed, move jumpers J27
and J28 to pins 2 and 3 and then
adjust R99. Reconnect J12.
7.
Move W2 on J5 to the Cutback
Enable position. Remove the input
video signal and verify that the
output of the transmitter drops to
25%. Adjust R71, the cutback level,
if necessary. Restore the input
video.
Note: The following step affects the
response of the entire transmitter.
8.
9.
Connect a video sweep signal to the
input of the tray. Monitor the
output of the system with a
spectrum analyzer. Adjust C71 with
R103 and C72 with R106, as
needed, to flatten the response.
C71 and C72 adjust for the
frequency of the correction notch
being applied to the visual response
of the transmitter. R103 and R106
are used to adjust the depth and
width of the correction notch.
Refer to Section 5.5 of this chapter
for the system alignment
procedures for the linearity
correctors. Controls R13, R18, and
R23, the magnitude controls,
should be set fully CW. Controls
R34, R37, and R40 are the linearity
cut-in adjustments.
325A, Rev. 0
Chapter 5, Detailed Alignment Procedures
5.12.11 (A14-A1) Channel Oscillator
Board, Dual Oven (1145-1201;
Appendix D)
This board is mounted in (A14) the
channel oscillator assembly (1145-1202).
To align the board:
1.
Connect the main output of the
channel oscillator (J1) to a
spectrum analyzer, tuned to the
crystal frequency, and peak tuning
capacitors C6 and C18 for
maximum output. Tune L2 and L4
for maximum output. The output
level should be about +5 dBm. The
channel oscillator should maintain
an oven temperature of 50° C.
If a spectrum analyzer is not
available, connect a digital
voltmeter (DVM) to TP1 on the x4
multiplier board. Tune capacitors C6
and C18 for maximum voltage,
then also tune L2 and L4 for a
maximum voltage output at TP1.
2.
Connect the sample output of the
channel oscillator (J2) to a suitable
counter and tune C11, coarse
adjust, and C9, fine adjust, to the
crystal frequency.
Note: Do not repeak C6, C18, L2, or
L4. This may change the output
level.
Note: While adjusting C9 and C11 to
the crystal frequency, the peak
voltage monitored at TP1 of the x4
multiplier board should not
decrease. If a decrease does occur,
there may be a problem with the
crystal. Contact Axcera Field
Support for further instructions.
Note: If the VCXO board (11451204) in the VCXO assembly (11451206) is used, the fine-frequency
adjust C9 is not located on the VCXO
board. Use R9 on the FSK with
EEPROM board.
5-15
500-Watt VHF Low Band Transmitter
3.
Reconnect the main output (J1) of
the channel oscillator to the input
(J1) of the x2 multiplier.
Chapter 5, Detailed Alignment Procedures
2.
5.12.12 (A11-A1) x2 Multiplier Board
(1172-1111; Appendix D)
While monitoring the board with a DC
voltmeter, maximize each test point
voltage by tuning the broadband
multipliers in the following sequence:
1.
2.
Monitor TP1 with a DVM and tune
C4 for maximum voltage. Monitor
TP2 with a DVM and tune C6 for
maximum voltage. Repeak C4 and
C6 for maximum voltage.
Connect a spectrum analyzer,
tuned to two times the crystal
frequency, to the x2 multiplier
output jack J2. While trying to
keep the out-of-band products to
a minimum, monitor the output
and peak the tuning capacitors for
maximum output.
The output of the x2 multiplier connects
to (A11-A1) the filter/mixer board.
5.12.13 (A11-A2) VHF Filter/Mixer
Board (1153-1101; Appendix D)
To align the board:
1.
Monitor J4, the LO output of the
board, with a spectrum analyzer
and adjust C12 and C18 for
maximum output (+14 dBm) at
the LO frequency and minimum
out-of-band products. Adjust C13
and C17 for the best frequency
response for the LO frequency.
325A, Rev. 0
Adjust C3 and C6 to determine
the center frequency. Use C2 and
C7 to locate the upper and lower
channel-edge shaping. C4 is used
to determine the channel
bandwidth.
5.12.14 (A11-A3) Low-Band VHF
Filter/Amplifier Board (1064251;
Appendix D)
The filter/amplifier board has been
factory swept and adjusted for a 6-MHz
bandwidth.
Note: This board should not be tuned
without the proper equipment.
The filtered output connects to J1 of the
board and is amplified by U1 to a
nominal +8 dBm visual and -2 dBm aural
level by adjusting R9. The output at J2 is
fed to J4 on the A11 enclosure and from
there to J15 on the rear of the tray.
To align the board, use a multiburst or
sweep video signal inserted into the
exciter tray.
Reconnect the cable from J6 to J1 on the
filter/amplifier board. Monitor J2, the RF
output of the board, and peak C17 for
the maximum signal level. Tune manual
gain adjust R9 for a +8 dBm peak visual
output.
This completes the detailed alignment
procedures for the 325A transmitter. If a
problem occurred during the alignment,
refer to the detailed alignment procedure
for that tray for more information.
5-16

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