Data Sheet Chapter 5

500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures
325A, Rev. 0 5-2
The 4.5-MHz composite input kit includes
a composite 4.5-MHz filter board (1227-
1244) and a 4.5-MHz bandpass filter
board (1265-1307).
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.
To operate the transmitter using the 4.5-
MHz 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.
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
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.
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.
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:
• 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.