Data Sheet Chapter 5

500-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures
425A, Rev. 0 5-3
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.
5.4 IF Phase Corrector Adjustment
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.
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 counter-
clockwise (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
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.
Note
:
Adjusting these pots changes
all visual parameters and should be
done cautiously.
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.