User's Manual

2650 Watt VHF High Band Transmitter Chapter 5, Detailed Alignment Procedures
435B, Rev. 0 5-13
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. 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 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%.
5.4 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. While
changing the input signals, switch the
transmitter to Standby. 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