Data Sheet Chapter 5

10-kW UHF Transmitter with Chapter 5, Detailed Alignment Procedures
Feedforward Drive
840A, Rev. 0 5-16
Gradually adjust pots R3, R11, and R19
CW, as needed, on the IF phase corrector
board to minimize the observed thickness
of the intermodulation as seen on the
display.
While performing the preceding
adjustments, the intermodulation beat
products between the colorburst and the
aural carrier at 920 kHz above visual
carrier should be observed on the
spectrum analyzer. 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
distortion, as displayed on the waveform
monitor, should be no more than 1 unit
of IRE. The pot R31 on the IF phase
corrector board is used for any extra
intermodulation corrections that may be
needed.
Note: Any adjustment to the above
pots affects other visual parameters
and some slight adjustments to all of
the pots may be needed to
simultaneously meet all of the
specifications.
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 UHF exciter tray
assembly drawing (1064946) and the
ALC board parts location drawing (1265-
5305) for 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.
To initially adjust the linearity correctors,
make sure that the transmitter is
operating at full power with the desired
A/V ratio. Check that the jumper W1 on
J4 on the ALC board is enabled, between
pins 1 and 2. Check that the ALC voltage
is set to +0.8 VDC as monitored on the
front panel meter in the ALC position.
Insert a modulated ramp video test
signal into the video input connector on
the remote interface panel of 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-
1305), preset pots R34, R37, and R40
(threshold) fully CCW, and the magnitude
adjustments R13, R18, and R23 fully CW.
Adjust pots R34, R37, and R40 CW on
the IF ALC board, as necessary, to give
correction at sync or at low luminance
levels; these are viewed at the right-
hand edge of the waveform monitor.
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 on the modulator board.
The sync stretch adjustment is R48 on
the sync tip clamp on the modulator
board.
Switch the transmitter to Standby.
5.6 (A11) UHF High-Power Tee
The inputs to the UHF tee (1227-1017,
low band; 1227-1018, mid band; or
1227-1019, high band) are the outputs
of the two UHF amplifier trays. The
inputs are 50-Ω impedances to match the
output impedance of the UHF amplifier
trays. The two inputs are combined and
then sent through a piece of transmission