User's Manual

2650 Watt VHF High Band Transmitter Chapter 5, Detailed Alignment Procedures
435B, Rev. 0 5-14
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.
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.
Insert a modulated ramp video test
signal into the transmitter. While
changing the input signals, switch the
transmitter to Standby. 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.
Set the waveform monitor to differential
step filter and the volts/division scale to
.1 volts. Center the display to
approximately the blanking level.
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.
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
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.
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 Calibration of the Forward
Output Power Level of the
Transmitter
NOTE: Only perform the following
procedure if the power calibration is
suspect.
Preset R51, the aural null pot, located on
the visual/aural metering board (1265-
1309) in the VHF exciter, 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)