Data Sheet Chapter 5

500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures
325A, Rev. 0 5-4
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. 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 Phase and Gain Adjustment of
the VHF Amplifier Trays
The following procedure was completed
at the factory and should only be
followed if one of the VHF amplifier trays
is replaced.
Preset the phase and gain potentiometer
on each VHF amplifier tray fully CCW.
Switch the transmitter to Operate and
adjust the gain pot on each tray for 25%
Output Power. Adjust the phase control
CW on the left VHF amplifier tray. If the
% Visual Output Power goes up, continue
to adjust the phase control until either
the peak is reached or the end-of-travel
is reached. If the % Output Power goes
down, reset the phase control on the VHF
amplifier tray fully CCW and repeat the
above procedure with the phase control
of the other amplifier tray.
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.