User's Manual

3300-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures
335B, Rev. 0 5-4
threshold point. Refer to the assembly
drawing for the ALC board (1265-5305),
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
on the ALC board is in the linearity
enabled position, 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 ALC
board (1265-1305), preset pots R34,
R37, and R40 (threshold, cut in) 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 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 (A6, A7 and A11) Low-Band VHF
Amplifier Tray (1304363; Appendix
C)
NOTE: The following procedure should
be followed only if complete alignment
of a VHF amplifier tray is needed.
The (A6, A7, and A11) low-band VHF
amplifier trays have been adjusted at the
factory to meet all specifications,
including phase adjustment to match the
multiple trays in an amplifier array when
they are combined. The trays should not
need to be adjusted to attain normal
operation. If necessary, any adjustments
to the boards in this tray should be
performed in the Manual Gain position,
with S1 on (A13) the AGC control board