Data Sheet Chapter 5
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures
325A, Rev. 0 5-5
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%.
5.7 Calibration of the Forward
Output Power Level of the
Transmitter
Note: Only perform the following
procedure if the power calibration is
suspect.
Switch the transmitter to Standby and
preset R51, the aural null pot on the
visual/aural metering board (1265-
1309), 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). Connect a sync and black
test signal to the video input jack of the
VHF exciter tray. Switch the transmitter
to Operate.
Set up the transmitter for the appropriate
average output power level: sync + black
0 IRE setup/wattmeter=298 watts; sync
+ black 7.5 IRE setup/wattmeter=273
watts.
Note: The transmitter must have 40
IRE units of sync.
Adjust R28, visual calibration, on (A19)
the visual/aural metering board (1265-
1309) for 100% on the front panel meter
in the % Visual Output position.
With the spectrum analyzer set to the
zero span mode, obtain a peak reference
on the screen. Reconnect jumper cable
W1 to J16 on (A5) the sync tip
clamp/modulator board. While in the
Visual Output Power position, adjust L3
for a minimum visual power reading.
Turn the power adjust pot on the front
panel until the original peak reference
level is attained. Peak L1 and C8 for a
maximum aural power reading and then
also adjust R20 for a 100% Aural Power
reading. Switch the transmitter to the
Visual Output Power position and adjust
R51, the aural null pot, for 100% Visual
Power.
5.8 Calibration of the Reflected
Output Level of the Transmitter
On the meter, in the Visual Power
position, turn the power adjust pot to
20%. Check that the jumper is in Manual
on the VHF filter/amplifier board
(1064251). Reverse the cables on A9-A5,
J3 and J4, and adjust R39 on the
visual/aural metering board (1265-1309)
for a 20% reading in the Reflected Power
position. At this 20% reference power
reading, the VSWR LED mounted on the
front panel of the exciter should be
illuminated. If this LED is not lit, adjust
R22 on the transmitter control board in
the VHF exciter tray until the VSWR LED
just turns on. Turn the power adjust pot
slightly CCW and the LED should go out.
Turn the pot CW until the LED just turns
on. The reflected output power is now
calibrated.
Switch the transmitter to Standby. Move
the cables on A9-A5, J3 and J4, to their
original positions.