Data Sheet Chapter 5
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures
325A, Rev. 0 5-8
5.11.7 (A3-A1, A3-A2, and A3-A3)
Low-Band Amplifier Board (1198-
1624 or 1198-1631; Appendix D)
Each of the (A3-A1, A3-A2, and A3-A4)
low-band amplifier boards have 20 dB of
gain and are biased at 2 amps of idling
current when they are used as a visual,
or visual + aural, amplifier, no RF drive
applied. To set the bias for the final
amplifier board (A3-A1), remove the RF
drive and switch the front panel current
meter to the I
1
position. Preset bias
adjust pot R4 CCW. Slowly tune bias
adjust pot R4 for 2 amps on the meter
for a visual, or visual + aural, amplifier.
To set the bias for the final amplifier
board (A3-A2), remove the RF drive and
switch the front panel current meter to
the I
2
position. Preset bias adjust pot R4
CCW. Slowly adjust bias adjust pot R4 for
2 amps on the meter for a visual, or
visual + aural, amplifier.
To set the bias for the final amplifier
board (A3-A3), remove the RF drive and
switch the front panel current meter to
the I
3
position. Preset bias adjust pots R4
CCW. Slowly adjust bias adjust pot R4 for
2 amps on the meter for a visual, or
visual + aural, amplifier.
Connect a spectrum analyzer to output
jack J2 on each of the boards and adjust
C1 on each board for peak output.
5.11.8 (A4-A1) 3-Way Combiner
Board (1198-1625 or 1198-1626;
Appendix D)
There are no adjustments to the (A4-A1)
3-way combiner board. The three +50
dBm inputs are combined to produce the
500 watts peak of sync output at J4 of
the combiner board.
5.11.9 Calibration of the Visual Plus
Aural Output Power and VSWR
Cutback
Check that a dummy load of at least 500
watts is connected to the output of the
tray that is to be calibrated. Place switch
S1 on the AGC control board in the
Manual position before beginning the
setup.
To adjust the visual output power levels:
1. Remove the J16 cable from (A5) the
sync tip clamp/modulator board
(1265-1302) in the exciter tray. Set
Manual AGC switch S1, on the (A13)
AGC control board (1142-1601) in the
600-watt amplifier, to the Manual
position. Turn the transmitter to the
Operate position.
2. Connect a sync and black test signal
to the video input jack of the remote
interface panel.
3. Adjust manual gain pot R5 on the
AGC control board for:
• Sync + black 0 IRE setup;
wattmeter=300 watts
• Sync + black 7.5 IRE setup;
wattmeter=270 watts
Note: The transmitter must have 40
IRE units of sync.
4. Obtain a zero span reference of the
visual-only carrier on a spectrum
analyzer. Replace the J16 connector
on the sync tip clamp/modulator
board in the exciter tray. Adjust R5
on the AGC control board until the
same visual reference is obtained.
Adjust R44 on the AGC control board
for 100% Forward Power.
Lower the forward power reading to 80%
on the front panel meter using R5, the
manual gain adjust on the AGC control
board. Adjust R65, the AGC fault adjust
on the AGC control board, until the green
Module LED DS3 on the front panel just
begins to light. Use R5 to readjust the
forward power to 100%.
Switch off the tray and reverse the J8
and J5 cables on the 3-way combiner
enclosure.