User's Manual

Digital UHF Transmitter Chapter 5, Detailed Alignment Procedures
DT835A, Rev. 1 5-10
5.3.12 (A7) Amplifier Protection
Board (1265-1412; Appendix D)
There are no adjustments to this board.
5.3.13 (A8) Amplifier Control Board
(1265-1414; Appendix D)
To check the operation of the overdrive
circuit, increase the gain pot of the UHF
amplifier tray to approximately 110%.
The Overdrive LED DS2 should light and
the output power should not increase
above the 110% level. If the LED does
not light, adjust R71 the overdrive
threshold as needed until the LED lights.
5.3.14 (A12) +26.5V/2000W
Switching Power Supply
This switching power supply does not
contain any customer-repairable items. If
the power supply should malfunction, do
not attempt to repair the power supply
without first consulting the Axcera Field
Support Department. The power supply
is adjusted to provide an output of +26.5
VDC.
5.3.15 Calibration of Output Power,
Reflected Power and VSWR Cutback
of the tray
To calibrate the forward output power
reading of the tray. Place a calibrated
coupler, average power wattmeter and
dummy load of at least 300 watts at the
output of the tray that is to be calibrated.
Switch the front panel meter to the %
Output Power position. Preset R16,
manual gain, on (A8) the amplifier
control board fully CCW. Move switch S1
on the amplifier control board to the
Manual position. Insert a digital test
signal and adjust R16 for 300 watts 64
QAM or 8-VSB digital on the wattmeter.
Calibrate the front panel output power
meter to 100% by adjusting the pot, R2,
forward calibration, located on the
amplifier control board. The forward
output power reading of the tray is now
calibrated.
To calibrate the reflected output power
reading and VSWR cutback of the tray.
Reduce manual gain pot R16 to a 50%
reading on the front panel meter in the
% Output Power position. Turn off the
tray. Remove the load from the output
of the tray and switch the front panel
meter to the Reflected Output Power
position. Switch on the tray. Adjust
reflected power adjust pot R22 on the
amplifier control board to a 50% reading.
Then adjust R29, the VSWR threshold
cutback pot, on the amplifier control
board until the VSWR Cutback LED DS1
on the front panel just lights. This sets
up the VSWR cutback circuitry.
After this calibration is completed,
reconnect the tray to the system and
move switch S1 on the amplifier control
board to the Automatic Gain Control
position. This is the normal operating
position for the switch.
The UHF amplifier tray is now aligned,
calibrated, and ready for normal
operation. Repeat as needed for the
other Amplifier Trays in the System
5.4 Phase and Gain Adjustment of
the UHF Amplifier Trays in each
Amplifier Array
Switch the Transmitter to Standby and
switch Off the Main AC Circuit Breaker
located on each of the AC Distribution
Assemblies in the Amplifier Array
Assemblies. Switch Off all the AC Circuit
Breakers, located on the AC Distribution
Assembly, for the individual Amplifier
Trays.
1. Adjust all gain controls located on
the UHF Amplifier Trays full CCW.
2. Switch On the Main AC Circuit
Breaker for the Side A Amplifier
Array Assembly and switch On the
AC Circuit Breaker for Amplifier #1.
3. Place the Transmitter in Operate
and adjust the Gain control on the