User's Manual

300-Watt Digital UHF Transmitter Chapter 5, Detailed Alignment Procedures
DT830A, Rev. 1 5-2
phasing adjustments to the four UHF
amplifier trays.
Verify that all red LEDs on the ALC board
are extinguished. The following details
the meaning of each LED when
illuminated:
• DS1 (input fault) – Indicates that
either abnormally low or no IF is
present at the input of the board
• DS2 (ALC fault) – Indicates that the
ALC circuit is unable to maintain the
signal level requested by the ALC
reference. This is normally due to
excessive attenuation in the linearity
signal path or the IF phase corrector
signal path, or that jumper W3 on J6
is in the Manual ALC Gain position.
• DS3 (modulation loss) – Indicates a
loss of modulation at the input of the
board
• DS4 (Mute) – Indicates that a Visual
Mute command is present (DS4 is not
used in this configuration.)
• DS5 (modulator enable) – Indicates
that the modulator IF output is
selected. This is only used if a
receiver tray is present in the system.
DS5 is always on with no receiver.
The ALC is muted when the transmitter is
in Standby. To monitor the ALC, turn off
the four amplifier on/off circuit breakers
on the AC input assembly in the rear of
the cabinet and switch the transmitter to
Operate. Adjust the power adjust gain
pot on the front panel of the UHF exciter
tray to obtain +0.8 VDC on the front
panel meter in the ALC position. On the
ALC board (1265-1305), move the
jumper W3 on J6 to the Manual position,
between pins 2 and 3, and adjust R87 on
the ALC board for +0.8 VDC on the front
panel meter in the ALC position. Move
the jumper W3 back to Auto, between
pins 1 and 2, which is the normal
operating position. The detected IF signal
level at J19-2 of the ALC board is
connected to the transmitter control
board. This board distributes the level to
the two UHF amplifier trays where it is
used as a reference for the automatic
gain control (AGC) in each amplifier tray.
5.1.1 Delay Equalization Adjustment
The procedure for performing a delay
equalization adjustment for the UHF
exciter tray is described in the following
steps:
1. Set J19 to the proper position (for
either a 50-Ω or 75-Ω input) and
monitor the output of the board at
J10 with a spectrum analyzer.
2. Bypass all attenuation and
equalizer sections, except delay
equalizer #1. With W12 removed,
tune L30 for the proper center
frequency as shown in Table 5-2.
Install W12 on J22 and adjust L21
and C43 for the best frequency
response across the band. Jumper
in attenuator equalizer #1.
3. Pull W14 from J26 and adjust L29
for the proper center frequency.
Install W14 and adjust L18 for the
best frequency response.
4. Repeat Steps 2 and 3 for each
delay/attenuation equalizer while
tuning the proper inductor for
each section.
5. After all five delay/attenuation
equalizers have been adjusted
according to Table 5-2,
individually jumper in all of the
sections and fine tune, as needed,
for the best group delay and
frequency response.