3
6
8
meter has been factory calibrated for 100% with the transmitter providing 500 watts peak visual
and 25 watts average aural. The following calibration procedures assume that the composite signal
from the transmitter has the aural carrier 13dB down from the peak visual with the visual carrier
having 87.5% video modulation and 0% (sync only) average picture level (APL). The power levels
stated in the steps below are those expected at the output of the transmitter. Therefore, when
measuring these power levels using the equipment shown in Figure 3
6
6, be sure to take into
account the attenuation factor provided by the 20dB directional coupler and the 20dB attenuator.
Power levels at 50% APL (5 Step/Ramp/50% Flat Field) are included in brackets following the
power level at 0% APL.
3.5a
Forward Power:
1.
Assuming the transmitter is in operation, place the Exciter’s OPERATE/STANDBY switch
to STANDBY and set up the test equipment shown in Figure 3
6
6.
2.
Using sync only video, verify that the modulator is providing 87.5% video modulation with
the aural carrier 13dB below the visual carrier. Release the Exciter AGC/MANUAL switch
to the MANUAL position and the transmitter RF POWER meter switch to FWD. Place the
Exciter OPERATE/STANDBY switch to OPERATE.
3.
Wait for the transmitter to come up to power and, using a small tuning tool, rotate the
OUTPUT LEVEL ADJUST for an external power meter reading of 323W [195W]. Notice
that 500 watts of peak visual power at 0% [50%] APL plus 25 watts of average aural power
at 13dB down equals 323W [195W] power on the external average power meter.
4.
To check and adjust visual to aural ratio, replace the power meter in Figure 3
6
6 with a
spectrum analyzer. Set the modulator’s aural carrier level adjustment for the desired visual/
aural ratio. Remove the spectrum analyzer, return the power meter to the transmitter output
and recheck power. Set the OUTPUT LEVEL ADJUST again for an external power meter
reading of 323W [195W].
5.
With the external power meter reading correctly, place the Control/Metering panel meter
switch to FWD and check the transmitter's RF POWER meter for a 100% indication. If this
reading is not obtained, adjust FORWARD POWER potentiometer A5PC1R32, located
behind the Control/Metering panel (A5) and accessible through the METER ADJUST hole
marked FWD, for the correct indication.
6.
Activate the transmitter’s output gain control by pushing in the Exciter AGC/MANUAL button
and recheck the power indication on the external meter for 323 watts [195W]. If the meter
reading is not correct, slowly turn the AGC LEVEL ADJUST (small tuning tool required) to
correct the transmitter output power. The output AGC circuit has a very long time constant
which requires the operator to wait for the transmitter power to settle after each small
movement of the AGC LEVEL ADJUST.