5.7.7.2
A/D Phasing Check
Make sure that the switches and jumpers are preset according to
the Frequency Tuning chart.
a. Operate the transmitter at 5 kW modulated at 100% with
a 10 kHz tone.
b. Feed the demodulated output of the Modulation monitor
to the Distortion Analyzer.
c. Measure the 10 kHz THD. It should typically be 1.5% or
less.
1. If the 10 kHz THD is 2% or higher, check 1 kHz THD.
If this, too, is 2% or higher, see the troubleshooting
section on “higher than normal audio distortion.”
2. If the distortion is 2% or less, the 10 kHz distortion can
be reduced by selecting different combinations of ca-
pacitance and inductance with S1 on the Analog to
Digital Converter.
NOTE
If during the final adjustment of the A/D phasing at full power,
the THD at 10 kHz is much higher than the 1 kHz THD, the
Modulated B- adjustment may not be correct and should be re-
checked. See the adjustment procedures under the Analog Input
Board and DC Regulator replacement sections in this section.
5.7.8
Tuning At High Power
a. Press MEDIUM then RAISE to slowly raise power to 15
kW. This should occur with approximately 14 PA Modules
turned ON.
b. Press HIGH then RAISE to slowly raise power to 25 kW.
c. At 25 kW power output, the PA Supply Current should be
between 105 and 122 amps with 23 PA Modules ON. Refer
to the Factory Test Data and use the following information
to help tune the transmitter:
Power Output (meter)
PA Voltage
PA Current
PA Efficiency
Antenna Null
Filter Null
# of Steps turned on
d. The TUNE control is adjusted for a peak in power output.
This control may be rather broad, especially at the low end
of the medium wave band.
e. The LOADING control adjusts the PA Current for a given
number of PA Modules ON.
f. Use the number of steps turned on from the Factory Test
Data to help determine if the LOADING needs adjusting.
1. If the desired # of steps is 23 and more than 25 are on,
press the LOWER button until 23 steps are ON and
increase the PA Current with the loading control.
2. If less than 23 PA Modules are ON, press RAISE until
23 PA Modules are turned ON and decrease the PA
Current with the loading control.
g. Perform A/D Phasing check at 25 kW. Refer to Figure 5-6.
h. Check the Oscillator Sync Adjustment. Refer to the Oscil-
lator adjustment procedures.
5.7.9
Completion of Basic Frequency Change of
Transmitter
The following procedures allow the transmitter to be checked and
adjusted for optimum performance. They are not critical for
reliability and basic performance of the transmitter.
5.7.9.1
Binary RF Amplifier Phase Alignment
Binary RF amplifier phase alignment is performed to achieve
optimum stereo performance. This alignment is performed by
measuring the phase difference between the Big Step and Binary
RF amplifiers at the output of each RF amplifier and adjusting
the tap on the Efficiency coil for minimum phase difference. The
taps on the Binary RF amplifier efficiency coils have already
been preset per the frequency tuning chart.
Because the Binary RF amplifiers B7-B12 are designed to pro-
duce output RF voltages less than that of the “Big Step” RF
amplifiers, they do not always operate at the same output phase
of RF as the Big Steps. This phase difference can be anywhere
from 0 to 30 degrees. Because the RF amplifiers operate at lower
voltage potentials, this situation does not affect reliability. How-
ever, it can have an affect on the amount of IPM (Incidental Phase
Modulation) products.
a. Connect the 10x probe, Harris part #610-1131-000, to the
drain TP1 test point of Q3 on PA Module RF33 by inserting
the probe through the ventilation slots on the interlocked
RF Amp Compartment door.
NOTE
THE 10X SCOPE PROBE, HARRIS PART #610-1131-000 MUST
BE USED. ALSO ENSURE THAT THE SCOPE PROBE IS
PROPERLY GROUNDED.
b. Set the scope on AC coupled, 50 Volts per division with the
trace centered on the screen.
c. Connect the external sync of the scope to J5 on the Oscil-
lator and make sure the scope sync is set to External.
d. Operate the transmitter at 25 kW with no modulation.
e. Adjust the Horizontal vernier on the scope so that one full
RF cycle occupies 9 divisions on the screen. Each division
now equals 40 degrees of phase shift.
f. Using the Horizontal positioning and triggering level on
the scope, place the transition time of the scope on the
center vertical line of the screen.
g. Increase the vertical sensitivity of the scope to expand the
waveform.
h. Switch the scope to the X10 position and readjust the
horizontal position so that the RF transition again crosses
the center line of the scope. This will be the reference for
the phase measurements. If another RF amplifier transition
occurs at the first large division on the right, this RF
amplifier is operating at 4 degrees lagging from the refer-
ence.
i. Check the drain of Q3 on RF33 to RF35 to verify that they
are all /-5 degrees of each other.
j. Check the drain of Q3 on the Binary RF amplifier B7. Note
that Q3 is now the right hand MOSFET when viewing the
module from the front.
Section V - Maintenance
Rev. Z1: 06-02-00 Additional info to ECN 46105
888-2297-002
5-19
WARNING: Disconnect primary power prior to servicing.
