c. Adjust the voltage of “REG B B- OUT” with R93 and
“REG A B-OUT” with R51 for approximately 2.5 VDC
with the Low Voltage ON.
d. Turn the transmitter ON and modulate 100% with a 1 kHz
tone at 25 kW.
e. Adjust the “A” and “B” voltages as needed to match those
recorded on the Test Data Sheet. These voltages will nor-
mally be between -4.5 and -5.3 Volts.
NOTE
Place P1 in the NORMAL position after adjustments are com-
pleted. This is important to prevent overheating of the regulator
transistors whan the transmitter is OFF and there is no air flow
through the compartment.
5.6.6
Output Monitor (A27)
The Output Monitor performs three main functions:
•
Forward and reflected power metering
•
VSWR overload sensing
•
Modulation monitor sample level adjustment
All of these functions must be calibrated for proper transmitter
operation. Set all jumpers and switches listed in Table 5-4 to the
same position as on the board to be replaced.
Since all of these circuits require adjustment while the transmit-
ter output network is set to 50 + j0 Ohms, it is preferred that the
transmitter be operated into a 50 Ohm load. This procedure can
be performed into the antenna, but operating the transmitter into
a load will make measurements easier due to the lack of interfer-
ence, compared to that existing on the antenna system.
5.6.6.1
DETECTOR NULL (Antenna) Adjustment
a. Set the PA TURN-OFF switch S2 on the Controller to the
OFF (up) position.
b. Depress the LOW power button. The PA Supply voltage
should be present but no power should be indicated on the
Forward Power meter.
c. Depress and hold the LOWER button for approximately
30 seconds.
d. Set the PA TURN-OFF switch S2 on the Controller to the
ON (down) position and hold the RAISE button until the
transmitter output power is approximately 2.5kW.
e. Using a Dual trace scope, connect a 10x probe on channel
1 to TP6 and a 10x probe on channel 2 to TP5. A signal
should be visible at both TP6 and TP5.
f. While depressing momentary button switch S5, set the
Normal/Calibrate switch S8 to the Calibrate position. Note
that the signal at TP5 has dropped in amplitude.
g. Adjust capacitor C29 for minimum signal at TP5. This
signal will contain mostly harmonics of the carrier fre-
quency. It may be necessary to add additional capacitance
with S9-1 and S9-3 at the low end of the band or additional
inductance with S9-2 and S9-4 at the high end of the band
to achieve a minimum signal.
h. Set the Normal/Calibrate switch S8 to the Normal position
and release momentary pushbutton switch S5. Make sure
that the vertical sensitivity of both channels of the scope
are the same.
i. Connect both scope probes to TP6 to ensure that both
traces are the same amplitude. Return the other probe to
TP5.
j. Set the time base on the scope to display 2 to 3 cycles of RF.
k. Adjust C15 to make the signal at TP6 the same amplitude
as TP5. Note that the two signals are probably not in phase
with each other. See Figure 5-4.
l. Using a non-inductive tuning tool, adjust L12 to phase
align the two signals. It may be necessary to readjust C15
to make the two signals equal in amplitude. Note that it
may not be possible to get both signals equal in amplitude
using C15 until some adjustment of L12 is made.
m. If, by adjusting L12, it is not possible to align the two
signals in phase, select a different value of capacitance
across L12 by switching in one or more sections of S6 then
readjusting L12 for an in phase signal.
n. Note that as the amplitude and phase of the two signals are
matched, the meter reading in the DETECTOR NULL
(Antenna) position will null. Fine adjustments of these
controls will be made at full power once the Bandpass
Filter controls are set.
5.6.6.2
DETECTOR NULL (Bandpass Filter) Adjustment
a. Using a Dual trace scope connect a 10x probe from channel
1 to TP10 on the Output Monitor. Connect a 10x probe
from channel 2 to TP1. A signal should be visible at both
TP1 and TP10.
b. While depressing the momentary pushbutton switch S5,
set the Normal/Calibrate switch S8 to the Calibrate posi-
tion. Note that the signal at TP10 has dropped in amplitude.
c. Adjust capacitor C21 for minimum signal at TP10. Also
note that the minimum residual signal will contain mostly
harmonics of the carrier frequency.
d. If a minimum cannot be achieved due to the capacitor C21
running out of range, use S1 to select a different value of
capacitance (C3 or C5), or a different value of inductance
(L2 or L3) to null out the signal at TP1. Note that some
frequencies may not require any added reactance. Nor-
mally capacitance is added at the low end of the frequency
band and inductance is added at the high end of the band.
JUMPER/SWITCH FUNCTION
P3
Directional Coupler select
P1
Directional Coupler select
S8
Normal/Calibrate
S6
Antenna VSWR Phasing
S7
Bandpass VSWR Phasing
S2
Bandpass VSWR Amplitude
S9
Antenna Null Detector Resonance
S1
Bandpass Filter Null Detector Resonance
Table 5-4. Output Monitor A27 Jumper/Switches
Section V - Maintenance
Rev. Z1: 06-02-00 Additional info to ECN 46105
888-2297-002
5-9
WARNING: Disconnect primary power prior to servicing.
