Harris DX 25U Technical Manual Download Page 68

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e. 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 is
the same.

f. Set the time base on the scope to display 2 to 3 cycles of RF.

g. Adjust C16 to make the signal at TP1 the same amplitude

as TP10, and also note that it may not be possible to get
both signals equal in amplitude using C16 until some
adjustment of L5 through L8 (selected by S7) is made.
Capacitance can be added with S2 if the signal cannot by
nulled with C16.

h. Note that the two signals are probably not in phase with

each other. See Figure 5-5.

i. Using a non-inductive tuning tool, adjust L5 through L8,

depending on which one is selected by the DIP switch S7,
to phase align the two signals. It may be necessary to
readjust C16 to make the two signals equal in amplitude.

j. If, by adjusting the selected variable inductor L5-L8, is not

possible to align the two signals in phase, select another
value of variable inductance with S7. Note that as the
amplitude and phase of the two signals are matched the
DETECTOR NULL (Filter) position on the Multimeter
will also null.

5.6.6.3

Fine Tuning

a. With the transmitter operating at 2.5 kW, both the DETEC-

TOR NULL (Antenna) and the DETECTOR NULL (Fil-
ter) positions on the front panel multimeter should indicate
near zero.

b. To prevent possible modulation monitor damage, turn both

the MEDIUM and HIGH power modulation monitor ad-
justment controls R7 and R8 full CCW.

c. Bring the transmitter to 25 kW and note the DETECTOR

NULL (Antenna) position on the multimeter. If the reading
is now above zero, null this reading using both C15 and
L12.

d. Note the DETECTOR NULL (Filter) indication on the

multimeter. If it is above zero, null it using C16 and L5
through L8, depending on what was selected by S7. The
final adjustments will be made into the antenna at full
operating power.

e. Modulate the transmitter with a 10 kHz tone, or one which

causes the greatest upward deflection on the DETECTOR
NULL (Antenna) meter reading, and recheck nulls.

f. Use a digital voltmeter or a dc coupled oscilloscope and

adjust for minimum voltage at TP8 and TP9 with reference
to ground.

5.6.6.4

Trip Threshold Adjustment

The overload settings for the Antenna and Bandpass circuit are
listed in the Factory Test Data sheet. After the replacement board
has been installed, set the overload settings as follows:

5.6.6.4.1

Antenna VSWR overload

a. Turn on the Low Voltage at CB1 and CB2.

b. Connect a voltmeter to TP4.

c. Adjust R24 until the voltage matches the Factory Test Data

sheet.

If the Factory Test Data sheet is unavailable or if it is necessary
to verify the original overload setting, use the following proce-
dure:

a. Verify that the DETECTOR NULL (Antenna) reading

on the front panel multimeter is nulled (zero) at full
power.

b. Press LOW power, and adjust the RF output for 3.5 kW.

c. Depress the OFF button.

WARNING

ENSURE ALL PRIMARY AC VOLTAGE HAS BEEN REMOVED
FROM TRANSMITTER AND A GROUNDING STICK IS USED TO
GROUND ALL POINTS WHERE AC OR RF POWER HAS BEEN
APPLIED BEFORE PROCEEDING WITH THE FOLLOWING PRO-
CEDURE.

d. Remove the rear panels from the Output Network Com-

partment.

e. Reverse the Antenna VSWR current sample by placing

P1 from 1-2 and P2 from 2-3 on the Output Sample
Board.

f. Replace the rear panels on the Output Network Compart-

ment.

g. Restore primary AC voltage at the main breaker.

h. Depress the LOW power button.

i. Switch the front panel multimeter to the DETECTOR

NULL (Antenna) position. Note that the meter reads up-
scale.

j. Adjust R24 until the transmitter indicates an ANTENNA

VSWR condition on the ColorStat™ panel.