Harris DX 25U Technical Manual Download Page 73

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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.

g. Remove the Driver Supply Regulator clear protective

cover and remove the external meter connections.

h. Replace the Driver Supply Regulator clear protective

cover.

5.7

Frequency Change Procedure

The following is a step by step procedure for changing the
frequency of the transmitter. If a complete frequency change is
desired this procedure can be followed in order. If only a specific
tuning procedure is desired (ex. Driver tuning) then only that
section needs to be looked at. In some cases, time is a considera-
tion in changing frequency of the transmitter. To allow a faster
frequency change, procedures which are performed to achieve
optimum transmitter performance, yet are not critical to the
reliable operation of the transmitter, are covered after the section
on “Basic Frequency Change.”

5.7.1

Test Equipment Required for Frequency Change

The following is a list of the test equipment required to perform
a frequency change. A frequency change should not be attempted
unless the proper equipment is used.

•

Audio Generator and Distortion Analyzer

•

Oscilloscope

•

Frequency Counter

•

Modulation Monitor

•

Digital Multimeter (preferred)

•

Vector Impedance meter or Impedance Bridge

•

Frequency programmable RF Generator (must operate up
to 3 times carrier frequency)

•

RF Load, 125 kW average dissipation

•

Function Generator (optional)

NOTE

Output Network setup can be performed with a Vector Impedance
Meter or Impedance Bridge and RF Generator. A Vector Imped-
ance Meter is usually faster but can be sensitive to interference.
An Impedance Bridge can be more effective in an RF environ-
ment.

5.7.2

Frequency Determined Components

Refer to the FD Chart, 839-7855-137, in the Drawing Package.
Install all the proper parts listed for the new desired frequency.
Oscillator crystal frequencies and part numbers are on drawing
817-1280-025.

Note that the FD chart for the output network components is
organized in bands A through L. The mounting hardware, straps,
plates, etc. are all listed on the parts list. When installing these
components, make sure all connections are tight. Use special care
when handling vacuum capacitors.

5.7.3

Frequency Determined Jumpers and Switches

Refer to the Tuning Chart, 839-7855-140, in the Drawing Pack-
age. Many of the frequency determined components on the
transmitter are permanently installed and are changed by moving
the desired jumper, coil tap or switch position. The Frequency
Tuning chart lists all jumpers, taps, and switches that need to be
set per frequency. Some of these settings are considered presets
and may need to change during a specific procedure. A list of
boards with frequency determined jumpers, taps, preset adjust-
ments, and switches is shown in Table 5-7.

5.7.4

Output Network Dry Tune

Preset all output network taps according to the Frequency Tuning
chart with all FD capacitors in place. If a high power dummy load
is not available, a small terminating 50 Ohm resistor can be used.

WARNING

Using a Vector Impedance meter or Impedance Bridge, adjust the
following output network sections to their appropriate X

or X

values at the desired carrier frequency. The sections should be
isolated (disconnected) from the rest of the other components
and only the necessary connecting straps and hardware should
be used to make the measurements. See table below.

2C2

∠

-90, 0 -j13 (2C2A and

2C2B capacitors only)

2C4

∠

-90, 0 -j75 (2C4A and

2C4B capacitors only)

2L3

∠

90, 0 + j42 (2L3 coil

only)