Harris DX 25U Technical Manual Download Page 74

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Example: for a carrier frequency of 1000 kHz; A =
100,000/(1.257 x 1000) + 42 = 121.5 = 121.5

∠

90 or 0 +

j121.5.

5.7.4.1

Third Harmonic Trap 2L3, 2C3

After setting the X

for 2L3, reconnect 2C3A (and 2C3B if

necessary), but still isolate the trap circuit from 2C2 and 2C4
sections. Set the Vector Impedance meter to 3f

and adjust 2C4A

for resonance (maximum impedance).

5.7.4.2

LOAD and TUNE

Reconnect all sections in the output network except the Com-
biner Output connection. Connect the Vector Impedance meter
at 2C6 with 2C6 disconnected from the combiner output pipe or
at 2L1 if 2C6 is not used at your frequency. Adjust the TUNE
and LOAD controls for 8

∠

0 or 8 +j0.

5.7.4.3

Half Frequency Trap 1L4, 1C4 (820 kHz - 1705 kHz)

Calculate the value to tap 1L4 as follows:

a. 1L4(Ohms) = 1000/(3.14 x Fo x C4)

b. Example: for Fo = 1000 kHz, C4 = 0.04 Mfd. 1L4(Ohms)

= 1000/(3.14 x 1000 x 0.04) = 7.96 Ohms.

Using a vector impedance meter, or RF generator and bridge,
adjust 1L4 tap for the correct resistance and install the 1L4/C4
combination in the transmitter.

5.7.5

RF Circuits Checkout

Before primary AC voltage is restored to the transmitter, make
sure all FD components have been properly installed and all FD
jumpers, coil taps, and switches have been properly set. Check
all connections in the output network for proper tightness, and
make sure that all panels removed for the frequency change have
been installed.

5.7.5.1

Oscillator A17

NOTE

Remember that the RF is held off by the VSWR-H input at J7-5
on the Oscillator until the Power Amplifier stage is turned on.
Use S1 on the LED Board to turn the output of the Oscillator ON
during troubleshooting and setup.

a. Connect a frequency counter to J5 (frequency monitor

sample).

b. Apply primary AC voltage to the transmitter and turn on

the low voltage at CB1 and CB2. Allow the Oscillator to
warm up for approximately 10 minutes.

c. Adjust trimmer C1 for the exact carrier frequency.

d. If a second crystal is installed in Y2, set the jumpers P1 and

P6 to positions 1-3. After a short warmup, adjust C3 to the
carrier frequency.

e. If an external RF source is used, i.e. stereo generator, verify

that it is setup for the correct frequency.

NOTE

Recheck the frequency after 30 minutes. This must be repeated
for both crystals.

5.7.5.2

Buffer Output (A16)

a. Turn off the low voltage to the transmitter at CB1 and CB2.

b. Locate the Predriver in the Driver Compartment.

c. Attach a 10x scope probe, Harris part #610-1131-000,

through the ventilation slots in the interlocked Driver
Compartment door to the anode of CR3.

d. Reapply the low voltage at CB1 and CB2.

e. Press S1 on the LED Board. Verify that the drive level to

the Predriver is approximately 8 to 12.0 Vp-p. See Figure
5-2.

f. The amplitude may be adjusted with R2, Buffer Voltage

adjust.

g. Move the probe and check the anode of CR4 on the

Predriver for the same level.

5.7.5.3

Predriver Tuning

WARNING

ENSURE ALL PRIMARY AC POWER IS REMOVED FROM TRANS-
MITTER AND THAT A GROUNDING STICK HAS BEEN USED TO
DISCHARGE ANY RESIDUAL POTENTIAL WHERE POWER HAS
BEEN APPLIED BEFORE PERFORMING THE FOLLOWING
STEPS.

a. Remove the rear access panels from the Driver Compart-

ment and the left rear RF Amp Compartment.

b. Remove the following PA Supply fuses:

1. A25: F1-F8

2. A24: F1-F10

3. F20: (on top of T1)

c. Make sure the proper value of C1 is installed and T8 is

tapped per the initial setting from the Tuning Chart.

d. Replace the rear access panels on the Driver Compartment

and the RF Amp Compartment

e. Reapply primary AC power to the transmitter.

f. Temporarily defeat the Underdrive Fault by connecting a