Harris DX 25U, Техническое руководство

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A.5.1.3.5
U5 faulty
The output of U5-7 should be 4-4.5 Vp-p square wave, and can
be checked at P3-3.
A.5.2
Symptom: No RF Output, External Oscillator
Used
A.5.2.1
Possible Cause: RF Input From External Oscillator
The fault could be either in the external oscillator or in coaxial
cables carrying the signal. Check for output from the external
oscillator, then trace the signal through each cable connection
point.
A.5.2.2
Possible Cause: External Input Impedance
When P5 is jumpered from 1 to 3, the external input is terminated
in 50 Ohms. For a 50 Ohm input, 0.23 Vrms to 4 Vrms should
be present at P5-1. For a TTL input, 4-4.5 Vp-p should be present
at P5-1 with P5 jumpered from 1 to 2.
A.5.2.3
Possible Cause: Q3 or U5 faulty
Using an oscilloscope, check the RF signal level at Q3 collector
and at P3-2. Both should be 4-4.5 Vp-p.
A.5.3
Symptom: Frequency Instability.
A.5.3.1
Possible Cause: Plug P6.
Make certain that both P1 and P6 are in the same position (jumper
from 1 to 3 on both, or from 1 to 2 on both).
A.5.3.2
Possible Cause: Crystal Oven Failure
The crystal oven which is operating should be hot.
A.5.3.3
Possible Cause: No -15 VDC Supply
Check for -15 VDC at P6-1. If not present, check both ends of
F2 for -22 VDC. If F2 is open, replace it. If the fuse opens again,
U6 is probably defective.
A.5.3.4 Possible Cause: Defective Crystal
Defective crystals may operate off frequency. If one crystal can
be adjusted to the correct frequency but the other cannot, the
off-frequency crystal is probably defective.
A.5.4
Symptom: Output At Incorrect Frequency
A.5.4.1 Possible Cause: Frequency Divider Jumper Plug P2
Check the position of the P2. The crystal frequency will be
divided by four if P2 is from 1-2, and will be divided by eight if
P2 is from 1-3. Check your crystal frequency and the jumper
position.
A.5.4.2
Possible Cause: Frequency Divider Fault
Normally, a frequency divider fault will result in complete loss
of RF at P2-1. The output of a frequency divider section will go
to either 0 VDC or to +5 VDC. Using an oscilloscope, check the
oscillator frequency at Q2 collector, and divider frequencies at
P2. Frequency at P2-2 should be 1/4 the oscillator frequency and
at P2-3, should be 1/8 the oscillator frequency.
A.5.5
Symptom: Oscillator LED on ColorStat™
panel is Red but transmitter operation is nor-
mal.
A.5.5.1 Possible Cause: RF Present circuit
CR6 and CR7 “RF Present Detectors.” Check for +4 to +5 VDC
at CR7 cathode and for -4 to -5 VDC at CR6 anode. If a normal
RF level is present at the Oscillator board output but one or both
of these DC voltages are missing, check and/or replace the
diode(s). Loss of one or both DC voltages will cause a RED
“OSCILLATOR” status indication. If voltage is present at one
side of R19 or R21 but not at the other side, a resistor failure or
a short after J7 is indicated. (In this case, the transmitter will
remain ON.)
If you measured correct DC voltages in the previous step, the red
“
OSCILLATOR
” Status LED indication is caused by a problem
on the LED Board or an open or short in an interconnecting cable.
Refer to SECTION Q, LED Board, and SECTION VI, Trou-
bleshooting, for additional information.
Section A - Oscillator (A17)
888-2297-002 A-3
WARNING: Disconnect primary power prior to servicing.