Tait TM8000 Series, Руководство по обслуживанию

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196 Diagnosis of Faults on Main Board TM8100 Mobile Radio Service Manual
May 2004 © Tait Electronics Limited
Task 4 —
Loop is Oscillating
If a check of the signal at the
TP
501 test point shows that the loop is
oscillating, proceed as follows:
1. If not already done, remove the
FCL TOP
can. Connect the
TP
501
test point [at 5F5/5G5] to ground by resoldering R527 [at
5E5/5F4] in the position shown in
Figure 9.11 . This forces the
VCXO loop voltage high.
2. Use the oscilloscope probe to check the VCXO output at C536
[at 5G8/5H6] — probe the via situated between C536 and R537
(see
Figure 9.11 ). The signal should be a sine wave with a
frequency of 13.017 MHz and an amplitude of about 1.1 V
pp
on
1.45 V DC. If the signal is correct, go to Step 4. If it is not, go
to Step 3.
3. The VCXO (Q501, Q503, XL501 and associated components) is
faulty. If not already done, remove the main-board assembly from
the chassis, and remove the
VCXO BOT
can. Locate and repair the
fault in the VCXO (see Figure 9.12 ), confirm the removal of the
fault, and go to Step 4. If the repair failed, return to
Subsection 8.1 and replace the complete main-board assembly.
4. Use the oscilloscope probe to check the difference frequency at
the
TP
502
test point [at 5F8/5J6] (see Figure 9.11 ). The signal is
SYN CDC FCL
and should be a sine wave with a frequency of about
17 kHz and an amplitude of about 1.3 V
pp
on 1.5 V DC. If the
signal is correct, go to Step 6. If it is not, go to Step 5.
5. The mixer circuitry (IC501 [at 5D7/5H4] and associated
components) or the LPF circuitry (IC502 pins 5 to 7 [at 5F7/5H5]
and associated components) under the
FCL TOP
can is faulty. Locate
the fault in the mixer or LPF (see Figure 9.11 ). Note that the
TCXO input to the mixer at R521 [at 5D7/5H3] (pin 4 of
IC501) should be a square wave with a frequency of
13000 000 Hz and an amplitude of 3 V
pp
. Also, the VCXO input
to the mixer at R522 [at 5D6/5G3] (pin 1 of IC501) should be a
sine wave of 20 mV
pp
(but the signal is noisy and difficult to
measure). Repair the circuitry, confirm the removal of the fault,
and go to Step 6. If the repair failed, return to
Subsection 8.1 and
replace the complete main-board assembly.
6. Resolder R527 [at 5E5/5F4] in its original position as shown in
Figure 9.11 . Replace all cans. Repeat Steps 1 to 3 of Task 3 to
confirm the removal of the fault. If the repair failed, return to
Subsection 8.1 and replace the complete main-board assembly.