TM8100 Mobile Radio Service Manual Diagnosis of Faults on Main Board 193
May 2004 © Tait Electronics Limited
Tas k 2 —
Check Signals at
TP501 and TP502
If the VCXO and TCXO outputs are correct, or any related faults were
rectified, check the signals at the
TP501 and TP502 test points:
1. Use the oscilloscope probe to check the difference frequency at
the
TP502 test point [at 5F8/5J6] (see Figure 9.11). The signal is
SYN CDC FCL and should be a sine wave of about 1.3 V
pp
on 1.5 V
DC. If it is, go to Step 4. If it is not, go to Step 2.
2. The mixer circuitry (IC501 [at 5D7/5H4] and associated
components) or the LPF (low-pass filter) circuitry (IC502 pins 5
to 7 [at 5F7/5H5] and associated components) under the
FCL TOP
can is faulty. Remove the
FCL TOP can.
3. Locate the fault in the mixer or LPF circuitry (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 4. If the repair failed, return to Subsection 8.1 and
replace the complete main-board assembly.
4. With the oscilloscope probe at the
TP501 test point [at 5F5/5G5]
(see Figure 9.11), check the DAC output
CDC VCXO MOD. If a
triangular wave is present, the loop is oscillating; go to Task 4.
Otherwise go to Task 3.
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