Tait TM8000 Series Service Manual page 193

Task 2 —
Check Signals at
501 and
502
TP TP
TM8100 Mobile Radio Service Manual
May 2004 © Tait Electronics Limited
If the VCXO and TCXO outputs are correct, or any related faults were
rectified, check the signals at the
1. Use the oscilloscope probe to check the difference frequency at
502 test point [at 5F8/5J6] (see
the
TP
and should be a sine wave of about 1.3 V
SYN CDC FCL
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
can is faulty. Remove the
3. Locate the fault in the mixer or LPF circuitry (see
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
to the mixer at R522 [at 5D6/5G3] (pin 1 of IC501) should be a
sine wave of 20 mV
measure). Repair the circuitry, confirm the removal of the fault,
and go to Step 4. If the repair failed, return to
replace the complete main-board assembly.
4. With the oscilloscope probe at the
(see Figure 9.11), check the DAC output
triangular wave is present, the loop is oscillating; go to Task 4.
Otherwise go to Task 3.
501 and 502 test points:
TP TP
Figure
can.
FCL TOP
. Also, the VCXO input
pp
(but the signal is noisy and difficult to
pp
501 test point [at 5F5/5G5]
TP
Diagnosis of Faults on Main Board
9.11). The signal is
on 1.5 V
pp
FCL TOP
Figure 9.11).
Subsection 8.1 and
. If a
CDC VCXO MOD
193