Tait TM8000 Series, Service Manual

Page: 207 / 244

TM8100 Mobile Radio Service Manual Diagnosis of Faults on Main Board 207
May 2004 © Tait Electronics Limited
Task 7 —
High Receiver
Distortion
If there is high receiver distortion, the TCXO is suspect.
1. Use the calibration application to check the TCXO calibration:
Open the
“Raw Data” page and click the “Voltage Ref, TCXO and
VCXO”
tab. Note the values listed in the
“Tx TCXO”
and
“Rx TCXO”
fields of the “TCXO” group box. The values should be between
+20 Hz and –20 Hz. If they are, go to Step 3. If they are not, go
to Step 2.
2. Remove the
CDC TOP
can. Check the TCXO components and
repair any fault (see Figure 9.16 ). Recalibrate the TCXO and
check the TCXO calibration values again as in Step 1. If the
values are now correct, go to Step 3. If they are not, go to Step 4.
3. Check if the distortion fault has been removed. If it has, return to
Subsection 8.1 . If it has not, go to Step 4.
4. Input a large unmodulated RF input signal exceeding –90 dBm at
the RF connector. Use a needle probe to measure the frequency
of the signal at the
QN
test point [at 4P1/4C12] — access is through
the hole in the
IF TOP
can (see Figure 9.13 ). The frequency is the
second IF and should be 64.000 kHz. If it is, go to Step 6. If it is
not, go to Step 5.
5. Recalibrate the TCXO and check if the distortion fault has been
removed. If it has, return to
Subsection 8.1 . If it has not, go to
Step 6.
6. Remove the
IF TOP
can. Check the components between T401 [at
4L6/4G10] and IC400 [at 4M1/4C10] — these form the
matching circuitry for the crystal filters XF400 [at 4N6/4G12]
and XF401 [at 4Q2/4D13] (see
Figure 9.13 ). Repair any fault,
confirm the removal of the fault, and return to Subsection 8.1 . If
the repair failed or the fault could not be found, return to
Subsection 8.1 and replace the complete main-board assembly.