Tait TM8235, Service Manual

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256 Receiver Fault Finding TM8100/TM8200 Service Manual
© Tait Electronics Limited June 2006
10.6 High Receiver Distortion
Introduction
If there is high receiver distortion, the TCXO is suspect, or alternatively, the
matching circuitry for the crystal filters XF400 and XF401. There are two
tasks:
■
Task 17 : TCXO calibration and repair of TCXO
■
Task 18 : second IF and repair of matching circuitry
Recalibrating the TCXO might often be sufficient to rectify the fault.
Task 17 —
TCXO Calibration
and Repair of TCXO
First check the TCXO calibration and, if necessary, repair the TCXO.
1. Use the calibration application to check the TCXO calibration:
Open the
“Raw Data” page and click the “Volt Ref/TCXO/VCO/VCXO”
tab.
2. Note the values listed in the “Tx TCXO” and “Rx TCXO” fields of the
“TCXO”
group box. The values should be:
3. If the calibration values are correct, go to Step 4 . If they are not,
recalibrate the TCXO and go to Step 8 .
4. Remove the
CDC TOP
can.
5. Check the components of the TCXO, which is based on XL500
(see Figure 10.4 ). Repair any fault.
6. Recalibrate the TCXO and check the TCXO calibration values
again as in
Step 1 and Step 2 .
7. If the calibration values are now correct, go to Step 8 . If they are not,
go to Task 18 .
8. Check if the distortion fault has been removed. If it has, go to “Final
Tasks” on page 157 . If it has not, go to Task 18 .
Tx TCXO and Rx TCXO values: b20Hz and –20Hz