216 Receiver Fault Finding TB7100 Service Manual© Tait Electronics Limited October 200512.6 High Receiver DistortionIntroduction If there is high receiver distortion, the TCXO is suspect, or alternatively, thematching circuitry for the crystal filters XF400 and XF401. There are twotasks:■ Task 17: TCXO calibration and repair of TCXO■ Task 18: second IF and repair of matching circuitryRecalibrating the TCXO might often be sufficient to rectify the fault.Task 17 —TCXO Calibrationand Repair of TCXOFirst 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 12.4). Repair any fault.6. Recalibrate the TCXO and check the TCXO calibration valuesagain 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 “FinalTasks” on page 123. If it has not, go to Task 18.Tx TCXO and Rx TCXO values: between +20Hz and –20Hz
