244 Frequency Synthesizer Fault Finding TM8100/TM8200 Service Manual© Tait Electronics Limited November 20079.11 VCXO and CODEC CircuitryIntroduction If the signals at the TP 501 and TP 502 test points are correct, two CCTMchecks will reveal any remaining faults. These possible faults concern theVCXO tank circuit and the CODEC 2 circuitry. There are therefore threeaspects, which are covered in Task 40 to Task 42:■ Task 40: CCTM checks■ Task 41: VCXO tank circuit■ Task 42: CODEC 2 circuitry.Following any repairs of the VCXO or CODEC 2 circuitry, Task 40 willneed to be repeated to confirm the removal of the fault.Task 40 —CCTM ChecksIf the signals at the TP 501 and TP 502 test points are correct, or any relatedfaults were rectified, perform the following CCTM checks:1. Enter the CCTM command 393 1 1900. Measure the voltage level atthe TP 501 test point (see Figure 9.18):2. Enter the CCTM command 72 and note the lock status.3. Enter the CCTM command 393 1 –1900. Again measure the voltagelevel at the TP 501 test point:4. Enter the CCTM command 72 and note the lock status.5. If the above voltage levels are not correct or if the FCL is out of lockin either or both of the above cases, investigate the VCXO tankcircuit; go to Task 41.If the voltage level remains fixed at about 1.5V DC, investigate theCODEC 2 circuitry; go to Task 42.If the voltage levels are all correct (following earlier repairs), the faulthas been removed; go to “Final Tasks” on page 163.Task 41 —VCXO Tank Circuit If the CCTM checks indicate that the VCXO tank circuit is faulty, repairthe circuit as follows:1. If not already done, remove the VCXO BOT can.2. Locate and repair the fault in the VCXO tank circuit (Q501, D501,D502, XL501 and associated components) (see Figure 9.19).3. Confirm the removal of the fault and go to Step 4. If the repair failed,replace the board and go to “Final Tasks” on page 163.TP501 test point: 1.3 ± 0.2V DC (after CCTM 393 1 1900)lock status=xyz (x=RF PLL; y=FCL; z=LO2) (0=not in lock; 1=in lock)TP501 test point: 2.1 ± 0.2V DC (after CCTM 393 1 –1900)
