198 Frequency Synthesizer Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
11.11 VCXO and CODEC Circuitry
Introduction If the signals at the TP501 and TP502 test points are correct, two CCTM
checks will reveal any remaining faults. These possible faults concern the
VCXO tank circuit and the CODEC 2 circuitry. There are therefore three
aspects, 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 will
need to be repeated to confirm the removal of the fault.
Task 40 —
CCTM Checks
If the signals at the TP501 and TP502 test points are correct, or any related
faults were rectified, perform the following CCTM checks:
1. Enter the CCTM command 393 1 1900. Measure the voltage level at
the
TP501 test point (see Figure 11.16):
2. Enter the CCTM command 72 and note the lock status.
3. Enter the CCTM command 393 1 –1900. Again measure the voltage
level at the
TP501 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 lock
in either or both of the above cases, investigate the VCXO tank
circuit; go to Task 41
.
If the voltage level remains fixed at about 1.5V DC, investigate the
CODEC 2 circuitry; go to Task 42
.
If the voltage levels are all correct (following earlier repairs), the fault
has been removed; go to “Final Tasks” on page 123.
TP501 test point: 1.3 ± 0.2 V 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 ± 0.2 V DC (after CCTM 393 1 –1900)
To Next Page
Loading...
Need help?
General
