Tait TM8235, Руководство по обслуживанию

Страница: 237 / 578

TM8100/TM8200 Service Manual Frequency Synthesizer Fault Finding 237
© Tait Electronics Limited June 2006
9.11 VCXO and CODEC Circuitry
Introduction
If the signals at the
TP
501 and
TP
502 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
TP
501
and
TP
502
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
TP
501 test point (see Figure 9.17 ):
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
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 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 157 .
Task 41 —
VCXO Tank Circuit
If the CCTM checks indicate that the VCXO tank circuit is faulty, repair
the 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 9.18 ).
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 157 .
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)