Tait TB7100, Service Manual

Page: 162 / 370

164 Frequency Synthesizer Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
Task 15 —
Check
DIG SYN FAST
Line
If the loop filter is suspect but the reference and feedback voltages are
correct, check the
DIG SYN FAST
line, which is input to the inverter.
1. Enter the CCTM command
389 1
to set the synthesizer mode to fast.
2. Measure the voltage at the collector of Q505 (see Figure 11.4 ).
The result should be:
3. Enter the CCTM command
389 0
to set the mode to slow.
4. Measure the voltage at the collector of Q505 (see Figure 11.4 ).
The result should be:
5. If the voltages measured in Step 2 and Step 4 are correct, go to
Task 16 . If they are not, go to Step 6 .
6. Remove R105 (see Figure 11.6 ).
7. Enter the CCTM command 389 1 to set the mode to fast.
8. Measure the voltage at the via between R105 and the digital board
(see
Figure 11.6 ). The result should be:
9. Enter the CCTM command 389 0 to set the mode to slow.
10. Measure the voltage at the via between R105 and the digital board
(see
Figure 11.6 ). The result should be:
11. If the voltages measured in Step 8 and Step 10 are correct, go to
Step 12 . If they are not, the digital board is faulty; replace the main-
board assembly and go to “Final Tasks” on page 123 .
12. Check and resolder R105 in position (see Figure 11.6 ), and check
for continuity between the collector of Q505 (see Figure 11.4 ) and
the digital board via R105.
13. If a fault is found, repair the circuit, and confirm that the voltages are
now correct. If they are, go to
“Final Tasks” on page 123 . If they are
not, or if no fault could be found, replace the main-board assembly
and go to
“Final Tasks” on page 123 .
Q505 collector: 14.2
±
0.3 V DC (after entry of CCTM 389 1)
Q505 collector: 0 V DC (after entry of CCTM 389 0)
via at R105: 0 V DC (after entry of CCTM 389 1)
via at R105: 3.3
±
0.3 V DC (after entry of CCTM 389 0)