Tait TB7100, Service Manual

Page: 175 / 370

TB7100 Service Manual Frequency Synthesizer Fault Finding 177
© Tait Electronics Limited October 2005
Task 24 —
Repair PLL feedback
If both the minimum and maximum transmit frequencies are correct, the
PLL feedback is suspect.
1. Resolder R542 in position (see Figure 11.9 ).
2. Remove the
VCO BOT
can.
3. Replace the components C570, R578 (see Figure 11.10 ) and
IC503 (see Figure 11.9 ).
4. Confirm that the fault in the radio has been removed. If it has, go to
“Final Tasks” on page 123 . If it has not, replace the main-board
assembly and go to “Final Tasks” on page 123 .
Task 25 —
Repair Transmit
VCO
If either or both the minimum and maximum transmit frequencies are
incorrect, the transmit VCO circuitry is faulty.
1. Remove the
VCO TOP
can.
2. Check the transmit VCO. The circuitry is based on Q510
(see
Figure 11.9 ).
3. If a fault is found, repair it and go to Step 4 . If no fault is found, go
to Step 6 .
4. Repeat the frequency measurements in Step 2 and Step 4 of Task 23 .
5. If the frequencies are now correct, resolder R542 in position
(see
Figure 11.9 ), and go to “Final Tasks” on page 123 . If they are
still not correct, go to Step 6 .
6. Resolder R542 in position (see Figure 11.9 ). Replace the main-
board assembly and go to “Final Tasks” on page 123 .