TB7100 Service Manual Frequency Synthesizer Fault Finding 163
© Tait Electronics Limited October 2005
Task 14 —
Check Feedback
Voltage
If the loop filter is suspect but the reference voltage is correct, check the
feedback voltage.
1. Measure the feedback voltage at pin 6 of IC505 (see Figure 11.4).
The result should be:
2. If the voltage is not correct, the loop filter is faulty; go to Step 3.
If the voltage is correct, resolder R542 in position (see Figure 11.7)
and go to Task 15
.
3. Check IC504, IC505, Q511 (see Figure 11.4), C5085 to C5089
(B1 band, see Figure 11.7) or C5085 and C5086 (H5, H6 bands
see Figure 11.7), and associated components.
4. If a fault is found, repair the circuit, repeat the measurement of the
feedback voltage in Step 1, and resolder R542 in position
(see Figure 11.7).
5. If the feedback voltage is now correct, go to “Final Tasks” on
page 123. If it is not, or if no fault could be found, replace the main-
board assembly and go to “Final Tasks” on page 123.
IC505 pin 6: 2.8 ± 0.1 V DC
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