TB7100 Service Manual Frequency Synthesizer Fault Finding 165
© Tait Electronics Limited October 2005
Task 16 —
Check
TP503 Test
Point
If the reference voltage, feedback voltage, and DIG SYN FAST line are all
correct, check the voltage at the
TP503 test point.
1. Measure the voltage at the
TP503 test point (see Figure 11.7).
The oscilloscope should show a DC level less than 3.0V with no sign
of noise or modulation.
2. If the correct result is obtained, go to Step 3. If it is not, go to Step 4.
3. The loop filter is faulty but the above measurements do not provide
more specific information. 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. Conclude with Step 9.
4. Remove R566 and R570 (see Figure 11.7), which provide a
modulation path to the VCO(s).
5. Repeat the measurement of Step 1.
6. If the correct result is now obtained, go to Step 7. If the correct result
is still not obtained, the CODEC and audio circuitry is suspect;
resolder R566 and R570 in position (see Figure 11.7), and go to
“CODEC and Audio Fault Finding” on page 343.
7. Resolder R566 and R570 in position (see Figure 11.7).
8. Check IC504 (pins 6, 8, 9) (see Figure 11.4) and the associated
components in the loop filter.
9. 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.
TP503 test point: < 3.0 V DC
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