Tait TP9100 - Page 182

182 Frequency Synthesizer Fault Finding TP9100 Service Manual
© Tait Electronics Limited May 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 9.8).
The oscilloscope should show a DC level less than 3.0V with no sign
of noise or modulation (Rx mode).
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 9.9), C548 (see Figure 9.8) and associated components.
Conclude with Step 9.
4. Remove R566 and R570 (see Figure 9.8), 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 9.8), and go to
“CODEC and Audio Fault Finding” on page 269.
7. Resolder R566 and R570 in position (see Figure 9.8).
8. Check IC504 (pins 6, 8, 9) (see Figure 9.9) 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 134. If they are
not, or if no fault could be found, replace the main board, and go to
“Final Tasks” on page 134.
TP503 test point: < 3.0V DC