TB7100 Service Manual CODEC and Audio Fault Finding 357
© Tait Electronics Limited October 2005
Task 9 —
Check LPF and
Buffer Amplifier
If there is no test signal at the internal-options connector, then either
CODEC 1 is faulty or there is a fault in the LPF or buffer amplifier.
1. Remove the
CDC BOT can.
2. Check the signal at the junction between R224 and IC204
(see Figure 15.7). This should be:
3. If the above signal is correct, go to Step 4. If it is not, CODEC 1
(IC204) is faulty; replace the main-board assembly and go to “Final
Tasks” on page 123.
4. Check the voltage at pin 1 of IC201 (see Figure 15.7).
5. If the voltage is correct, go to Step 6. If it is not, check the LPF circuit
based on IC201 (pins 1 to 3) (see Figure 15.7). Repair any fault and
conclude with Step 9.
6. Check the voltage at pin 7 of IC201 (see Figure 15.7).
7. If the voltage is correct, go to Step 8. If it is not, check the buffer
amplifier based on IC201 (pins 5 to 7) (see Figure 15.7). Repair any
fault and conclude with Step 9.
8. Check R207 and R208 (see Figure 15.7). Repair any fault and
conclude with Step 9.
9. Confirm the removal of the fault and go to Step 10. If the repair failed
or the fault could not be found, replace the main-board assembly and
go to “Final Tasks” on page 123.
10. If there is also a fault with the transmit modulation, notwithstanding
modulation at the auxiliary connector, go to “Faulty Modulation
Using Auxiliary Connector” on page 366. If there is no other fault,
go to “Final Tasks” on page 123.
junction of R224 and IC204: 0.7V
pp
with 2.4V DC offset
pin 1 of IC201: 1.2V
pin 7 of IC201: 2.4V
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