TB7100 Service Manual CODEC and Audio Fault Finding 359
© Tait Electronics Limited October 2005
15.4 Faulty Receiver
Introduction This section covers the case where the receiver does not operate, although
there is no apparent fault in the receiver circuit itself. There are two tasks:
■ Task 10: check level shifter
■ Task 11: check QN test point
The latter check will isolate the module at fault if the level shifter is not the
cause of the problem.
Task 10 —
Check Level Shifter
Check the operation of the base-band CODEC and receiver AGC as
described below. This concerns the level-shifter circuit. It is assumed that the
receiver and power-supply circuitry were checked and no faults were found.
1. If not already done, remove the
CDC BOT can.
2. With no RF signal applied, check the voltage at pin 14 of IC201
(see Figure 15.7):
3. If the above voltage is correct, go to Task 11
. If it is not, go to Step 4.
4. Check the voltage at pin 12 of IC201 (see Figure 15.7):
5. If the above voltage is correct, go to Step 8. If it is not, go to Step 6.
6. Check for and repair any shorts to ground at the junction of R238
and pin 12 of IC201 (see Figure 15.7).
7. Confirm the removal of the fault and go to “Final Tasks” on
page 123. If the repair failed or the fault could not be found, replace
the main-board assembly and go to “Final Tasks” on page 123.
8. Check the circuitry (R238, R239, R240) around pins 12, 13 and 14
of IC201 (see Figure 15.7). Repair any fault.
9. Confirm the removal of the fault and go to “Final Tasks” on
page 123. If the repair failed or the fault could not be found, replace
the main-board assembly and go to “Final Tasks” on page 123.
pin 14 of IC201: more than 2.5V DC
pin 12 of IC201: more than 1V DC
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