Tait TP9100 Service Manual

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282 CODEC and Audio Fault Finding TP9100 Service Manual© Tait Electronics Limited May 200512.4 Faulty ReceiverIntroduction This section covers the case where the receiver does not operate, althoughthere is no apparent fault in the receiver circuit itself. There are two tasks:■ Task 9: check level shifter■ Task 10: check QN signalThe latter check will isolate the module at fault if the level shifter is not thecause of the problem.Task 9 óCheck Level Shifter Check the operation of the base-band CODEC and receiver AGC asdescribed below. This concerns the level-shifter circuit. It is assumed that thereceiver 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 and in receive mode, check the voltage atpin 14 of IC201 (see Figure 12.5):3. If the above voltage is correct, go to Task 10. If it is not, go to Step 4.4. Check the voltage at pin 12 of IC201 (see Figure 12.5):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 R238and pin 12 of IC201 (see Figure 12.5).7. Confirm the removal of the fault and go to “Final Tasks” onpage 134. If the repair failed or the fault could not be found, replacethe main board go to “Final Tasks” on page 134.8. Check the circuitry (R238, R239, R240) around pins 12, 13 and 14of IC201 (see Figure 12.5). Repair any fault.9. Confirm the removal of the fault and go to “Final Tasks” onpage 134. If the repair failed or the fault could not be found, replacethe main board and go to “Final Tasks” on page 134.pin 14 of IC201: more than 2.5V DCpin 12 of IC201: more than 1V DC