396 CODEC and Audio Fault Finding TM8100/TM8200 Service Manual© Tait Electronics Limited June 200613.4 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 10: check level shifter■ Task 11: check QN test pointThe latter check will isolate the module at fault if the level shifter is not thecause of the problem.Task 10 —Check Level ShifterCheck 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, check the voltage at pin 14 of IC201(see Figure 13.7):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 13.7):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 R238and pin 12 of IC201 (see Figure 13.7).7. Confirm the removal of the fault and go to “Final Tasks” onpage 157. If the repair failed or the fault could not be found, replacethe board and go to “Final Tasks” on page 157.8. Check the circuitry (R238, R239, R240) around pins 12, 13 and 14of IC201 (see Figure 13.7). Repair any fault.9. Confirm the removal of the fault and go to “Final Tasks” onpage 157. If the repair failed or the fault could not be found, replacethe board and go to “Final Tasks” on page 157.pin 14 of IC201: more than 2.5V DCpin 12 of IC201: more than 1V DC
