348 CODEC and Audio Fault Finding TB7100 Service Manual© Tait Electronics Limited October 2005Task 4 —Check AudioPower AmplifierIf there is no fault with the power supplies, check the inputs to the audio PAas follows. This check is only applicable, however, if the output of the voice-band CODEC is correct and the signal level varies as the volume is varied.1. Use the programming application to find the frequency selected forchannel 1.2. In user mode apply an on-channel RF signal of –47dBm with 60%,1kHz deviation. The channel must not have signalling enabled.Set the volume to maximum.3. Use an oscilloscope probe to check the output of the voice-bandCODEC at the TP 200 test point (see Figure 15.2). The signalshould be:4. If the above signal is correct, go to Step 5. If it is not, go to Task 7.5. Vary the volume control. This should cause the signal level at theTP 200 test point (see Figure 15.2) to vary. If it does, go to Step 6.If it does not, go to Task 6.6. Check the voltage at pin 11 of IC202 (see Figure 15.2):7. If the voltage is correct, go to Step 9. If it is not, check for and repairany faults in the level-translation circuits incorporating Q200 andQ201 (see Figure 15.2).8. Confirm the removal of the fault and go to “Final Tasks” onpage 123. If the repair failed, replace the main-board assembly and goto “Final Tasks” on page 123.9. Check the digital signals DIG AUD PA EN1 at R200 and DIG AUD PA EN2 atR202 (see Figure 15.2):10. If the signals are correct, go to Task 5. If they are not, check theprogramming and test set-up; otherwise the digital board is faulty;replace the main-board assembly and go to “Final Tasks” onpage 123.TP200 test point: sine wave of 100mV pp with 0.6V DC offsetpin 11 of IC202: at least 8V DCR200 (DIG AUD PA EN1): 3.3V DCR202 (DIG AUD PA EN2): 0.0V DC
