Tait TP9100 - Page 274

274 CODEC and Audio Fault Finding TP9100 Service Manual
© Tait Electronics Limited May 2005
Task 4 —
Check Audio
Power Amplifier
If there is no fault with the power supplies, check the inputs to the audio PA
as 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 for
channel 1. Ensure that the channel is programmed as an analog chan-
nel.
2. In user mode apply an on-channel RF FM signal of –47dBm with
60% deviation, 1kHz tone. The channel must not have signaling
enabled. Set the volume to maximum.
3. Use an oscilloscope probe to check the output of the voice-band
CODEC at C238 (see Figure 12.1). The signal should be:
4. If the above signal is correct, go to Step 7. If it is not, go to Step 5.
5. Check the signal in Step 3 at R273 (see Figure 12.3).
6. If the above signal is correct, repair R273 and go to Step 10.
Otherwise the main board is faulty; replace the main board and go to
“Final Tasks” on page 134.
7. Vary the volume control. This should cause the signal level at the
C238 (see Figure 12.1) to vary. If it does, go to Step 8. If it does not,
go to Ta sk 6
.
8. Check the voltage at pin 1 of IC200 (see Figure 12.1):
9. If the voltage is correct, go to Step 11. If it is not, check for and repair
any faults in the circuits incorporating Q200 (see Figure 12.1).
10. Confirm the removal of the fault and go to “Final Tasks” on
page 134. If the repair failed, replace the main board and go to “Final
Tasks” on page 134.
11. Check the digital signals
DIG AUD PA EN at R252 and DIG INT SPK INH at
R253 and the level-shifted signal at R255 (see Figure 12.1):
12. If the signals are correct, go to Task 5
. If they are not, check the
programming and test set-up; otherwise the main board is faulty;
replace the main board and go to “Final Tasks” on page 134.
C238: sine wave of 100mV
pp
with 1.2V DC offset
pin 1 of IC200: less than 0.5V DC
R252 (DIG AUD PA EN): 3.3V DC
R253 (DIG INT SKP EN): 0V DC (if internal speaker is enabled)
3.3V DC (if internal speaker is disabled)
R255: 0V DC (if internal speaker is disabled)
>10V DC (if internal speaker is enabled)