236 Diagnosis of Faults on Main Board TM8100 Mobile Radio Service Manual
May 2004 © Tait Electronics Limited
9.7 CODEC and Audio Circuitry
Fault Conditions This subsection covers the diagnosis of faults in the CODEC and audio
circuitry. There are four conditions that indicate a possible fault in the
circuitry:
■ no speaker audio or speaker audio is distorted
■ receiver does not operate
■ no transmit modulation or modulation is distorted
■ no receive audio at auxiliary connector
■ no transmit modulation despite modulation at auxiliary connector
In the first case regarding the speaker audio, the green
STATUS LED will
be operating correctly and all unmute criteria will be satisfied. In the
third case regarding the transmit modulation, the radio will be
transmitting the correct amount of RF power. In the fourth case the
receiver will be operating normally and in the fifth case the transmitter
will be.
Fault-diagnosis
Procedures
The procedures for diagnosing the above faults are given below in Tasks
2 to 6 respectively. In each case, however, first carry out Task 1. Also
note that the fourth and fifth conditions mentioned above can both
occur at the same time. In this case car r y out b o t h Ta s k 5 an d Ta s k 6 .
Task 1 —
Check Power Supply
Before searching for faults in the CODEC and audio circuitry, first check
the 2.5 V DC supply. The procedure is given in Task 3 of Subsection 9.1
on the power-supply circuitry.
Task 2 —
Faulty Speaker Audio
If the green STATUS LED is operating correctly and all unmute criteria are
satisfied, but there is either no speaker audio or the speaker audio is
distorted, proceed as follows:
1. In user mode apply an on-channel RF signal of –47 dBm with
60%, 1 kHz deviation. (Use the programming application to find
the frequency selected for channel 1.) The channel must not have
signalling enabled. Set the volume to maximum. Use an
oscilloscope probe to check the output of the voice-band
CODEC at the
TP200 test point [at 2D2/2D4] (see Figure 9.30).
The signal should be a sine wave of 100 mV
pp
with an offset of
0.6 V DC. If it is, go to Step 2. If it is not, go to Step 9.
2. Vary the volume control. This should cause the signal level at the
TP200 test point to vary. If it does, go to Step 3. If it does not, go
to Step 7.
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