Tait TB7100, Service Manual

Page: 360 / 370

362 CODEC and Audio Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
15.5 Faulty Modulation
Introduction
This section covers the case where the radio transmits the correct amount of
RF power, but there is either no modulation or the modulation is distorted.
There are three tasks:
■
Task 12 : initial checks
■
Task 13 : check 2.3 V DC supply
■
Task 14 : check bias network
The initial checks will determine whether the frequency synthesizer, the
2.3V supply, or the bias network is at fault.
Task 12 —
Initial Checks
Carry out the following checks to isolate the part of the circuitry that is
faulty.
1. Apply a 1kHz audio signal of 20 mV
pp
at the microphone input.
2. Enter the CCTM command
33
to place the radio in transmit mode.
(The frequency is that of channel 1.)
3. Check that the 1 kHz signal appears at the
TP
503 test point
(see Figure 15.8 ).
4. Enter the CCTM command 32 to place the radio in receive mode.
5. If the 1 kHz signal is present, go to “Frequency Synthesizer Fault
Finding” on page 143 . If it is not, go to Step 6 .
6. With no microphone connected, check the voltage at the junction of
C708
and C732 (
CH MIC AUD
) (see Figure 15.8 ):
7. If the above voltage is correct, go to Task 14 ; the bias network is
suspect. If it is not, go to Task 13 ; the 2.3 V supply is suspect.
TP503 test point: 1kHz signal
junction of C708 and C732: approximately 3V