Faulty Modulation - Tait TM8235 Service Manual

13.5

Faulty Modulation

Introduction
Task 12 —
Initial Checks
398 CODEC and Audio Fault Finding
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 2.3V 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.
Carry out the following checks to isolate the part of the circuitry that is
faulty.
1. Apply a 1kHz audio signal of 20 mV
the control head.
Enter the CCTM command 33 to place the radio in transmit mode.
2.
(The frequency is that of channel 1.)
3. Check that the 1kHz signal appears at the
(see Figure 13.8).
TP503 test point: 1kHz signal
Enter the CCTM command 32 to place the radio in receive mode.
4.
5. If the 1kHz signal is present, go to
Finding" on page
6. With no microphone connected, check the voltage at the junction of
C708 and C732 (
junction of C708 and C732: approximately 3V
7. If the above voltage is correct, go to
suspect. If it is not, go to
pp
"Frequency Synthesizer Fault
179. If it is not, go to
) (see Figure
CH MIC AUD
Task
Task 13; the 2.3V supply is suspect.
at the microphone input on
503 test point
TP
Step 6.
13.8):
14; the bias network is
TM8100/TM8200 Service Manual
© Tait Electronics Limited June 2006