Tait TM8000 Series, Service Manual

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TM8100 Mobile Radio Service Manual Diagnosis of Faults on Main Board 241
May 2004 © Tait Electronics Limited
Task 4 —
Faulty Modulation
If the radio transmits the correct amount of RF power, but there is either
no modulation or the modulation is distorted, proceed as follows:
1. Apply a 1 kHz audio signal of 20 mV
pp
at the microphone input
on the control head. Enter the CCTM command
33
to place the
radio in transmit mode. (The frequency is that of channel 1.)
Check that the 1 kHz signal appears at the
TP
503
test point [at
5N9/5J12] (see Figure 9.31 ). If it does, go to Subsection 9.3 on
the frequency synthesizer. If it does not, go to Step 2.
2. With no microphone connected, check that the voltage at the
junction of C708 and C732 [at 7C0/7B3] (
CH MIC AUD
) is
approximately 3 V (see Figure 9.31 ). If it is, go to Step 4. If it is
not, go to Step 3.
3. Remove the
CDC BOT
can. Check that the voltage across C202 [at
2D7/2H3] is 3 V (see Figure 9.32 ). If it is, go to Step 4. If it is
not, check the soldering of R209 [at 2D8/2J3] (see Figure 9.32 ),
and check for shorts to ground at C202. Repair any fault.
Confirm the removal of the fault and return to
Subsection 8.1 . If
the repair failed or the fault could not be found, return to
Subsection 8.1 and replace the complete main-board assembly.
4. Remove the
CDC TOP
can. Check that the voltage at the junction
of R229 and R232 [at 2J5/2G8] is 1.5 V DC (see Figure 9.31 ). If
it is, then IC204 is suspect; return to Subsection 8.1 and replace
the complete main-board assembly. If it is not, go to Step 5.
5. Check the soldering of R229 and R232 [at 2J5/2G8], and check
for shorts across R232 (see
Figure 9.31 ). Repair any fault.
Confirm the removal of the fault and return to Subsection 8.1 . If
the repair failed or the fault could not be found, return to
Subsection 8.1 and replace the complete main-board assembly.