190 Diagnosis of Faults on Main Board TM8100 Mobile Radio Service Manual
May 2004 © Tait Electronics Limited
Task 4 —
Incorrect VCO Tuning
Voltages
If the VCO tuning voltages are incorrect, rectify the fault as follows:
1. Remove the
SYN TOP and SYN BOT cans.
2. There are two basic faults: Firstly, the VCO tuning voltage at
C565 is low, whereas the voltage at pin 7 of IC505 is much higher;
in this case go to Step 3. Secondly, the tuning voltage is either
near 0 V or near the VCL supply voltage measured in Step 3 of
Task 1; in this case go to Step 4.
3. Check for damage, shorts, open circuits, and dry solder joints in
the circuitry along the path between pin 7 of IC505 [at 5J5/5F8]
and C565 [at 5M5/5F11] (see Figure 9.9 and Figure 9.10). If a
fault is found, repair it, 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. Check for open circuits, short circuits, and faulty components in
the circuitry in the feedback path for the operational amplifier
IC505 [at 5J5/5F8] (Q511, IC504, R554, R556, C548, C555)
(see Figure 9.9 and Figure 9.10). If no fault is found, go to Step
5. If a fault is found, repair it, confirm the removal of the fault,
and return to Subsection 8.1. If the repair failed, return to
Subsection 8.1 and replace the complete main-board assembly.
5. Check the reference input signal at pin 8 of IC503 [at 5H7] of the
PLL (see Figure 9.9). The signal should be about 13 MHz and
1V
pp
. If it is, return to Subsection 8.1 and replace the complete
main-board assembly. If it is not, go to Step 6.
6. Remove IC503. Repeat the above check of the reference input
signal. It should now be about 1.5 to 2.0 V
pp
. If it is not, the FCL
requires investigation; go to Subsection 9.4. If it is, replace IC503,
confirm the removal of the fault, and return to Subsection 8.1. If
the repair failed, return to Subsection 8.1 and replace the
complete main-board assembly.
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