Tait TM8100 mobiles, Service Manual

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204 Frequency Synthesizer Fault Finding TM8100/TM8200 Service Manual
© Tait Electronics Limited November 2007
9.4 Loop Filter
Introduction
If the power supplies for the frequency synthesizer are correct, and the PLL
is functioning properly, check the loop filter next:
■
Task 11 : check loop voltage
■
Task 12 : VCO fault
■
Task 13 : check reference voltage
■
Task 14 : check feedback voltage
■
Task 15 : check
DIG SYN FAST
line
■
Task 16 : check
TP
503
test point.
The test and measurement points for diagnosing faults concerning the loop
filter are summarized in
Figure 9.6 .
Task 11 —
Check Loop Voltage
Check whether the loop filter is functioning correctly by measuring the
loop voltage at the output of the filter at C565.
1. If not already done, remove the board from the chassis, remove the
SYN TOP
can, and place the radio in CCTM.
2. Remove R542 (see Figure 9.8 ).
3. Using an oscilloscope, proceed as follows to observe the voltage at
C565 before and after grounding the junction between C541 and
R547 (see Figure 9.8 ):
While holding the oscilloscope probe at C565, use a pair of tweezers
to momentarily ground the junction. The voltage should change to
the following value (if it is not already at this value):
4. If the loop voltage is correct, go to Step 5 . If it is not, the loop-filter
circuitry is suspect; go to Task 13
.
5. Proceed as follows to observe the voltage at C565 before and after
applying 3V DC to the junction of C541 and R547 ; there is a
convenient 3V level at R544 (see Figure 9.8 ):
While holding the probe at C565, use the tweezers to momentarily
apply 3V DC to the junction; do not touch the board with your
hand, and do not allow the tweezers to touch any cans when you
remove them. The voltage should change to:
6. If the loop voltage is correct, go to Task 12 . If it is not, the loop-filter
circuitry is suspect; go to Task 13 .
C565: 13.3
±
0.3V DC
C565: < 0.5V DC