Loop Filter; Check Loop Voltage - Tait TM9100 Service Manual

9.4

Loop Filter

If the power supplies for the frequency synthesizer are correct, and the PLL
Introduction
is functioning properly, check the loop filter next:
The test and measurement points for diagnosing faults concerning the loop
filter are summarized in
Check whether the loop filter is functioning correctly by measuring the
Task 11 —

Check Loop Voltage

loop voltage at the output of the filter at C565.
1.
2.
3.
4.
5.
6.
TM9100 Service Manual
© Tait Electronics Limited August 2005
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
503 test point
Task 16: check
TP
Figure
If not already done, remove the main-board assembly from the
chassis, remove the
SYN TOP
Remove R542 (see Figure
Using an oscilloscope, proceed as follows to observe the voltage at
C565 before and after grounding the junction between C541 and
R547 (see Figure 9.7):
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):
C565: 13.3 ± 0.3 V DC
If the loop voltage is correct, go to
circuitry is suspect; go to
Proceed as follows to observe the voltage at C565 before and after
applying 3 V DC to the junction of C541 and R547; there is a
convenient 3 V level at R544 (see
While holding the probe at C565, use the tweezers to momentarily
apply 3 V 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:
C565: < 0.5 V DC
If the loop voltage is correct, go to
circuitry is suspect; go to
line
9.5.
can, and place the radio in CCTM.
9.7).
Step 5. If it is not, the loop-filter
Task 13.
Figure 9.7):
Task 12. If it is not, the loop-filter
Task 13.
Frequency Synthesizer Fault Finding 187