TM9100 Service Manual Frequency Synthesizer Fault Finding 187© Tait Electronics Limited August 20059.4 Loop FilterIntroduction If the power supplies for the frequency synthesizer are correct, and the PLLis 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 pointThe test and measurement points for diagnosing faults concerning the loopfilter are summarized in Figure 9.5.Task 11 —Check Loop Voltage Check whether the loop filter is functioning correctly by measuring theloop voltage at the output of the filter at C565.1. If not already done, remove the main-board assembly from thechassis, remove the SYN TOP can, and place the radio in CCTM.2. Remove R542 (see Figure 9.7).3. Using an oscilloscope, proceed as follows to observe the voltage atC565 before and after grounding the junction between C541 andR547 (see Figure 9.7):While holding the oscilloscope probe at C565, use a pair of tweezersto momentarily ground the junction. The voltage should change tothe 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-filtercircuitry is suspect; go to Task 13.5. Proceed as follows to observe the voltage at C565 before and afterapplying 3 V DC to the junction of C541 and R547; there is aconvenient 3 V level at R544 (see Figure 9.7):While holding the probe at C565, use the tweezers to momentarilyapply 3 V DC to the junction; do not touch the board with yourhand, and do not allow the tweezers to touch any cans when youremove them. The voltage should change to:6. If the loop voltage is correct, go to Task 12. If it is not, the loop-filtercircuitry is suspect; go to Task 13.C565: 13.3 ± 0.3 V DCC565: < 0.5 V DC
