198 Frequency Synthesizer Fault Finding TM8100/TM8200 Service Manual© Tait Electronics Limited November 20079.3 Phase-locked LoopIntroduction If there is no fault with the power supplies, check the critical output from,and inputs to, the PLL:■ Task 7: supply for charge pump■ Task 8: reference frequency input■ Task 9: DIG SYN EN line input■ Task 10: SYN LOCK line output.The measurement points for diagnosing faults concerning the PLL inputsand output are summarized in Figure 9.6.Task 7 —Supply forCharge PumpFirst check the supply for the charge pump of the PLL.1. Measure the supply for the charge pump at pin 16 of IC503(see Figure 9.2).2. If the voltage is correct, go to Task 8. If it is not, go to Step 3.3. Check the C-multiplier Q512 (see Figure 9.2) and check IC503itself; if necessary, replace the transistor or IC.4. If there is a fault, repair the circuit, confirm the removal of the fault,and go to “Final Tasks” on page 163. If the repair failed or no faultcould be found, replace the board and go to “Final Tasks” onpage 163.pin 16 of IC503: 5.0 ± 0.3V DC
