TM9100 Service Manual Frequency Synthesizer Fault Finding 221© Tait Electronics Limited August 20059.10 Signals at TP501 and TP502Introduction If the VCXO and TCXO outputs are correct, the next stage is to check thesignals at the TP 501 and TP 502 test points. The procedure is divided intothree tasks:■ Task 37: check signal at TP 502■ Task 38: check signal at TP 501 and ground TP 501 if loop is oscillating■ Task 39: check signal at TP 502 with TP 501 groundedThese checks will reveal any faults in the mixer and LPF circuitry, and anyadditional fault in the VCXO circuitry.Task 37 —TP502 Test Point Check the signal at the TP 502 test point to determine if there is a fault in themixer or LPF (low-pass filter) circuitry:1. Use the oscilloscope probe to check the difference frequency at theTP 502 test point (see Figure 9.16). The signal is SYN CDC FCL andshould be:2. If the signal is correct, go to Task 38. If it is not, go to Step 3.3. The mixer or LPF circuitry under the FCL TOP can is faulty. Removethe FCL TOP can.4. Locate the fault in the mixer (IC501 and associated components) orLPF circuitry (IC502 pins 5 to 7, and associated components)(see Figure 9.16).5. Repair the circuitry. Note that the TCXO input to the mixer atR521 (pin 4 of IC501) (see Figure 9.16) should be:Also, the VCXO input to the mixer at R522 (pin 1 of IC501)(see Figure 9.16), although noisy and difficult to measure, should be:6. Confirm the removal of the fault and go to Task 38. If the repairfailed, replace the main-board assembly and go to “Final Tasks” onpage 147.TP502 test point: sine wave of 1.1 ± 0.2 Vpp on 1.5 ± 0.1 V DCTCXO input at R521: square wave with frequency of 13000000 Hzand amplitude of 3.0 ± 0.2 V ppVCXO input at R522: sine wave of 20 ± 10 mV pp
