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TM9100 Service Manual Frequency Synthesizer Fault Finding 221

9.10 Signals at TP501 and TP502

Introduction If the VCXO and TCXO outputs are correct, the next stage is to check the

signals at the

TP501 and TP502 test points. The procedure is divided into

three tasks:

■ Task 37: check signal at TP502

■ Task 38: check signal at TP501 and ground TP501 if loop is oscillating

■ Task 39: check signal at TP502 with TP501 grounded

These checks will reveal any faults in the mixer and LPF circuitry, and any

additional fault in the VCXO circuitry.

Task 37 —

TP502 Test Point

Check the signal at the TP502 test point to determine if there is a fault in the

mixer or LPF (low-pass filter) circuitry:

1. Use the oscilloscope probe to check the difference frequency at the

TP502 test point (see Figure 9.16). The signal is SYN CDC FCL and

should 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. Remove

the

FCL TOP can.

4. Locate the fault in the mixer (IC501 and associated components) or

LPF 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 at

R521 (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 repair

failed, replace the main-board assembly and go to “Final Tasks” on

page 147.

TP502 test point: sine wave of 1.1 ± 0.2 V

on 1.5 ± 0.1 V DC

TCXO input at R521: square wave with frequency of 13000000 Hz

and amplitude of 3.0 ± 0.2 V

VCXO input at R522: sine wave of 20 ± 10 mV

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