TB7100 Service Manual Frequency Synthesizer Fault Finding 195
© Tait Electronics Limited October 2005
11.10 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 11.16). 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 11.16).
5. Repair the circuitry. Note that the TCXO input to the mixer at
R521 (pin 4 of IC501) (see Figure 11.16) should be:
Also, the VCXO input to the mixer at R522 (pin 1 of IC501)
(see Figure 11.16), 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 123.
TP502 test point: sine wave of 1.1 ± 0.2 V
pp
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
pp
VCXO input at R522: sine wave of 20 ± 10 mV
pp
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