Tait TP9100 - Page 211

TP9100 Service Manual Frequency Synthesizer Fault Finding 211
© Tait Electronics Limited May 2005
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 9.21). The signal is SYN CDC FCL and
should be:
2. If the signal is correct, go to Task 3 8
. If it is not, go to Step 3.
3. The mixer or LPF circuitry under the
FCL TOP can is faulty.
4. Locate the fault in the mixer (IC501 and associated components) or
LPF circuitry (IC502 pins 5 to 7, and associated components)
(see Figure 9.19).
5. Repair the circuitry. Note that the TCXO input to the mixer at
R521 (pin 4 of IC501) (see Figure 9.19) should be:
Also, the VCXO input to the mixer at R522 (pin 1 of IC501)
(see Figure 9.19), although noisy and difficult to measure, should be:
6. Confirm the removal of the fault and go to Task 3 8
. If the repair
failed, replace the main board and go to “Final Tasks” on page 134.
TP502 test point: sine wave of 1.1 ± 0.2V
pp
on 1.5 ± 0.1V DC +12kHz
TCXO input at R521: square wave with frequency of 13000000 Hz
and amplitude of 3.0 ± 0.2V
pp
VCXO input at R522: sine wave with frequency of 13.012±0.002MHz
and amplitude of 20 ± 10mV
pp