TB7100 Service Manual Frequency Synthesizer Fault Finding 197
© Tait Electronics Limited October 2005
Task 39 —
TP502 Test Point
(TP501 Grounded)
If the loop was oscillating, Task 38 will have revealed any fault in the
VCXO circuitry. If there was no fault, or if the circuit was repaired, a check
at the
TP502 test point is now required. This will show if there are any
additional faults in the mixer or LPF 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 Step 6. If it is not, go to Step 3.
3. The mixer circuitry (IC501 and associated components) or the LPF
circuitry (IC502 pins 5 to 7, and associated components) under the
FCL TOP can is faulty (see Figure 11.16). Locate the fault.
4. 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:
5. Confirm the removal of the fault, and go to Step 6. If the repair
failed, resolder R527 in its original position as shown in
Figure 11.16, replace the main-board assembly and go to “Final
Tasks” on page 123.
6. Resolder R527 in its original position as shown in Figure 11.16.
7. Replace all cans.
8. 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:
9. If the signal is correct, the fault has been removed; go to “Final
Tasks” on page 123. If the signal is not correct, the repair failed;
replace the main-board assembly and go to “Final Tasks” on
page 123.
TP502 test point: sine wave with frequency of at least 15 kHz and
amplitude 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
TP502 test point: sine wave of 1.1 ± 0.2 V
pp
on 1.5 ± 0.1 V DC
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