Tait TP9100 - Page 213

TP9100 Service Manual Frequency Synthesizer Fault Finding 213
© Tait Electronics Limited May 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 9.21). 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 9.19). Locate the fault.
4. 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:
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 9.19,
replace the main board and go to “Final Tasks” on page 134.
6. Resolder R527 in its original position as shown in Figure 9.19.
7. Replace all cans.
8. 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:
9. If the signal is correct, the fault has been removed; go to “Final
Tasks” on page 134. If the signal is not correct, the repair failed;
replace the main board and go to “Final Tasks” on page 134.
TP502 test point: sine wave with frequency of at least 15kHz and
amplitude of 1.1 ± 0.2V
pp
on 1.5 ± 0.1V DC
TCXO input at R521: square wave with frequency of 13000000Hz
and amplitude of 3.0 ± 0.2V
pp
VCXO input at R522: sine wave of 20 ± 10mV
pp
TP502 test point: sine wave of 1.1 ± 0.2V
pp
on 1.5 ± 0.1V DC
(approx. 12kHz)