Tait TB7100, Service Manual

Page: 194 / 370

196 Frequency Synthesizer Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
Task 38 —
TP501 Test Point
If the signal at the
TP
502 test point is correct, check the signal at the
TP
501
test point:
1. With the oscilloscope probe at the
TP
501
test point
(see Figure 11.16 ), check the DAC output
CDC VCXO MOD
. If a
triangular wave is present, go to Step 2 . Otherwise go to “VCXO and
CODEC Circuitry” on page 198 .
2. A fault is causing the loop to oscillate. If not already done, remove the
FCL TOP
can.
3. Check the waveform at the
TP
500
test point (see Figure 11.16 ).
The waveform should be an amplified and inverted version of the
waveform at the
TP
501
test point .
4. If the waveform is correct, go to Step 5 . If it is not, there is a fault in
the modulator buffer amplifier (IC502 pins 1 to 3, and associated
components) (see
Figure 11.16 ). Rectify the fault and return to
Step 1 .
5. Connect the
TP
501 test point to ground by resoldering R527 in the
position shown in Figure 11.16 . This forces the VCXO loop voltage
high.
6. Use the oscilloscope probe to check the VCXO output at C536 —
probe the via next to C536 (see
Figure 11.16 ). The signal should be:
7. If the signal is correct, go to Task 39 . If it is not, go to Step 8 .
8. The VCXO circuitry is faulty. If not already done, remove the
VCXO
BOT
can.
9. Locate and repair the fault in the VCXO circuitry (Q501, Q503,
XL501
and associated components) (see Figure 11.17 ).
10. Confirm the removal of the fault, and go to Task 39 . If the repair
failed, replace the main-board assembly and go to “Final Tasks” on
page 123 .
VCXO output at C536: sine wave with frequency of 13.017 MHz and
amplitude of 1.1
±
0.2 V
pp
on 1.4
±
0.2 V DC