TM8100/TM8200 Service Manual Frequency Synthesizer Fault Finding 233
© Tait Electronics Limited June 2006
9.10 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:
â– Task 37: check signal at TP502
â– Task 38: check signal at TP501 and ground TP501 if loop is oscillating
â– 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 9.17). The signal is SYN CDC FCL and
should be:
2. If the signal is correct, go to Tas k 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 9.17).
A4 band: Check the mixer (IC501, IC506 and associated
components) or LPF circuitry (IC502 pins 5 to 7, and associated
components) (see Figure 9.17). Also check L501 and associated
components and buffer Q504, IC509 and associated components.
5. Repair the circuitry. Note that the TCXO input to the mixer
(see Figure 9.17) should be:
TP502 test point: sine wave of 1.1 ± 0.2 V
pp
on 1.5 ± 0.1 V DC
A4 band: The signal should be:
TP502 test point: triangular wave of 1.5 ± 0.2 V
pp
on 1.5 ± 0.1 V DC
TCXO input at R521 (pin 4 of IC501):
TCXO input: square wave with frequency of 13 MHz
and amplitude of 3.0 ± 0.2 V
pp
A4 band: TCXO input at R520 (pin 4 of IC506):
TCXO input: square wave with frequency of 10.4MHz
and amplitude of 3.0 ± 0.2 V
pp
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