188 Frequency Synthesizer Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
Task 33 —
Check Buffer
Amplifier
If no VCO frequency is detected but the switching network is not faulty,
check the buffer amplifier. If the amplifier is not faulty, there might be a fault
in the VCO that was not detected earlier.
1. Enter the CCTM command 335 0 to set the transmit-receive switch
off.
2. Measure the voltage at pin 3 of D5004 (see Figure 11.13).
(Some RF noise might be observed.)
3. Measure the voltage at pin 1 of Q5001 (see Figure 11.13).
4. If the voltages measured in Step 2 and Step 3 are not correct, go to
Step 5. If they are, check the VCO circuitry based on Q5000
(see Figure 11.13). Conclude with Step 6.
5. The buffer amplifier is suspect. Check the buffer circuitry (based on
Q5001) (see Figure 11.13).
6. If a fault is found, repair the circuit, and confirm that the voltages are
now correct. If they are, go to “Final Tasks” on page 123. If they are
not, or if no fault could be found, replace the main-board assembly
and go to “Final Tasks” on page 123.
pin 3 of D5004: 4.2 ± 0.2 V DC
pin 1 of Q5001: 0.7 ± 0.2 V DC
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