Tait TB7100, Service Manual

Page: 186 / 370

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