Tait TM8100 mobiles, Service Manual

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TM8100/TM8200 Service Manual Frequency Synthesizer Fault Finding 233
© Tait Electronics Limited November 2007
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 9.15 ). (Some RF
noise might be observed.)
3. Measure the voltage at pin 1 of Q5001 (see Figure 9.15 ).
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 9.15 ). Conclude with Step 6 .
5. The buffer amplifier is suspect. Check the buffer circuitry (based on
Q5001 ) (see Figure 9.15 ).
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 163 . If they are
not, or if no fault could be found, replace the board and go to “Final
Tasks” on page 163 .
pin 3 of D5004: 4.2
±
0.2V DC
pin 1 of Q5001: 0.7
±
0.2V DC