190 Frequency Synthesizer Fault Finding TP9100 Service Manual
© Tait Electronics Limited May 2005
Task 22 —
Check Receive
Buffer Amplifier
If no receive frequency is detected but the switching network is not faulty,
check the receive buffer amplifier. If the amplifier is not faulty, there might
be a fault in the receive VCO that was not detected earlier.
1. Remove the
VCO BOT can.
2. Check the receive buffer amplifier in receive mode: Enter the CCTM
command 335 0 to set the transmit-receive switch off.
3. Measure the voltages at the collector of Q514 (see Figure 9.13) and
pin 4 of IC404 (see Figure 9.12).
4. Then check the receive buffer amplifier in transmit mode: Enter the
CCTM command 335 1 to set the transmit-receive switch on.
5. Again measure the voltage at Q514 and IC404.
6. If the voltages are correct, the receive VCO is suspect; go to Step 7.
If they are not, the receive buffer amplifier is suspect; go to Step 9.
7. Remove the
VCO TOP can.
8. Check the receive VCO circuitry based on Q504 (see Figure 9.11).
Conclude with Step 10.
9. Check the first buffer stage (based on Q514) (see Figure 9.13) and
the second stage (based on IC404) (see Figure 9.12).
10. 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 134. If they are
not, or if no fault could be found, replace the main board, and go to
“Final Tasks” on page 134.
collector of Q514: 0.7 ± 0.1V DC (receive mode)
pin 4 of IC404: 2.0 ± 0.1V DC (receive mode)
collector of Q514: 0V DC (transmit mode)
pin 4 of IC404: 0V DC (transmit mode
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