Tait TB7100, Service Manual

Page: 177 / 370

TB7100 Service Manual Frequency Synthesizer Fault Finding 179
© Tait Electronics Limited October 2005
Task 27 —
Check Transmit
Buffer Amplifier
If no transmit frequency is detected but the switching network is not faulty,
check the transmit buffer amplifier. If the amplifier is not faulty, there might
be a fault in the transmit VCO that was not detected earlier.
1. Remove the
VCO BOT
can.
2. Check the transmit buffer amplifier in receive mode: Enter the
CCTM command
335 0
to set the transmit-receive switch off.
3. Measure the voltage at pin 6 of Q5001 (see Figure 11.10 ).
4. Then check the transmit buffer amplifier in transmit mode: Enter the
CCTM command
335 1
to set the transmit-receive switch on.
5. Again measure the voltage at Q5001.
6. If the voltages are correct, the transmit VCO is suspect; go to Step 7 .
If they are not, the transmit buffer amplifier is suspect; go to Step 9 .
7. Remove the
VCO TOP
can.
8. Check the transmit VCO circuitry based on Q510
(see
Figure 11.9 ). Conclude with Step 10 .
9. Check the buffer circuitry based on Q5001 (see Figure 11.10 ).
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 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 6 of Q5001: 0 V DC (receive mode)
pin 6 of Q5001: 0.7
±
0.1 V DC (transmit mode)