Tait TB7100, Service Manual

Page: 172 / 370

174 Frequency Synthesizer Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 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 base of Q5100 and at pin 4 of IC5100
(see
Figure 11.10 ).
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 voltages of Q5100 and IC5100.
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 11.9 ).
Conclude with Step 10 .
9. Check the first buffer stage (based on Q5100) and the second stage
(based on IC5100) (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 .
base of Q5100: 0.7
±
0.1V DC (receive mode)
pin 4 of IC5100: 2.0
±
0.5V DC (receive mode)
base of Q5100: 0V DC (transmit mode)
pin 4 of IC5100: 0V DC (transmit mode)