Tait TP9100, Service Manual, Page: 188 / 326

188 Frequency Synthesizer Fault Finding
TP9100 Service Manual
© Tait Electronics Limited May 2005
Task 19 —
Repair Receive VCO
If either or both the minimum and maximum receive frequencies are
incorrect, the receive VCO circuitry is faulty.
1. Remove the
VCO TOP
can.
2. Check the receive VCO. The circuitry is based on Q504
(see Figure 9.11 ).
3. If a fault is found, repair it and go to Step 4 . If no fault is found, go
to Step 6 .
4. Repeat the frequency measurements in Step 2 and Step 4 of Task 17 .
5. If the frequencies are now correct, resolder R542 in position
(see Figure 9.11 ), and go to “Final Tasks” on page 134 . If they are
still not correct, go to Step 6 .
6. Resolder R542 in position (see Figure 9.11 ). Replace the main
board and go to “Final Tasks” on page 134 .
Task 20 —
Check Switching
to Receive Mode
If no receive frequency is detected in the check of the receive VCO, first
check that the transmit-receive switch is functioning correctly.
1. Resolder R542 in position (see Figure 9.11 ).
2. Enter the CCTM command
335 0
to switch on the supply to the
receive VCO.
3. Measure the voltage at the first collector (pin 3) of Q506
(see Figure 9.11 ). The voltage should be:
4. Enter the CCTM command 335 1 to switch off the supply.
5. Again measure the voltage at the first collector of Q506 .
6. If the voltages measured in Step 3 and Step 5 are correct, go to
Task 22 . If they are not, the switching network is suspect; go to
Task 21 .
pin 3 of Q506: 5.0
±
0.3V DC (after entry of CCTM 335 0)
pin 3 of Q506: 0V DC (after entry of CCTM 335 1)