Tait TB7100, Service Manual

Page: 166 / 370

168 Frequency Synthesizer Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
Task 17 —
Check Receive VCO
Check that the correct receive frequency is synthesized. This is the
frequency of the receive VCO output
SYN RX LO
1
at the
RX
port shown in
Figure 11.9 .
1. Enter the CCTM command 335 0 to set the transmit-receive switch
off (receive mode).
2. Using a frequency counter, proceed as follows to observe the receive
frequency at the
RX
port before and after grounding the junction
between C541 and R547 (see Figure 11.9 ):
While holding the probe from the counter on the
RX
port, use a pair
of tweezers to momentarily ground the junction. The frequency
should change to:
The loop filter will hold its output steady at 13.3 V. This should result
in a frequency equal to the maximum given in
Table 11.2 .
3. If the receive frequency measured in Step 2 is correct, go to Step 4 .
If it is incorrect, go to Task 19 , but if no frequency is detected, go to
Task 20 .
4. Proceed as follows to observe the receive frequency at the
RX
port
before and after applying 3 V DC to the junction of C541 and R547;
there is a convenient 3 V level at R544 (see
Figure 11.9 ):
While holding the probe on the
RX
port, use the tweezers to
momentarily apply 3 V DC to the junction; do not touch the board
with your hand, and do not allow the tweezers to touch any cans
when you remove them. The frequency should change to:
The loop filter will hold its output steady at about 0V. This should
result in a frequency equal to the minimum given in
Table 11.2 .
5. If the receive frequency measured in Step 4 is correct, go to Task 18 .
If it is incorrect, go to Task 19 . If no frequency is detected, go to
Task 20 .
RX port: maximum receive frequency (see Table 11.2 )
RX port: minimum receive frequency (see Table 11.2 )