Tait TB7100, Service Manual

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182 Frequency Synthesizer Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
Task 28 —
Check VCO
Check that the correct receive and transmit frequencies are synthesized.
The receive frequency is that of the VCO output
SYN RX LO
1
at the
RX
port
shown in Figure 11.12 . The transmit frequency is that of the output
SYN TX
LO
at the
TX
port.
1. Enter the CCTM command
335 1
to set the transmit-receive switch
on (transmit mode).
2. Using a frequency counter, proceed as follows to observe the transmit
frequency at the
TX
port before and after grounding the junction
between C541 and R547 (see Figure 11.12 ):
While holding the probe from the counter on the
TX
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 11.4 .
3. If the maximum frequency measured in Step 2 is correct, go to
Step 4 . If it is incorrect, go to Task 30 , but if no frequency at all is
detected, go to Task 31 .
4. Enter the CCTM command
335 0
to set the transmit-receive switch
off (receive mode).
5. 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 11.4 .
6. If the minimum frequency measured in Step 5 is correct, go to
Task 29 . If it is incorrect, go to Task 30 . If no frequency is detected,
go to Task 31 .
TX port: maximum VCO frequency (see Table 11.4 )
RX port: minimum VCO frequency (see Table 11.4 )