Tait TM8100 mobiles, Service Manual

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226 Frequency Synthesizer Fault Finding TM8100/TM8200 Service Manual
© Tait Electronics Limited November 2007
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 9.14 . 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 9.14 ):
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 13.3V. This should result
in a frequency equal to the maximum given in
Table 9.5 .
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 3V DC to the junction of C541 and R547 ;
there is a convenient 3V level at R544 (see Figure 9.11 ):
While holding the probe on the
RX
port, use the tweezers to
momentarily apply 3V 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 9.5 .
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 9.5 )
RX port: minimum VCO frequency (see Table 9.5 )