Tait TB7100, Service Manual

Page: 174 / 370

176 Frequency Synthesizer Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
Task 23 —
Check Transmit VCO
Check that the correct transmit frequency is synthesized. This is the
frequency of the transmit VCO output
SYN TX LO
at the
TX
port shown in
Figure 11.9 .
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 11.9 ):
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 11.3 .
3. If the transmit frequency measured in Step 2 is correct, go to Step 4 .
If it is incorrect, go to Task 25 . If no frequency is detected, go to
Task 26 .
4. Proceed as follows to observe the transmit frequency at the
TX
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
TX
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 11.3 .
5. If the transmit frequency measured in Step 4 is correct, go to Task 24 .
If it is incorrect, go to Task 25 . If no frequency is detected, go to
Task 26 .
TX port: maximum transmit frequency (see Table 11.3 )
TX port: minimum transmit frequency (see Table 11.3 )