Tait TP9100 - Page 185

TP9100 Service Manual Frequency Synthesizer Fault Finding 185
© Tait Electronics Limited May 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 LO1 at the RX port shown in
Figure 9.11.
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 of T401 (see Figure 9.12) before and after
grounding the junction between C541 and R547 (see Figure 9.11):
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 9.2.
3. If the receive frequency measured in Step 2 is correct, go to Step 4.
If it is incorrect, go to Ta sk 1 9
. If no frequency is detected, go to
Task 2 0
.
4. 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
(see Figure 9.11); there is a convenient 3V level at R544
(see Figure 9.11):
While holding the probe on the
RX port, use the tweezers to momen-
tarily 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 9.2.
5. If the receive frequency measured in Step 4 is correct, go to Task 1 8
.
If it is incorrect, go to Ta sk 1 9
. If no frequency is detected, go to
Task 2 0
.
RX port: maximum receive frequency (see Table 9.2)
RX port: minimum receive frequency (see Table 9.2)