Tait TP9100 Service Manual

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TP9100 Service Manual Frequency Synthesizer Fault Finding 185© Tait Electronics Limited May 2005Task 17 óCheck Receive VCOCheck that the correct receive frequency is synthesized. This is thefrequency of the receive VCO output SYN RX LO1 at the RX port shown inFigure 9.11.1. Enter the CCTM command 335 0 to set the transmit-receive switchoff (receive mode).2. Using a frequency counter, proceed as follows to observe the receivefrequency at the RX port of T401 (see Figure 9.12) before and aftergrounding the junction between C541 and R547 (see Figure 9.11):While holding the probe from the counter on the RX port, use a pairof tweezers to momentarily ground the junction. The frequencyshould change to:The loop filter will hold its output steady at 13.3 V. This should resultin 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 Task 19. If no frequency is detected, go toTask 20.4. Proceed as follows to observe the receive frequency at the RX portbefore 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 yourhand, and do not allow the tweezers to touch any cans when youremove them. The frequency should change to:The loop filter will hold its output steady at about 0V. This shouldresult 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 18.If it is incorrect, go to Task 19. If no frequency is detected, go toTask 20.RX port: maximum receive frequency (see Table 9.2)RX port: minimum receive frequency (see Table 9.2)