162 Frequency Synthesizer Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
Task 12 —
VCO Faulty
If the loop voltage is correct, the loop filter is functioning properly.
The VCO and related circuitry is therefore suspect. The section to proceed
to depends on the type of the radio and the nature of the fault.
1. With a UHF radio go to Step 2. With a VHF radio go to “VCO and
Related Circuitry (VHF Radios)” on page 180.
2. If a UHF radio exhibits a lock error
or a receive fault. the receive
VCO is suspect; go to “Receive VCO and Related Circuitry (UHF
Radios)” on page 166.
If it exhibits a system error
or a transmit fault, the transmit VCO is
suspect; go to “Transmit VCO and Related Circuitry (UHF
Radios)” on page 175.
Task 13 —
Check Reference
Voltage
If the loop-filter circuitry is suspect, first check the reference voltage for the
filter.
1. Remove the
SYN BOT can.
2. Measure the reference voltage at pin 5 of IC505 (see Figure 11.4).
The result should be:
3. If the voltage is correct, go to Task 14
. If it is not, the reference-
voltage circuitry is suspect; go to Step 4.
4. Resolder R542 in position and check the C-multiplier Q512
(see Figure 11.7).
5. If a fault is found, repair the circuit, and confirm that the reference
voltage is now correct. If it is, go to “Final Tasks” on page 123. If it
is not, or if no fault could be found, replace the main-board assembly
and go to “Final Tasks” on page 123.
IC505 pin 5: 2.8 ± 0.1 V DC
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