TB7100 Service Manual Frequency Synthesizer Fault Finding 153
© Tait Electronics Limited October 2005
Task 5 —
5V Power Supply
If the SMPS output and 6V DC supply are correct, check the +5V DEC
supply next.
1. Remove the
SYN TOP can.
2. Measure the supply +5
V DEC at pin 4 of Q508 (see Figure 11.2).
3. If the voltage is correct, go to Task 6
. If it is not, go to Step 4 (UHF
radios) or Step 5 (VHF radios).
4. With a UHF radio check for faults in the C-multiplier Q508 (pins 3,
4, 5) and the 5V and transmit-receive switches based on Q506, Q507
and Q508 (pins 1, 2, 6) (see Figure 11.2). Replace any suspect
transistor. Conclude with Step 6.
5. With a VHF radio check for faults in the C-multiplier and 5V switch
based on Q508 and Q5004 (see Figure 11.2). Remove the
VCO BOT
can and check the transmit-receive switch based on Q5002 and
Q5003 (see Figure 11.4). Replace any suspect transistor. Conclude
with Step 6.
6. If a fault is found, repair the circuit, confirm the removal of the fault,
and go to “Final Tasks” on page 123. If the repair failed or no fault
could be found, replace the main-board assembly and go to “Final
Tasks” on page 123.
pin 4 of Q508: 5.3 ± 0.3V DC
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