Tait TB7100, Service Manual, Page: 151 / 370

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 +5
V 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 5 V 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