Tait TB7100, Service Manual

Page: 224 / 370

226 Transmitter Fault Finding (25W) TB7100 Service Manual
© Tait Electronics Limited October 2005
Task 2 —
Check Switch Circuit
If the power supply to the drains of the PAs and PA driver is not correct, the
switch circuit is suspect. Check the circuit as follows:
1. Measure the voltage at the point 1 on R350 shown in Figure 13.3 .
The voltage should be:
2. If the voltage measured in Step 1 is correct, go to Step 3 . If it is not,
check for continuity between R350 and the power connector. Repair
any fault and conclude with
Step 8 .
3. Measure the voltage at R339 as shown in Figure 13.3 . The voltage
should be:
4. If the voltage measured in Step 3 is correct, go to Step 5 . If it is not,
go to Task 3 and check the 9 V power supply.
5. Measure the voltage at the point 2 on R350 shown in Figure 13.3 .
The voltage should be:
6. If the voltage measured in Step 5 is correct, go to Step 7 . If it is not,
replace Q308 — see Figure 13.3 — and conclude with Step 8 .
7. Remove the heat-transfer block from the main board. Replace Q311
(situated on the bottom-side of the main board next to the power
connector). Replace the heat-transfer block, and conclude with
Step 8 .
8. Repeat Task 1 to confirm the removal of the fault, and go to “Final
Tasks” on page 123 . If the repair failed or the fault could not be
found, replace the main-board assembly and go to “Final Tasks” on
page 123 .
Task 3 —
9V Power Supply
If the supply from the power connector is correct, check the 9 V DC supply.
1. Enter the CCTM command
326 1
to set the transmitter power level
very low.
2. Enter the CCTM command
33
to place the radio in transmit mode.
3. Measure the supply voltage between the 9
V
0
TX
test point and the
GND
test point (see Figure 13.4 ).
4. Enter the CCTM command
32
to place the radio in receive mode.
5. If the supply measured in Step 3 is correct, go to Task 4 in
“Transmitter RF Power” on page 230 . If it is not, the 9V regulator
IC601 and the associated switching circuitry Q603 are suspect; go to
Task 3 of “Power Supply Fault Finding” on page 132 .
point 1 on R350: 13.8V DC
R339: 9V DC
point 2 on R350: < 5V DC
supply 9V0 TX: 9.0
±
0.5V DC