TB7100 Service Manual Power Supply Fault Finding 131
© Tait Electronics Limited October 2005
2. Disconnect the 13.8V supply at the power connector. Remove R199
(see Figure 9.1). Reconnect the 13.8V supply.
3. With the probe of the multimeter on the 3
V3 test point, press the
ON/OFF key. If the voltage is now 3.3 ± 0.1V, the digital board is faulty;
replace the main-board assembly and go to “Final Tasks” on
page 123. If the voltage is still not correct, go to Step 4.
4. If the digital board is functional, the fault is on the main board.
Replace R199. Disconnect the 13.8V supply. Use the multimeter to
measure the resistance between the 3
V3 test point and ground.
If there is a short circuit, continue with Step 5. If there is no short
circuit (but the voltage is wrong), go to Step 7.
5. Search for shorts to ground in the components C603, C612, C613,
C618, D606 of the SMPS circuitry (see Figure 9.1) as well as in the
CODEC and interface circuitry. Repair any fault and repeat the
resistance measurement of Step 4 to confirm the removal of the fault.
If there is no fault, go to Step 6. If the fault remains, the repair failed;
replace the main-board assembly and go to “Final Tasks” on
page 123.
6. Reconnect the 13.8V supply. Press the
ON/OFF key. If the radio powers
up, return to “Initial Tasks” on page 111. If the radio fails to power
up, disconnect the 13.8V supply and go to Step 7.
7. Measure the resistance of L601 (see Figure 9.1). The resistance
should be virtually zero. If it is, go to Step 8. If it is not, replace L601.
Reconnect the 13.8V supply and press the
ON/OFF key. If the radio
powers up, return to “Initial Tasks” on page 111. If the radio fails to
power up, disconnect the 13.8V supply and go to Step 8.
8. Remove the
CDC BOT can. Remove IC603 (3.0V regulator) and
IC604 (2.5V regulator) (see Figure 9.3). Reconnect the 13.8V
supply and press the
ON/OFF key. If the 3.3V supply is restored, go to
Task 3
to check each regulator (3.0V and 2.5V) in turn. If the 3.3V
supply is not restored, continue with Step 9.
9. Suspect IC602. Disconnect the 13.8V supply. Replace IC602 with a
spare (see Figure 9.1). Resolder IC603 and IC604 in position
(see Figure 9.3). Reconnect the 13.8V supply and press the
ON/OFF
key. If the radio powers up, return to “Initial Tasks” on page 111.
If the radio fails to power up, the repair failed; replace the main-board
assembly and go to “Final Tasks” on page 123.
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