TB8100 Service Manual Power Amplifier Fault Finding 157
© Tait Electronics Limited September 2006
Task 3 —
Check Driver
Transistor
Also carry out this task if you have tried to calibrate the PA bias in Ta sk 4
on page 143 and the driver calibration failed.
1. Disconnect the DC supply from the PA.
2. Measure the DC resistance between the gate of Q103 and ground. If
the resistance is greater than 5k ohms, go to Step 4. If it is less than
5k ohms, replace the 6W board.
3. Reconnect the DC supply to the PA.
4. Check that 28V is present on the drain of Q103. If it is, go to Step 5.
If it is not, replace the 6W board.
5. Connect an oscilloscope across pin 3 of SK103
(DRIVER-BIAS-CONT) and ground.
6. Use the Calibration Kit to calibrate the PA bias. Check that the
DRIVER-BIAS-CONT voltage ramps from 0V to 5V during
calibration. If it does, replace the 6W board. If it does not, replace
the control board and go to Task 4
.
Task 4 —
Check Tx Key
1. Check that the PA is set to its rated output power (i.e. 5W, 50W, or
100W according to model).
2. Put the base station in Run Mode and key the PA using the TX KEY
switch on the CTU.
3. Measure the RF detect voltage at pin 2 of SK102. The voltage should
be less than 0.8V. Measure the PA_KEY_COAX voltage at pin 3 of
SK102 (or TP100). The voltage should be less than 0.5V. If both
voltages are correct, go to Step 6. If one of these voltages is incorrect
(or both are incorrect), go to Step 4.
4. Check that both the RF signal and DC key voltage are present on the
coaxial cable from the reciter. If they are, go to Step 5. If they are
not, check the coax and/or reciter and then repeat the test.
Note A DC bias voltage is present on the reciter’s RF output signal.
Take care that this does not damage your test equipment.
5. Check that the SMA RF input connector and nearby components
(e.g. C100, L100) are correctly soldered and undamaged. If they are,
replace the 6W board. If they are not, repair and repeat the test.
6. Check that the +10V_SWITCHED_CONT voltage is present on
pin 4 of SK103. If it is, go to Step 7. If it is not, go to “Control
Board” on page 148.
7. Measure the voltage at TP101. The voltage should be between 2.9V
and 3.4V. If it is, go to Step 8. If it is not, and the control board is
not faulty, replace the 6W board.
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