Tait TB7100, Service Manual

Page: 295 / 370

TB7100 Service Manual Transmitter Fault Finding (50W/40W) 297
© Tait Electronics Limited October 2005
14.3 Biasing of PA Driver and PAs
Introduction
The measurements of the transmitter RF output power in “Transmitter RF
Power” might indicate a need to check the biasing of the two PAs and the
PA driver. The procedure is covered in this section. There are thirteen tasks
grouped as follows:
■
Task 12 : prepare to check biasing
■
Task 13 to Task 15 : check biasing of first PA
■
Task 16 to Task 18 : check biasing of second PA
■
Task 19 and Task 20 : check biasing of PA driver
■
Task 21 to Task 24 : repair circuitry
The test and measurement points for diagnosing faults in the biasing of the
PAs and PA driver are summarized in
Figure 14.10 .
Task 12 —
Prepare to
Check Biasing
If the transmitter is not being inhibited, check the biasing of the two PAs
and the PA driver. First make the following preparations:
1. Set the current limit on the DC power supply to 3 A.
2. Enter the CCTM command 331 to check the DAC value of final bias
1 (
CDC TX FIN BIAS
1
) at maximum power. Record the value
x
returned.
3. Enter the CCTM command 332 to check the DAC value of final bias
2 (
CDC TX FIN BIAS
2
) at maximum power. Record the value
y
returned.
4. Enter the CCTM command 304 to check the DAC value of the
clamp current at the driver gate. Record the value
z
returned.
5. Enter the CCTM command 33 to place the radio in transmit mode.
6. Switch off all biases by entering the following CCTM commands in
sequence:
■
331 1
■
332 1
■
304 1
■
114 1023
■
334 0
■
335 0
7. Note the current reading on the DC power supply. This will be less
than 500 mA.
8. With the radio still in transmit mode, check the biasing of the PAs and
PA driver, beginning with
Task 13 .