Tait TB7100, Service Manual

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TB7100 Service Manual Transmitter Fault Finding (50W/40W) 299
© Tait Electronics Limited October 2005
Task 13 —
Check Biasing
of First PA
Check the biasing of the first PA (Q310).
Important
Ensure that the current limit on the DC supply is 3 A.
And, when entering the CCTM command
331 x , do not
specify a value
x
higher than that recorded in Task 12 . Fail-
ure to do so might result in the destruction of the PAs.
1. Use a multimeter to measure the voltage at pin 14 of IC301
(see
Figure 14.11 and Figure 14.12 ). The voltage should be:
2. Note the current reading on the DC power supply. As mentioned in
Step 7 of Task 12 , this will be less than 500mA.
3. Enter the CCTM command 331 x (where x was recorded in Task 12 ).
4. Check that the voltage changes to:
5. Also note the current reading. This should increase by an amount
approximately equal to the offset given in
Table 14.6 .
6. If the voltage and current are both correct, go to Step 7 . If the voltage
is correct but not the current, go to Task 14 . If neither the current
nor the voltage is correct, go to Task 15 .
7. Enter the CCTM command 331 1 to switch off final bias 1, and go to
Task 16 .
pin 14 of IC301: < 100mV (initially)
pin 14 of IC301: 2 to 5V (after entry of CCTM 331 x )
Table 14.6 Gate biases for the PAs and PA driver at high power
Frequency band
Offset currents in mA
First PA Second PA PA driver
B1 1690 1690 150
H5 1800 1800 400
H7 1800 1800 600