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246
Transmitter Fault Finding (25W)
TB7100 Service Manual
© Tait Electronics Limited
October 2005
Task 16 —
Check Biasing
of Second PA
If the biasing of the first PA is correct, check that of the second PA (Q309).
Important
Ensure that the current limit on the DC supply is 2A.
And, when entering the CCTM command
332 y
, do not
specify a value
y
higher than that recorded in
Failure to do so might result in the destruction of the PAs.
1.
Use the multimeter to measure the voltage at pin 8 of IC301
(see
). The voltage should be:
2.
Note the current reading on the DC power supply. As mentioned in
, the current will be less than 500mA.
3.
Enter the CCTM command
332 y
(where
y
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
6.
If the voltage and current are both correct, go to
. If the voltage
is correct but not the current, go to
. If neither the current
nor the voltage is correct, go to
7.
Enter the CCTM command
332 1
to switch off final bias 2, and go to
.
pin 8 of IC301: < 100mV (initially)
pin 8 of IC301: 2 to 5V (after entry of CCTM 332
y
)
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