344 Transmitter Fault Finding (25W) TM8100/TM8200 Service Manual
© Tait Electronics Limited June 2006
Task 1 3 —
Check Biasing
of First PA
Check the biasing of the first PA (Q310).
Important Ensure that the current limit on the DC supply is 2A.
And, when entering the CCTM command 331 x, do not
specify a value x higher than that recorded in Task 12.
Failure 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 12.8). The voltage should be:
2. Note the current reading on the DC power supply. As mentioned in
Step 7 of Ta s k 1 2
, 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 12.6.
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 1 5
.
7. Enter the CCTM command 331 1 to switch off final bias 1, and go to
Tas k 16
.
pin 14 of IC301: < 100mV (initially)
pin 14 of IC301: 2 to 5V (after entry of CCTM 331 x)
Table 12.6 Gate biases for the PAs and PA driver at high power
Frequency band
Offset currents in mA
First PA Second PA PA driver
A4 750 750 300
B1 750 750 300
C0 750 750 300
D1 800 800 300
H5 1000 1000 450
H6 1000 1000 450
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