Tait TB7100, Service Manual

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306 Transmitter Fault Finding (50W/40W) 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 3A.
And, when entering the CCTM command
332 y , do not
specify a value
y
higher than that recorded in Task 12 .
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
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 , the current will be less than 500mA.
3. Enter the CCTM command 332 y (where y 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 17 . If neither the current
nor the voltage is correct, go to Task 18 .
7. Enter the CCTM command 332 1 to switch off final bias 2, and go to
Task 19 .
pin 8 of IC301: < 100mV (initially)
pin 8 of IC301: 2 to 5V (after entry of CCTM 332 y )