Tait TB7100, Service Manual

Page: 294 / 370

296 Transmitter Fault Finding (50W/40W) TB7100 Service Manual
© Tait Electronics Limited October 2005
Task 11 —
Power and Current
Are Low
If the RF output power and the supply current are uniformly low at all
frequencies, one of the PAs is suspect or the input to the PAs is reduced.
Check each PA in turn:
1. For the first PA (Q310), enter the CCTM command
331
to check the
DAC value of final bias 1 (
CDC TX FIN BIAS
1 ). Record the value x
returned.
2. Note the current reading on the DC power supply.
3. Enter the CCTM command
331 1
to turn off final bias 1.
4. Enter the CCTM command 33 to place the radio in transmit mode.
5. Note the RF output power measured at the test set. This should be
as shown in
Table 14.5 .
6. If the RF power is correct, go to Step 7 to repeat the check with the
second PA. If it is not, enter the CCTM command 32 to place the
radio in receive mode, and carry out Task 12 and then Task 13 .
7. For the second PA (Q309), enter the CCTM command 332 to check
the DAC value of final bias 2 (
CDC TX FIN BIAS
2
). Record the value
y
returned.
8. Note the current reading on the DC power supply.
9. Enter the CCTM command 332 1 to turn off final bias 2.
10. With the radio still in transmit mode, note the RF output power
measured at the test set. This should be as shown in
Table 14.5 .
11. Enter the CCTM command
32
to place the radio in receive mode.
12. If the RF power measured in Step 10 is correct, go to “RF Signal
Path” on page 320 . If it is not, carry out Task 12 and then Task 16 .
Table 14.5 RF output power of individual RF power amplifiers at different frequencies
Frequency band
Frequency within band
Lowest frequency Centre frequency Highest frequency
B1
38 ± 5 W 48 ± 5 W 33 ± 5 W
H5
16 ± 5 W 17 ± 5 W 21 ± 5 W
H7
25 ± 5 W 32 ± 5 W 40 ± 5 W