Tait TB7100, Service Manual

Page: 252 / 370

254 Transmitter Fault Finding (25W) TB7100 Service Manual
© Tait Electronics Limited October 2005
Task 22 —
Directional Coupler
and Buffer
Amplifier
Following the checks in Task 19 to Task 21 , locate the fault and repair the
circuitry as described in the remaining tasks of the section. In this task any
faults in the directional coupler or buffer amplifier will be located.
1. Cycle the power.
2. Enter the CCTM command 326 5 to set the transmitter to maximum
power.
3. Enter the CCTM command 33 to place the radio in transmit mode.
4. Measure the voltage at pin 9 of IC303 in the power-control circuit
(see
Figure 13.9 ).
5. The above voltage should be as given in Table 13.7 . If it is, go to
Task 24 . If it is not, go to Step 6 .
6. Check the voltage at pin 5 of IC303 (or use the
FWD PWR
test point )
(see Figure 13.9 ). Note that the probe impedance might affect the
measurement.
7. Enter the CCTM command 32 to place the radio in receive mode.
8. The voltage measured in Step 6 should be as given in Table 13.7 .
If it is not, go to Step 9 . If it is, go to Step 11 .
9. Remove the
DIRC TOP
can.
10. Check the components of the directional coupler (see Figure 13.12 )
and go to Step 12 .
11. Check R340 between pins 6 and 7 of IC303 in the buffer amplifier
(see
Figure 13.13 ), and then go to Step 12 .
12. Repair any fault revealed by the above checks. Replace IC303 if none
of the other components is faulty (see
Figure 13.9 ).
Table 13.7 Voltages at IC303 at maximum power (40 W)
Frequency band Frequency (MHz)
Voltage (V)
Pin 9 Pin 5 (
FWD PWR
)
B1
136
155
174
2.2 ± 0.5
2.3 ± 0.5
2.5 ± 0.5
1.9 ± 0.5
2.1 ± 0.5
2.3 ± 0.5
H5
400
435
470
3.4 ± 0.5
3.8 ± 0.5
4.0 ± 0.5
3.3 ± 0.5
3.7 ± 0.5
3.9 ± 0.5
H6
450
490
530
3.9 ± 0.5
4.2 ± 0.5
4.7 ± 0.5
3.8 ± 0.5
4.1 ± 0.5
4.6 ± 0.5