Tait TM8105 - Page 297

TM8100/TM8200 Service Manual Transmitter Fault Finding (40W/50W) 297
© Tait Electronics Limited June 2006
Task 22 —
Directional Coupler
and Buffer
Amplifiers
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 the buffer amplifiers will be located.
1. Cycle the power.
2. Enter the CCTM command 326 5 to set the transmitter to maximum
power. Enter the CCTM command 33 to place the radio in transmit
mode.
3. Measure the voltage at pin 9 of IC303 in the power-control circuit
(see Figure 11.8 and Figure 11.9).
4. The above voltage should be as given in Table 11.7. If it is, go to
Tas k 24
. If it is not, go to Step 5.
5. Check the voltage at the
FWD PWR test point (pin 5 of IC303) and at
the
REV PWR test point (pin 3 of IC303) (see Figure 11.8 and
Figure 11.9). Note that the probe impedance may affect these
measurements.
6. Enter the CCTM command 32 to place the radio in receive mode.
7. The voltages measured in Step 5 should be as given in Table 11.7.
If they are, go to Step 10. If the
FWD PWR voltage is incorrect, go to
Step 8. If the
REV PWR voltage is incorrect, go to Step 9.
8. Remove the
DIRC TOP can. Check the components of the directional
coupler (see Figure 11.13) and go to Step 11.
9. Remove the
DIRC TOP can. Check D305 and R3035 (VHF) or R383
(UHF) (see Figure 11.13). If there is no fault, the PIN switch or
LPF or both are suspect; go to Task 33. If there is a fault, go to
Step 11.
10. In the buffer amplifiers, check R340 (see Figure 11.8 for VHF and
Figure 11.14 for UHF) and R341 (see Figure 11.14 and
Figure 11.15).
Table 11.7 Voltages at IC303 at maximum power (70 W for VHF, and 60W for UHF)
Frequency band Frequency (MHz)
Voltage (V)
Pin 9 Pin 3 (
REV PWR) Pin 5 (FWD PWR)
B1 136
155
174
2.6 ± 0.5
2.9 ± 0.5
3.2 ± 0.5
0.4 ± 0.3
0.4 ± 0.3
0.5 ± 0.3
3.1 ± 0.5
3.4 ± 0.5
3.9 ± 0.5
H5 400
435
470
2.8 ± 0.5
3.0 ± 0.5
3.3 ± 0.5
0.6 ± 0.4
0.6 ± 0.4
0.5 ± 0.4
3.3 ± 0.5
3.7 ± 0.5
3.9 ± 0.5
H7 450
485
520
3.9 ± 0.5
4.1 ± 0.5
4.4 ± 0.5
0.6 ± 0.4
0.8 ± 0.4
0.8 ± 0.4
4.4 ± 0.5
4.6 ± 0.5
5.0 ± 0.5