September 5, 2008 6881076C25-E
4-62 Troubleshooting Procedures: Power Amplifier Procedures
3. Power Intermittently Low (or Zero) and Current Less than 1 A. When Power Drops
- Check LLA stage.
4. Power Zero and Current Greater Than 2 A.
- Check harmonic filter, antenna switch, matching circuits between driver and final stages,
and matching circuits beyond final stage.
5. Power Zero and Current Less Than 1 A.
- Check LLA/pre-driver circuitry.
4.5.2.2.4 Isolating Failures
Methods of analyzing individual stages of the power amplifiers are detailed below. Most of the stages
are Class C and must be analyzed under relatively high RF power levels. Generators capable of
such levels may not be available in all service shops, therefore the tests below are arranged in order
of ascending power. This tends to allow the preceding stage to be the source of RF power for testing
the next stage.
1. Testing Low-Level Amplifier (LLA) Circuitry
The required DC and RF conditions are defined in Table 4-19 on page 4-58. Measure LLA
voltages according to Table 4-22.
If the above DC bias conditions are correct, check to see if the LLA is providing drive power to
the pre-driver, Q5803. Do so by checking Q5803 collector current under normal drive
conditions, as follows:
- Remove R5810 and L5806 (Be sure to reinstall after testing.)
- Solder wires to the remaining pads.
- Place an ammeter in series with Q5803 collector.
- Check for 0.2 to 0.5 A. (depending on control voltage).
NOTE: With no RFdrive to the input of the PA, Q5803 collector current should be zero.
Table 4-22. LLA and Pre-Driver Typical Voltages
CONTROL
VOLTAGE
RF DRIVE OFF RF DRIVE ON
10.0 V 6.0 V 10.0 V 6.0 V
Q5801
Base
Collector
–
0.7
8.1
–
0.7
9.1
–
0.7
8.0
–
0.3
8.8
Q5800
Base
Collector
Emitter
–
7.6
2.3
8.1
–
8.5
1.4
9.1
–
7.4
2.8
8.0
–
8.3
1.1
8.8
Q5806
Base
Collector
Emitter
–
6.4
7.6
5.7
–
3.8
8.5
3.2
–
6.4
7.4
5.7
–
3.9
8.3
3.2
Q5803
Base
Collector
–
0.6
9.6
–
0.6
9.6
–
0.0
9.5
–
0.3
9.5
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