Table 4-18. Lla And 2Nd Stage Typical Voltages - Motorola ASTRO Digital Spectra Plus Service Manual

4-52
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. The following information
should help in isolation and repair of the majority of transmitter failures.
Testing Low-Level Amplifier (LLA) Circuitry
Proper operation of the LLA can be checked by monitoring the voltage across resistor R5805. The
voltage should measure in the range of 0.4 to 1.2 V, depending on the value of control voltage. A 0.4-
V reading corresponds to a low control voltage (4 to 5 V) and a 1.2-V reading corresponds to a high
control voltage (up to control voltage limit).
Measure LLA voltages according to
the LLA is providing drive power to 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 RF drive to the input of the PA, Q5803 collector current should be zero.
CONTROL
VOLTAGE
Q5801
Base
Collector
Q5800
Base
Collector
Emitter
Q5806
Base
Collector
Emitter
Q5803
Base
Collector
NOTE: The LLA voltages change with different control voltages. An example of LLA voltages with
control voltage equal to 10.0 V and 6 V is shown.
If Q5803 draws no current under normal conditions, then check for short or open input cable, or for
defective parts in the transmit injection filter or matching circuitry between Q5801 and Q5803.
Testing Second Stage Circuitry Q5803
The second stage is a typical class-C stage, except the base is biased with resistors R5809 and
R5806. The necessary conditions for proper operation of this stage are input drive power, and bias
conditions as shown in
October 28, 2002
Troubleshooting Procedures: Power Amplifier Procedures
Table 4-18. If the DC bias conditions are correct, check to see if

Table 4-18. LLA and 2nd Stage Typical Voltages

RF DRIVE OFF
10.0 V 6.0 V
— —
0.7 0.7
8.1 9.1
— —
7.6 8.5
2.3 1.4
8.1 9.1
— —
6.4 3.8
7.6 8.5
5.7 3.2
— —
0.6 0.6
9.6 9.6
Table 4-18.
RF DRIVE ON
10.0 V 6.0 V
— —
0.7 0.3
8.0 8.8
— —
7.4 8.3
2.8 1.1
8.0 8.8
— —
6.4 3.9
7.4 8.3
5.7 3.2
— —
0.0 0.3
9.5 9.5
6881076C25-D