Motorola ASTRO Digital Spectra, Detailed Service Manual, Page: 207 / 442

6881076C25-E September 5, 2008
Troubleshooting Procedures : Power Amplifier Procedures 4-65
3. Control-Voltage-Limit Circuitry Test
Disconnect J5901 (transmitter injection) from the internal transceiver chassis. This will
require removal of the power amplifier assembly. With all other connections in normal
condition, key the transmitter and monitor the control voltage at the node of R5811,
C5814,L5808, and R5808. If the voltage exceeds 10.0 V, troubleshoot the control voltage
limit circuitry.
4. Current-Limiting Circuitry Test
When ready to adjust current limit, decrease the relative current limit value with the keyboard
per instructions. After several decrements, the current limit should begin to reduce power in
0. 1- to 0.5-Watt increments. After this test, reset the current limit to its original value. If the
circuitry does not perform as indicated, troubleshoot the current limit circuitry.
5. Power-Leveling Circuitry Test
With the radio connected for power measurements, vary the line voltage from 12.5 to
16 V. The power should not vary more than 2 Watts. At a line voltage of 13.6 V, vary the
frequency using the three test modes. If power varies more than 2 Watts, measure the
detected voltage on P0853, pin 9. 1 this voltage varies more than 0.2 V over line and
frequency variations, the power control circuitry (most of which is located on the command
board) may be malfunctioning. If the detected voltage varies less than 0.2 V, the problem is
likely in diode CR5900, the harmonic filter, the antenna switch, or the output coax. Check
continuity through the 12-pin DC connector P0853 on the PA board; check digital/analog
circuitry, and check 5-V regulator operation. See
Table 4-20 on page 4-59 for typical values.
With the radio connected for power measurements and, disconnected TX injection coax, the
detected voltage a P0853, pin 9, should measure approximately 1.3 V.
NOTE: If any part of the power leveling circuitry is replaced, perform the power set procedure. See
the
ASTRO Digital Spectra and Digital Spectra Plus Mobile Radios Basic Service Manual
(6881076C20) for details.