Motorola ASTRO Digital Spectra Service Manual page 178

4-46
Temperature Sense Circuit Test
Temporarily place a leaded 6.8k ohm resistor in parallel with RT3875. Key the transmitter and
monitor the output power. The power meter should read approximately 1/2 the rated power
(25 Watts).
Control-Voltage-Limit Circuitry Test
Disconnect the 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. It the voltage exceeds 9.0 V, troubleshoot the control voltage limit
circuitry.
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. After this test,
reset the current limit to its original value. If the circuitry does not perform as indicated, troubleshoot
the current limit circuitry.
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 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. If 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 probably in diode CR3900, the harmonic filter, the antenna switch, or the output
coax. Check continuity through the 12-pin connector P0853 on the PA board; check digital/analog
circuitry, and check 5-V regulator operation. See
With the radio connected for power measurements and a disconnected TX injection coax, the
detected voltage at 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
(68P81076C20) for details.
July 1, 2002
Troubleshooting Procedures: Power Amplifier Procedures
Table 4-15 for typical values.
68P81076C25-C