Power Control And Protection Circuitry - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

Troubleshooting Procedures: Power Amplifier Procedures
- Check for proper DC current through the PIN diodes; correct current is indicated if
approximately 1.5 V is present at the junction of C3900 and L3900 during transmit mode.
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W A R N I N G

4.5.1.2.5 Power Control and Protection Circuitry

Localizing Problems to a Circuit
Power leveling and current limiting are set to values detailed in the ASTRO Digital Spectra and
Digital Spectra Plus Mobile Radios Basic Service Manual (6881076C20). These values will vary from
unit to unit, depending on the unique variations of each unit. If symptoms indicate that either of these
circuits have failed, verify that the radio has been properly aligned before investigating the circuitry.
Temperature sense and control voltage limit are fixed by design and are not influenced by the
alignment of the radio. If symptoms indicate that these circuits have failed, then troubleshoot the
circuit.
The tests that follow are intended to provide a convenient means of verifying that a particular circuit
is functioning properly. These tests will isolate the failure to a minimum number of components.
Refer to the Theory of Operation and the schematic for information needed to identify the failed
component(s).
Temperature Sense Circuit Test
Temporarily install a 6.8k ohm resistor in parallel with RT3876. Key the transmitter and monitor the
output power. The power meter should read approximately one-half the rated power (12 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. If the voltage exceeds 9.2 V, troubleshoot the control voltage limit
circuitry.
Current-Limiting Circuitry Test
Refer to Chapter 6 of the ASTRO Digital Spectra and Digital Spectra Plus Mobile Radios Basic
Service Manual (6881076C20) for current limit setting instructions. 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.
6881076C25-E
DO NOT measure bias directly at the PIN diodes while in
transmit mode unless TX injection is removed.
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September 5, 2008