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

4-28
Additional antenna switch tests are:
• Check CR3901, CR3902, and CR3903 using the diode check function of a multimeter. Note
that CR3903 is on the bottom side of the board. This diode affects the receive path only and is
unrelated to transmitter problems.
• Check for proper DC current through the PIN diodes; correct current is indicated if
approximately 1.5 V is present at the junction of R3900 and L3900 during transmit..
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W A R N I N G

4.5.1.1.3 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 2.2k ohm resistor in parallel with RT3842. Key the transmitter and monitor the
output power. The power meter should read approximately one-half the rated power.
Control-Voltage-Limit Circuitry Test
Disconnect the transmitter injection cable from J3850. With all other connections in normal condition,
key the transmitter and monitor the control voltage at J1 pin 2. If 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 in 0.5 to
1.0 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.
Directional Coupler and Power-Leveling Test
The directional coupler combined with the RPCIC form a closed-loop power leveling circuit. This
circuit keeps forward power essentially constant under variations of line voltage, frequency, and
VSWR.
September 5, 2008
DO NOT measure bias directly at the PIN diodes while in
transmit mode unless TX injection is removed.
Troubleshooting Procedures: Power Amplifier Procedures
6881076C25-E